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Page 1: [Pergamon Materials Series] Multinuclear Solid-State NMR of Inorganic Materials Volume 6 || Chapter 10 NMR of other quadrupolar nuclei

Chapter 10

NMR of Other Quadrupolar Nuclei

The quadrupolar nuclei of greatest importance to materials science (27A1 and 170) have been dealt with in Chapters 5 and 6 respectively. Two other important quadrupolar nuclei (23Na and liB) have also been treated separately in Chapter 7. The present chap- ter deals with a number of the other quadrupolar nuclei encountered in solid state NMR studies of inorganic materials.

10.1. 6Li AND 7Li NMR

10.1.1 Generalconsiderations Both the lithium nuclides are suitable for NMR spectroscopy. The spin = 3/2 nucleus VLi is commonly used since it has a high natural abundance (92.5%) and favourable receptivity, but the quadrupole moment ( -4 .0 • 10 -3~ e m 2) can give rise to relatively broad lines from Li in non-symmetrical sites. The Larmor frequency of the spin = 1 nucleus 6Li is about 2.6 times smaller than 7Li and it has a much lower natural abun- dance (7.5%) and hence a less favourable receptivity, but its quadrupole moment is also significantly lower than 7Li and its homonuclear dipole-dipole interactions are much weaker, giving narrower resonance lines under MAS conditions. Thus, although 6Li has in the past been less widely used in solid-state NMR studies, its use can be preferable in circumstances requiring the resolution of two close resonances. Because of its smaller quadrupolar interaction, the 6Li shift may also more closely approximate the isotropic chemical shift and provide a better measure of the Li bonding environ- ment, but these benefits may be offset by the relaxation times which are often very long, and require longer data acquisition times. Comparing 2 isotopes with very dif- ferent interactions can also be used to understand the source of relaxation and hence the motion responsible. Figure 10.1A shows the 7Li MAS NMR spectrum of Li-con- taining beryl, one of the few materials in which the 7Li-VLi homonuclear dipole-dipole interactions are sufficiently small to be removed by magic angle spinning, revealing the second-order quadrupolar lineshape. In most other materials, dipolar-dipolar broaden- ing renders the 7Li lineshape broad and featureless even under MAS conditions. By contrast, the 6Li MAS NMR spectrum of the same sample (Figure 10.1B) shows a sharp resonance very close to the isotropic chemical shift, since the second-order quadrupolar shift is negligible. 6'7Li NMR chemical shifts are commonly measured with respect to aqueous LiC1 solution.

629

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630 M u l t i n u c l e a r Sol id-S ta te N M R o f l n o r g a n i c Mater ia l s

A B

7Li 6Li

ated

~observed . . . . . . _ ~ ' ' ' ' ~ . . . . I . . . . , . . . . . . . . . I . . . . . �9 . . . . . . . . I . . . . . . . . . . . . . . l . . . . . . . . . .

2.4 0.8 -0.8 6 3 0 -3

Li shif t ( p p m ) w.r . t . L iCI

Figure 10.1. A. 7Li MAS NMR observed and simulated spectra of Li-substituted beryl, spinning speed 10 kHz. Simulation parameters XQ = 0.66 MHz, r I = 0.2. B. 6Li MAS NMR

spectrum of the same sample, spinning speed 6 kHz. From Xu and Stebbins (1995), by permission of the copyright owner.

10.1.2 6'7Li NMR of crystalline solids A se lec t ion of 6Li and 7Li N M R shifts for l i th ium c o m p o u n d s are s h o w n in Tab le 10.1.

Table 10.1. 6'7Li NMR parameters for lithium compounds.

Compound ~iso (ppm)* Reference

6Li LiI - 2.312

LiBr - 2.152, - 0.359 LiA1Si4Olo (petalite) 0.1

LiA1Si206 (spodumene) - 1.0 LiA1SiO4 (eucriptite) 0.2

Nao.3(Mg,Li)3SiaOlo(F,OH)2 (hectorite) - 0.8 K(Li,A1)3(Si,A1)40~o(F,OH)2 (lepidolite) - 0.8 to - 1.0

BeAlzSi6018 (beryl) 0 . 9 , - 1 . 0 , - 1.5 LiA1Si206.H20 (bikitaite) 0.1

LiAla(Si3A1)Olo(OH)8 (cookeite) - 0.6 Laponite - 0.735 LizSiO3 0.200, 0.44

Li2Si205 0.2 Li4SiO4 1.5, 0.8, 0.2, - 0.7

NaLiSiO4 - 0.69 NaLi3SiO4 0.76

LiSiON 0.27I" LiSieN3 1.315"

Bond et al. (1991) Bond et al. (1991)

Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995)

Bond et al. (1991) Bond et al. ( 1991),

George et al. (1998) Xu & Stebbins (1995) Xu & Stebbins (1995)

Gee et al. (1997) Gee et al. (1997)

Kempgens et al. (1999) Kempgens et al. (1999)

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Table 10.1. (Continued)

N M R o f O t h e r Q u a d r u p o l a r N u c l e i 631

Compound ~iso ( p p m ) * Reference

NaLiSO4 NaLi3SiO4 LisB7S13

LIPS3 Li4P2S6 Li3PS4 LivPS6 LiSiON LiSizN3

7Li

- 0.69 0.47 - 2

1.76"* 1.45"* 2.80** 2.08** 0.17t 1.27t

Gee et al. (1997) Gee et al. (1997)

Griine et al. (1995) Eckert et al. (1990) Eckert et al. (1990) Eckert et al. (19900 Eckert et al. (1990)

Kempgens et al. (1999) Kempgens et al. (1999)

* Chemical shifts quoted with respect to LiC1 solution. ** With respect to solid LiC1 ~ determined at 7.05 T

Lithium orthosilicate, Li4SiO4, has an interesting structure containing LiO4, LiO5 and LiO6 units. One of the LiO4 sites has one very long Li-O bond, making it effectively LiO3. These polyhedra are connected by edge-sharing to form the three-dimensional structure. The 6Li MAS NMR spectrum of an enriched sample (Figure 10.2A) shows 3 well-resolved peaks at 1.5 ppm (LiO3), 0.8 ppm (LiO4), - 0.7 ppm (LiO6) and a shoulder at about 0.2 ppm attributed to LiO5 units (Xu and Stebbins 1995). As the temperature of the sample is raised, these resolved spectral peaks merge into a single broad line due to hopping of the Li + between all the sites (Xu and Stebbins 1995a). Two-dimensional 6Li NMR exchange spectra (Figure 10.2 B) have been used to provide a detailed picture of the hopping rates of the Li + among the various sites. The two-dimensional spectra are determined at various mixing times. If the mixing time of the experiment is faster than the rate of exchange between the sites, the normal one-dimensional spectrum appears on the diagonal and there are no other peaks. As the mixing time becomes longer, cross- peaks appear at the coordinates of the 2 peaks involved in Li + exchange, allowing exchange rates and activation energies to be determined for the various sites (Xu and Stebbins 1995a).

The static 7Li NMR spectra of LizSiO3 recorded as a function of temperature from ambient to 1150~ show a well-defined quadrupolar lineshape at all temperatures, indicating that the Li in this compound is not sufficiently mobile to display an averaged isotropic environment even at temperatures 50~ below the melting point (George et al.

1998). The room-temperature second-order quadrupolar lineshape can be simulated by assuming XQ = 0.15 MHz and xl = 0.65, but above 600~ an additional narrower quadrupolar lineshape appears, with XQ = 0.03 MHz and xl = 0. These results have been tentatively explained in terms of a partial dynamical averaging of the spectra caused by Li + exchange from one position to another. Complete averaging, evidenced

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632 Multinuclear Solid-State NMR of Inorganic Materials

LiO 4

uo~/i

3 I -1

6Li shift (ppm) w.r.t. LiC!

B 4 6

3

4

~ 4 - 6 3-6

Figure 10.2. A. 6Li MAS NMR spectra of 6Li-enriched Li4SiO4, from Xu and Stebbins (1995), with permission of the copyright owner. B. Two-dimensional pure-absorption exchange 6Li NMR

spectra of Li4SiO4 at two different mixing times. Note at the slower mixing time of 47 ms (top spectrum) only the peaks corresponding to the primary units are seen. At the faster mixing time of

188 ms (lower spectrum) peaks corresponding to Li exchange between the various sites appear. The horizontal axis is to2. Adapted from Xu and Stebbins (1995a).

by the collapse of the spectrum to a single narrow peak is not observed, suggesting that the Li motion does not sample a wide enough set of site geometries and orientations for their time average to achieve spherical symmetry (George et al. 1998). The 6Li MAS NMR spectrum of LizSiO3 consists of a single narrow peak with a typically tetrahedral shift (0.44 ppm) but the relaxation time (hundreds of seconds) is too long for most practical purposes (George et al. 1998).

Lithium aluminate, LiA1508 with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. 6Li and 7Li NMR has been used to measure the spin-lattice relaxation of lithium in this com- pound (Stewart et al. 1995). The results indicate that 6Li relaxes most significantly through interactions with paramagnetic impurities, whereas 7Li relaxes much more strongly through dipole-dipole interactions.

Small unexpected differences in the crystal structure of LiKSO4 have been revealed by a single-crystal 7Li NMR study which has indicated XQ and xl values of 0.025 MHz and 0.15 respectively for this compound (Lim et al. 1996, Lim and Jeong 2001). These results, taken together with 39K parameters determined for the same sample, indicate a non-axially symmetric EFG tensor, suggesting that the LiO4 unit is slightly distorted from its expected symmetry, possibly resulting from an artifact of the crystal growing conditions. The temperature dependence of the 7Li XQ and xl values of single-crystal LiKSO4 indicates the occurrence at 190 K of a first-order transition to ferroelastic domains characterised by lowering of the Li site symmetry (Lim et al. 1997). The

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NMR of Other Quadrupolar Nuclei 633

7Li and 39K NMR spectra of this material, measured at 180 K, have allowed the structure of the ferroelastic phase to be directly inferred from the domain pattern of the low-temperature NMR spectra (Lira and Jeong 2000).

The temperature dependence of the 7Li NMR spectrum of single-crystal LiRbSO4 has been determined in the temperature range 140-400 K. The room-temperature value of XQ (20.4 kHz) decreases with increasing temperature and has been interpreted in terms of the torsional frequency of Li-O (Lim et al. 1997a). The 7Li NMR spectrum of single-crystal LiCsSO4 has also been determined, and shows 3 sets of signals explained in terms of 3 types of growth twin-domains rotated with respect to each other by 60 ~ around the c-axis (Lira and Jeong 1999). The temperature dependence of these spectra indicates a second-order phase transition to ferroelastic domains with lowered Li site symmetry below 200 K (Lira and Jeong 1998).

Measurements of the temperature dependence of the 7Li NMR spectrum of single- crystal LiNHaSO4 demonstrate the occurrence of a first-order phase transition at 285 K (Lim et al. 2000a). The room temperature values of • and Xl (25 kHz and 0.22 respec- tively) increase with decreasing temperature, explained in terms of a change in the Li-O torsional frequency. Differences in the temperature dependence of the 7Li XQ val- ues for the series of single crystals LiXSO4 (where X = K, Rb, Cs and NH4) have been explained in terms of differences in the torsional motion of the LiO4 tetrahedra about the x-axis of the EFG tensor, which, in turn, can be related to differences in the atomic weight of the X ion (Lim et al. 2000).

Laponite, [Mg,Li]6SisOzo(OH)4Nao.sv.nH20, a synthetic form of hectorite, is a tri- octahedral layer silicate with the structure of talc in which some octahedral Mg is sub- stituted by Li, the charge balance provided by interlayer Na + or Ca 2+. These substitutions give the material useful cation exchange properties and also influence its thermal decomposition behaviour. The thermal decomposition of laponite has been studied by a combination of VLi, 298i and 25Mg MAS NMR (MacKenzie and Meinhold 1994). Loss of interlayer water at about 200~ produces very little change in the 7Li spectra, but just prior to dehydroxylation at 650~ discontinuities in the downfield trend of the 7Li shift with increasing temperature suggest the movement of interlayer Na closer to the tetrahedral sheets, influencing the Li in the octahedral sites (MacKenzie and Meinhold 1994). A similar downfield shift with increasing temperature reported in the 6Li spectra of heated laponite has been ascribed to the movement of Li from trioctahedral sites to edge sites (Bond et al. 1991).

By contrast with the spinel LiTi204, which contains only tetrahedral Li sites, the superconducting spinel phase Li~.33Ti1.6704 contains both tetrahedral and octahedral Li sites, but the chemical shift difference between them is too small to be resolved even with very high MAS spinning speeds. The static 7Li spectrum is considerably broader than that of LiTi204 due to dipolar broadening between 7Li sites. This observation has led to an interesting experiment in which the Li-Li separation was increased by

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634 Multinuclear Solid-State NMR of lnorganic Materials

isotopically diluting the sample with enriched 6Li (Dalton et al. 1994). The resulting 7Li MAS NMR spectrum showed structure resolvable into 2 Gaussian lineshapes with shifts of 0.2 and - 0.09 ppm. These resonances were identified as the tetrahedral and octahedral resonances respectively, by comparison with the known Li shifts in other compounds (8 for octahedral Li = - 0.1 to - 0.6 ppm, 8 for tetrahedral Li = 2.4 ppm)

(Dalton et al. 1994). The lithium silicon nitride phases LiSiON and LiSi2N3 have been studied by 6Li and

7Li NMR at 2 magnetic field strengths (Kempgens et al. 1999). The spectra of both compounds consist of an intense central resonance with associated spinning side band manifolds. Although the difference between the isotropic chemical shift is small (Table 10.1), the 2 phases can readily be distinguished by their 7Li XQ and x I values (130 kHz and 0.55 for LiSiON and 100 kHz and 0.9 for LiSizN3). The quadrupolar interaction in the 7Li NMR spectra is much larger than the chemical shift interaction, making it difficult to accurately determine the small CSA and the relative orientation of the 2 interactions by 7Li NMR. These parameters can, however, be determined much more accurately from the 6Li NMR spectra (Kempgens et al. 1999).

10.1.3 Relation between 6Li chemical shifts and structure The 6Li NMR spectra of silicates are dominated by chemical shift effects, and this, together with their superior resolution makes them potentially useful for providing structural information. The 6Li chemical shifts (peak positions) of a series of silicate and aluminosilicate minerals derived from MAS NMR measurements have been shown to correlate well with the Li coordination numbers derived from single-crystal X-ray measurements (Figure 10.3A) (Xu and Stebbins 1995). This systematic decrease in the 6Li chemical shift with increasing oxygen coordination number (CN) resulting from increased shielding can be described by the linear relationship:

6 (rLi) = -0.608(CN) + 2.91 (~o.1)

This trend, which is in the same direction as for 27A1, 29Si, 23Na and 25Mg, appears to

be related to the increase in Li-O bond length and Li + ionicity with increasing coordi- nation number. A similar trend in the 6Li isotropic shift with ionicity has also been

reported in sulphide-based glasses (Eckert et al. 1990). This simple relationship between the 6Li shifts and Li coordination number has been

found not to hold for crystalline lithium phosphates, indicating that other factors must be taken into account (Alam et al. 1999). By analogy with 23Na NMR (Chapter 7), Alam et al. (1999) sought a relationship between the 6Li shift and the average degree of polymerisation in the phosphate tetrahedra as reflected by the number of non-bridging

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NMR of Other Quadrupolar Nuclei 635

oxygen atoms per phosphate tetrahedron. The scatter in this relationship indicates that it does not satisfactorily describe the situation in the lithium phosphates. However, the chemical shift parameter (A) derived for 23Na shifts by Koller et al. (1994) in terms of the bond valences of the neighbouring oxygens was found by Alam et al. (1999) to give a good linear relationship with 6Li shifts (Figure 10.3B), described by

6Li + (ppm) -- 4.30A - 5.85 (10.2)

Similarly, a linear relationship was found for the Li sites in crystalline Li4SiO4

(Figure 10.3B), described by

6Li ~ (ppm) -- 7.50A - 8.52 (10.3)

By contrast with the results for 23Na (Chapter 7), both these lines have a positive slope. This unexplained descrepancy suggests that the formalism for defining the parameter A requires refinement, although the treatment is potentially useful within limited groups of similar compounds.

o 0

-1

A B

_~ �9

. . . . i . . . . I . . . . i . . . . i . . . . i . . . . l .+++:+ _

4 6 8

Li c o o r d i n a t i o n n u m b e r

2 / /

p /

/ /

~, ] ~o /

/

-2 I I v P : 4 1.1 1.3 1.5

Chemical shift parameter A

Figure 10.3. A. Relationship between the 7Li chemical shift of lithium silicates and aluminosilicates and the Li coordination number (CN). The open circles indicate the probable peak

assignments of Li4SiO4, crosses indicate the hypothesized LiO6 and LiOs sites in Li-substituted beryl, with all other samples indicated by +. From Xu and Stebbins (1995), with permission of the copyright owners. B. Relationship between the 6Li chemical shift and the chemical shift parameter

A (defined by equation 7.7, Chapter 7) for crystalline lithium phosphates (solid circles) and crystalline Li4SiO4 (open circles). From Alam et al. (1999), by permission of Elsevier Science.

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636 Multinuclear Solid-State NMR of Inorganic Materials

10.1.4 6'7Li N M R o f fas t l i thium ion conductors

One of the most important practical applications of lithium compounds is as fast ion conductors with potential electronic applications such as solid electrolytes for lithium batteries. Li20 is a fast ion conductor in which the Li ions occupy a simple cubic sub- lattice with the antifluorite structure. Both MAS and static 7Li NMR spectra of Li20 have been reported, the former recorded as a function of temperature up to 1000 K (Xie et al. 1995). The effect of introducing vacancies on the Li sites by doping with LiF has been studied by high-temperature static 7Li NMR, which reveals the interaction of the Li defects > 600 K and the appearance of 2 distinct quadrupolar interactions at about 900 K. Measurements of the relative intensities of the satellite peaks as a function of temperature have provided evidence of thermal dissociation of an impurity-vacancy complex (Xie et al. 1995).

The mechanism of Li motion in the novel thioborate LisBvS13 has been investigated by measuring the relaxation rates of 7Li as a function of temperature up to 650 K (Grtine et al. 1995). This compound displays pronounced Li + mobility but with rather complex relaxation behaviour indicating the operation of 3 different processes by which Li ions move within the crystal. Below room temperature, the lithium ions move in the extended channels in the structure, unhindered by the presence of S atoms such as those located in the channels of related thioborate compounds. A second process becoming significant at about 200 K involves jumping of the Li + between the holes of the porous anionic net- work, while the third process, above about 300 K, results from the movement of Li + between more isolated sites via pathways which become increasingly accessible because of thermal activation (Grtine et al. 1995).

Lithium intercalation of compounds such as SnS2 are of technical interest for pho- tochromic display materials and lithium electrodes which reversibly take up and release Li +. 6'7Li and ll9Sn NMR has been used to investigate the location of the Li insertion sites in this material (Pietrass et al. 1997). The 7Li spectra show a central tran- sition which can be decomposed into 2 components with different XQ values corre- sponding to Li in octahedral and tetrahedral interlayer sites. As the Li concentration increases, the additional ions enter tetrahedral intralayer sites surrounded by 3 tin and 4 sulphur atoms and characterised by a broad VLi NMR spectral component. Further insertion of Li results in the material becoming amorphous by rupture of the layers (Pietrass et al. 1997).

LiCoO2, an important electrode material for secondary lithium batteries, occurs in 2 polytypes, both of which have been investigated by 6'7Li and 59Co NMR at 3 mag- netic fields (Siegel et al. 2001). Both polytypes show only 1 Li resonance correspond- ing to lithium in octahedral coordination with oxygen, with similar 7Li XQ values (25-36 kHz for the 02 polytype and 31-39 kHz for the 03 polytype).

The superionic compound Li3Scz(PO4)3, studied by 7Li NMR up to 575~ (Vash- man et al. 1992) has revealed the operation of three types of Li ion motion and allowed

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NMR of Other Quadrupolar Nuclei 637

their activation energies to be determined. Both the 7Li quadrupole parameter and spin- lattice relaxation rate change abruptly in the vicinity of a phase transformation at about 530 K. The 6Li NMR spectra of a 6Li-enriched sample were also measured as a func- tion of temperature, and it was possible to derive a precise value of the quadrupole moment for 7Li of (2.56 + 0.05) • 10 -2 barn from observation of the 6Li and 7Li quadrupole spectra in the same compound (Vashman et al. 1992). The 7Li NMR spectra of the superionic conducting compound Li3Inz(PO4)3 have been obtained at tempera- tures up to 520 K, and, together with the corresponding 31p NMR spectra, provide evi- dence of phase transitions in this material at about 380 K and 420 K, at which the lithium ions are re-distributed between the different crystallographic sites (Pronin et al. 1990). Measurements of the 7Li relaxation rates indicate the presence of Li+-Li + contact pairs in the crystal lattice, leading to a suggested model for Li ion transport involving the movement of an interstitial configuration of metastable Li + pairs (Pronin et al.

1990). Lithium-doped BPO4, another candidate ceramic electrolyte material for lithium

batteries has been studied by 7Li NMR relaxation and linewidth measurements of sam- ples with Li doping levels up to 20 mol % (Dodd et al. 2000). Comparison of the NMR data with values of the second moment calculated for both random and homogeneous models of Li distribution indicate the existence of Li clusters with an internuclear distance of --~ 3A, possibly consisting of 1 Li ion fixed at a boron vacancy with addi- tional 2 Li ions in the conduction channels surrounding the vacancy. The atomic jump time, determined from measurements of the 7Li motional narrowing behaviour, indicate a maximum in the Li ionic mobility at the 10 mol % doping level (Dodd et al. 2000).

7Li NMR has been used to study the processes by which Li ions move through the structures of the ionic conducting ceramic materials lithium lanthanum titanate and lithium aluminium titanium phosphate (Nairn et al. 1996). The 7Li static NMR spectra of Lio.33Lao.svTiO3 show the quadrupolar powder pattern associated with significant Li ionic mobility, with a room-temperature XQ value of 900 Hz. A second Li site which becomes apparent in these spectra at higher temperatures has been attributed to the presence of less mobile defects. The 7Li NMR spectrum of Lil.3Alo.3Til.7(PO4)3 shows a powder pattern with a large room-temperature • (about 45 kHz) increasing smoothly to about 54 kHz at 400 K probably due to a temperature-induced lattice dis- tortion (Nairn et al. 1996).

Lithium vanadate bronzes are intercalated compounds with potential applications for lithium battery technology, since Li can be reversibly inserted into these structures by electrochemical reaction. 7Li NMR has been used to study the structure of "y-Lio.95V205 (Cocciantelli et al. 1992) and a series of related bronzes LixV205 (Cocciantelli et al.

1992a). The 7Li NMR spectrum of the ~/-phase indicates the presence of a single Li site, but as the Li content is increased beyond x = 1, new lines can be resolved, corresponding

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638 Multinuclear Solid-State NMR of Inorganic Materials

to the Li sites in the 8-phase ( - 12 ppm) and g-phase ( - 10 ppm) which co-exist with the y-phase (5-15 ppm) in these compositions (Cocciantelli et al. 1992, 1992a).

10.1.5 6, 7Li N M R o f glasses

A series of binary lithium silicate glasses have been studied by 6Li MAS NMR, showing a linear relationship between the average isotropic chemical shift and the glass composi- tion (Figure 10.4A) (Gee et al. 1997). This relationship, which is similar to that found for 23Na isotropic shifts in binary sodium silicate glasses, indicates that for both nuclei the isotropic chemical shift becomes more positive (i.e. the bonding becomes more covalent) as the concentration of non-bridging oxygen species in the glass increases (Gee et al.

1997). Both 6Li and 7Li NMR spectra have been reported for binary lithium silicate glasses and their crystallisation products (Dupree et al. 1990). On thermal recrystallisa- tion of the glasses, the widths of the 6Li NMR spectra decrease, indicating a significant contribution by chemical shift dispersion to the linewidth of the glass. The static 7Li NMR spectra of the crystallised glasses exhibit splitting due to dipolar coupling of iso- lated pairs of 7Li nuclei with a separation of -~ 2.1/k (Dupree et al. 1990).

The "mixed alkali" effect in silicate glasses refers to the observation that systems containing more than 1 alkali cation show ionic conductivity and dielectric behaviour which does not follow a simple linear combination of the properties of the pure com- ponents, but can often show a marked minimum in these properties at about the equiatomic composition. 7Li, 23Na and 29Si MAS NMR has been used to investigate this effect in (Li,Na) disilicate glasses (Ali et al. 1995). The 7Li and 23Na linewidths and shifts were found to change continuously as a function of composition, suggesting that the alkali ions are uniformly mixed rather than segregated into Li and Na-rich domains. This conclusion, which contradicts previous glass structure models, has been confirmed by 23Na-{ 7Li} Spin Echo Double resonance (SEDOR) studies (Gee and Eckert 1996), and by 29Si{VLi} and 29Si{23Na} Rotational Echo Double Resonance (REDOR) NMR results (Gee et al. 1997). The REDOR experiments were used to selectively enhance those silicon sites most strongly coupled to either Li or Na ions, allowing a comparison of their spectroscopic parameters. The isotropic chemical shifts of both the 7Li and 6Li MAS NMR spectra of (Na,Li) disilicate glasses have been found to become more positive with increasing Na content of the glasses (Figure 10.4B), fol- lowing a similar trend found for the 23Na shifts (although the chemical shift range in 6'7Li is smaller, making the relationship with composition more subtle). These mono- tonic compositional dependencies of the alkali chemical shifts provide further evi- dence against glass structural models involving cation clustering (Gee et al. 1997).

Both 6Li and 7Li MAS NMR has been used to investigate the local Li coordination environment in a series of binary lithium phosphate glasses (Alam et al. 1999). The 6Li chemical shifts, which approximate closely to the isotropic chemical shifts, increase

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NMR of Other Quadrupolar Nuclei

A B

639

0.4 ~ 0.6] -~

0.3 w ~., _t_ ~ 0.4 "

0.2 ~ - G < : ) . . - q

. p . m

o.1 ~.~ 0.2

0 - , ~ , ! , i ' , i , - - 1 , J o i , - i ~ ~

0 10 20 30 40 0.2 0.6 1.0

Mol % Li20 Na/(Na + Li)

Figure 10.4. A. Relationship between the 6Li isotropic chemical shift and the composition of lithium silicate glasses. B. Relationship between the VLi and 6Li isotropic chemical shift and the

composition of a series of (Li,Na) disilicate glasses. The open circles denote the 7Li shifts, the filled squares denote the 6Li shifts. From Gee et al. (1997), by permission of Elsevier Science.

with increasing Li20 content, reflecting increased cross-linking of the tetrahedral phosphate network by Li-O-Li bridges. The 6Li chemical shifts were also found to vary monotonically through the anomalous glass transition minimum in this system, show- ing that this phenomenon is not related to any abrupt changes in the Li coordination envi- ronment. The NMR results for this series of phosphate glasses indicate that the average coordination number of the Li atoms is 4-5 (Alam et al. 1999).

10.2. 9Be NMR

9Be is a spin = 3/2 nucleus with a relatively small quadrupolar moment (5.3 • 10 -3o m2),

good receptivity and 100% natural abundance. Despite its suitability for NMR studies, recent solid-state 9Be NMR studies are relatively rare, probably due to the extreme tox- icity of the element and its compounds, but possibly also because the narrow chemical shift range of 9Be detracts from its utility as a characterisation tool. Most of the solid state 9Be NMR studies reported to date have been of minerals which are comparatively non- toxic. 9Be chemical shifts are commonly reported relative to solid BeO.

The 9Be MAS NMR spectra of BeO, beryl (AlzBe3Si6018) and tugtupite (NasAlzBezSisO24C12) have been obtained by Skibsted et al. (1995). Because of the small quadrupole moment of 9Be, measurements of the satellite transitions (SATRAS) were used to determine the quadrupolar parameters of these materials. A considerable contribution from the central transition to the intensities of the first and second-order spinning side bands of BeO was ascribed to incomplete averaging of the 6 9Be-9Be dipolar couplings for each Be. The value of Xe for BeO obtained from the SATRAS measurements (0.039 MHz) is in good agreement with a previous single-crystal value of 0.0394 MHz (Thorland et al. 1972).

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640 Multinuclear Solid-State NMR of lnorganic Materials

The 9Be XQ value obtained from SATRAS measurements of beryl (0.495 MHz)

agrees with the value of 0.504 MHz obtained from a previous single-crystal study (Brown and Williams 1956) and is more than an order of magnitude larger than in BeO,

reflecting the more distorted BeO4 tetrahedra in beryl (Skibsted et al. 1995). The corre- sponding parameter for tugtupite (0.035 MHz) is very similar to that of BeO, consistent

with the axial symmetry of the Be sites in both compounds. A single crystal 9Be NMR study of the gemstone alexandrite, BeAll.98Cro.o204, has

shown the presence of 2 magnetically inequivalent Be sites which are, however, chem- ically equivalent (Yeom et al. 1995). The temperature dependence of • was also determined, and was found to increase with increasing temperature, by contrast with the more usual decrease found in other compounds, but the asymmetry parameter

decreases with increasing temperature (Yeom et al. 1995). The 9Be MAS NMR chemical shifts have been measured for a number of beryllium

sodalite framework structures of general formula M8[BeZO4]6X2 where M = Cd or Zn, Z = Si or Ge and X - S, Se or Te (Dann and Weller 1997). All the spectra show a single sharp 9Be resonance corresponding essentially to the giso value. The chemical shifts show linear correlations with the Be-O-Si and Be-O-Ge angles (Figure 10.5A) given

by

9Be 6iso = -0 .0626 Be-O-Si angle (~ + 7.91 (10.4)

9 B e ~iso -- -0 .0617 Be-O-Ge angle (~ + 8.25 (10.5)

A B

0.8

o r ~

-0.8

silicates

4

0.8 a tes

~u o

"'~~ -0.8 silicates

-1.6

120 130 140 8.1 8.5 Be-O-Z angle (o) Lattice parameter (~ )

8.9

Figure 10.5. A. Relationship between the 9Be chemical shift and the tetrahedral Be-O-Z angle for a series of sodalite framework structures, where Z -- Si or Ge. B. Relationship between the 9Be

shift and the lattice parameter for the same series of sodalites. From Dann and Weller (1997), by permission of the copyright owner.

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N M R o f O ther Q u a d r u p o l a r Nuc le i 641

Linear correlations were also found between ~iso and the sodalite lattice parameter a

(Figure 10.5B) given by

9 B e 6 i s o - - - 2 . 4 4 a (A) + 20.0 (beryllosilicates) (10.6)

9 B e 6 i s o - - 1.96a (* ) + 17.1 (beryl logermanates) (10.7)

The 9Be N M R interaction parameters in Be compounds are shown in Table 10.2.

Small amounts of beryl l ium can be substituted into the tetrahedral f ramework sites

of Z S M - 5 zeolite by t reatment with ammon ium tetrafluoroberyllate. Al though the

amounts of Be substituted were too small for detection by thermal analysis and des-

orption measurements , the presence of Be 2+ in f ramework sites was detected by 9Be

MAS NMR, which showed a resonance at - 5.0 ppm with respect to aqueous BeSO4

Table 10.2. 9Be NMR interaction parameters for beryllium compounds.

Compound ~iso XQ (MHz) x I Reference (ppm)*

BeO 0 0.039, 0.0394 0.15 Skibsted et al. (1995), Thorland et al. (1972)

A12Be3Si6018 - 1.9, 0.495, 0.14, Skibsted et al. (1995), (beryl) - 2.4 0.504 0.09 Brown & Williams (1956)

tugtupite - 3.0, 0.035 0.14 Skibsted et al. (1995), - 2.1 Xu & Sherriff (1994)

alexandrite ND 0.3178 0.904 Yeom et al. (1995) Be3(AsOa)z.2H20 - 1.4"* ND ND Harrison et al. (1994) BezAsOaOH.4H20 - 0.77** ND ND Harrison et al. (1993)

Zn8[BeSiO4]6S2 0.15 ~ ND ND Dann & Weller (1997) Zn8[BeSiO4]6Se2 0.08 t ND Dann & Weller (1997) Zns[BeSiO4]6Te2 0.03' ND Dann & Weller (1997) Cds[BeSiO4]6S2 - 0.59 t ND ND Dann & Weller (1997) Cdg[BeSiO4]6Se2 - 0.73* ND ND Dann & Weller (1997) Cds[BeSiO4]6Te2 - 0.80 t ND ND Dann & Weller (1997) Zn8[BeGeO4]6S2 0.83 t ND ND Dann & Weller (1997) Zns[BeGeO4]6Sez 0.77 * ND ND Dann & Weller (1997) Zns[BeGeO4]6Te2 0.64* ND ND Dann & Weller (1997) Cds[BeGeO4]6S2 0.27* ND ND Dann & Weller (1997) Cds[BeGeO4]6Se2 0.07 t ND ND Dann & Weller (1997) Cds[BeGeO4]6Tea - 0.03 t ND ND Dann & Weller (1997)

Mg19Na58(BePO4)96 - 1.6"* ND ND Nenoff et al. (1992)

* chemical shifts quoted with respect to solid BeO t these shifts quoted with respect to aqueous BeC12 solution ** these shifts quoted with respect to aqueous Be(NO3)2 solution

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642 Multinuclear Solid-State NMR of Inorganic Materials

(Han et al. 1993). This shift value is in fair agreement with another reported value of - 5.8 ppm for Be inserted in ZSM-5 during synthesis (Romannikov et al. 1985). The 9Be MAS NMR spectrum of the dehydrated faujasite analogue Mg-exchanged sodium beryllophosphate has been reported to contain a single sharp resonance at - 1.6 ppm with respect to aqueous Be(NO3)2 solution (Nenoff et al. 1992).

9Be NMR has been used to detect slow atomic motion of beryllium in Zr-Ti-Cu-Ni- Be metallic glasses. The results, obtained by a spin alignment echo technique, are consistent with Be diffusion occuring by a mechanism involving thermal fluctuations of the spread-out free volume rather than by vacancy-assisted or interstitial diffusion mechanisms (Tang et al. 1998).

10.3. SlV NMR

10.3.1 Genera lcons idera t ions

5~V is a favourable nucleus for solid state NMR since it has a 99.76% natural abun- dance, a large magnetic moment and generally short relaxation times because of the nuclear quadrupole interaction of this spin = 7/2 system. Although the static spectra are often broad, MAS satisfactorily averages the first-order anisotropic shielding and quadrupolar interactions, yielding useful spectral information. In materials science, the principal use of 5~V NMR has been to study the industrially important families of V2Os-containing catalysts.

Although the 5~V NMR spectra of vanadium compounds are very sensitive to the oxygen environment of the V atom, the 5~V ~iso values are less diagnostic than the chem- ical shift anisotropies (CSA). However, ~iso values for compounds with the same first coordination sphere are sensitive to the nature of the atoms in the second coordination sphere (Lapina et al. 1992).

10.3.2 st V NMR of vanadium oxides and the vanadates The structure of V205 is built up from VO5 square pyramidal units sharing edges and comers. A 5~V MAS NMR study of V205 has shown a single resonance from the unique vanadium site, with a pattern of spinning sidebands which have been simulated to provide information about the relative orientation of the principal axes of the 2 anisotropic interactions (Fernandez et al. 1994). A partial structural phase transition in B-type VO2 between 300-180 K has been studied by 5lV NMR in which the identifi- cation of a spin-singlet state confirmed the operation of V 4+-V 4 + pairing in half the V sites of the low-temperature modification (Oka et al. 1993). Similar spin-pairing behaviour is also known in the structurally-related oxide phase V6013.

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NMR of Other Quadrupolar Nuclei 643

Vanadium in the alkali metal orthovanadates M3VO4 and pyrovanadates M4V207 is in nearly regular tetrahedral coordination, giving almost isotropic 51V NMR spectra illustrated by T13VO4 (Figure 10.6A). The distortion from tetrahedral symmetry pro- gressively increases in the other T1 vanadates T14V207 and T1VO3 containing tetrahe- dral chains, finally assuming a distorted octahedral symmetry in T12V6016. The orthovanadates of the divalent metals show 51V isotropic chemical shifts of about - 5 2 0 to - 560 ppm (with respect to VOC13) where the cation radius is < 1A, and about - 6 0 0 ppm for cations of radius > 1A,. All divalent orthovanadates have CSAs of 100

ppm (Lapina et al. 1992). Divalent metal pyrovanadates have 8iso values > - 6 0 0 ppm where r < 1A, and 8iso < - 6 0 0 ppm where r > 1A. The CSA of the divalent pyrovanadates varies from 80 to 300 ppm. The 51V NMR spectra of the divalent metavanadates M(VO3)2, in which the vanadium is in a distorted trigonal bipyramidal environment, have nearly axial CSAs varying from 500 to 700 ppm, considerably less than the CSA value for V205 (960 ppm) (Lapina et al. 1992). The 5~V NMR spectra of the trivalent metal vanadates show narrow, almost isotropic lines with 8iso values rang- ing from - 4 0 0 to - 780 ppm and CSA values < 140 ppm (Lapina et al. 1992). A num- ber of vanadates have structures with V in both distorted tetrahedral and octahedral environments. The S~V NMR spectra (Figure 10.6B) show the superposition of the fully anisotropic line from V in the distorted tetrahedron with a line of axial symmetry from

the V atoms in the distorted octahedra.

A B

TI 4 V 1 0 ~ ~

1 | ! 0 -~obo

Tet Oct ,--'--, Cs2V~

TI3 ~ ~

Rb3V5Ol4 y I~___

_ ! | _ I . I i _

o -looo

SlY shift (ppm) w.r.t VOCI2

Figure 10.6. A. Selection of 5~V NMR spectra of thallium vanadates showing the effect of increasing distortion of the VO4 tetrahedra in progressing from T13VO4 to T1VO3. The spectrum of

TlzV6016 arises from V in a distorted octahedral environment with almost axial symmetry. B. A series of 5~V NMR spectra of vanadates with V in both distorted tetrahedral and distorted octahedral environments. From Lapina et al. (1992), by permission of Elsevier Science.

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644 M u l t i n u c l e a r S o l i d - S t a t e N M R o f I n o r g a n i c M a t e r i a l s

The 5~V N M R in te rac t ion p a r a m e t e r s for a n u m b e r o f v a n a d i u m c o m p o u n d s are

co l l ec t ed in T a b l e 10.3.

Table 10.3. 5~V NMR interaction parameters of vanadium compounds.

Compound 6i~o (ppm)* XQ (MHz) rl Reference

Li3VO4 - 544 1.52 ND Na3VO4 - 545 ND ND K3VO4 - 560 ND ND C s 3 V 0 4 - 576 ND ND T13VO4 - 480 ND ND

KCaVO4 site 1 - 5 8 0 ND ND site 2 - 589 ND ND site 3 - 616 ND ND site 4 - 623 ND ND

Mg3(VO4)2 - 557, ND, ND, - 554 0.49 0.63

C a 3 ( W O 4 ) 2 - 615 2.05 ND S r 3 ( V O 4 ) 2 - 610 0.53 ND Ba3(VO4)2 - 605 0.75 ND Z n 3 ( g o 4 ) 2 - 522 ND ND

AIVO4 site 1 - 668 ND ND site 2 - 747 ND ND site 3 - 780 ND ND

BiVO4 - 420 ND ND YVO4 - 664 4.75 0 LaVO4 - 609 5.21 0.69 LuVO4 - 663 4.23 0

Na4V207 site 1 - 560 ND ND site 2 - 575 ND ND

K 4 V 2 0 7 - 578 ND ND Cs4V207 site 1 - 543 ND ND

site 2 - 567 ND ND T14V207 - 504 ND ND

o~-Mg2V207 site 1 - 5 5 5 , - 551 ND ND site 2 - 617, - 605 ND, 4.6 ND, 0.55

13-Mg2V207 site 1 - 6 8 0 , - 642 ND ND site 2 - 650, - 497 ND, 9.5 ND, 0.70

C a 2 V 2 0 7 site 1 - 574 ND ND site 2 - 578 ND ND

Sr2V207 site 1 - 557 ND ND site 2 - 582 ND ND site 3 - 588 ND ND site 4 - 592 ND ND

Ba2V207 site 1 - 579 ND ND site 2 - 588 ND ND site 3 - 600 ND ND

ZnV207 - 625 ND ND

Lapina et al. (1992) Eckert & Wachs (1989)

Lapina et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Laplna et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Laplna et al. (1992), Occelli et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992)

Eckert & Wachs (1989) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992)

Gubanov et al. (1977) Laplna et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapina et al. (1992), Occelli et al. (1992) Lapina et al. (1992), Occelli et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992)

Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992)

Eckert & Wachs (1989)

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Table 10.3. (Continued). N M R o f O t h e r Q u a d r u p o l a r N u c l e i 645

Compound ~iso (ppm)* Xe (MHz) qq Reference

Cd2V207 Pb2V207 ZrV207 LiVO3

NH4VO3

ot-NaVO3

[3-NaVO3

T1VO3

CsVO 3

RbVO3 KVO3

KV308 site 1 site 2

K2V6016 Rb2V6016 Cs2V6016 T12V6016 K2V8021

KVO3H20 NaVO32H20 o~-Mg(VO3)2

Ca(VO3)2

Ba(VO3)2 Zn(VO3)2 Pb(VO3)2 Cd(VO3)2 VOPO 4

VOAsO4 VOC13 ( - 170~

- 579 - 522 - 774

- 573.4, - 577.1

- 572, - 569.5, - 571.5

- 582, - 572.7, - 578.2

- 510.4, - 516.4

- 528, - 529.1 - 5 8 3

ND - 548, - 552.7,

- 557.7

- 548.1 - 510.0 - 503 - 503 - 508 - 700 - 570 - 606 - 530 - 5 7 6 - 5 7 5

- 660 - 5 1 7 - 5 3 3 - 500 - 734 - 6 1 7

6

ND ND ND 3.18

2.88, 2.95, 2.76, 2.95

3.7, 3.65, 3.15, 3.94, 3.80 4.20, ND

ND, 3.67 3.92, 3.84 4.33 4.21, 4.35, 4.06, 4.34, 4.20

2.45 3.03 ND ND ND ND ND ND 3.94 6.79 3.30, 3.16, 2.81 ND ND ND ND ND ND

7.5, 5.7, 2.98, 5.4

ND Eckert & Wachs (1989) ND Eckert & Wachs (1989) ND Lapina et al. (1992) 0.87 Skibsted et al. (1993),

Hayashi & Hayamizu (1990) 0.3, 0.19, 0.37, Baugher et al. (1969),

0.30 Segel & Creel (1970), Skibsted et al. (1993),

Hayashi & Hayamizu (1990) 0.52, 0.6, 0.64, Baugher et al. (1969),

0.64, 0.46 Spegel & Creel (1970), Skibsted et al. (1993),

Hayashi & Hayamizu (1990) 0.55, Skibsted et al. (1993), ND Hayashi &

Hayamizu (1990) Lapina et al. (1992), Skibsted et al. (1993) Segel & Creel (1970)

ND, 0.71 0.62, 0.63 0.72

0.65, 0.75, 0.76, 0.77, 0.80

0.44 0.89 ND ND ND ND ND ND 0.64 0.63

0.8, 0.6, 0.6

ND ND ND ND ND ND 0.08

Lapina et al. (1992) Gomostansky &

Stager (1968) Baugher et al. (1969), Segel & Creel (1970), Skibsted et al. (1993),

Hayashi & Hayamizu (1990) Skibsted et al. (1993) Skibsted et al. (1993) Lapina et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Laplna et al. (1992) Laplna et al. (1992) Laplna et al. (1992) Laplna et al. (1992) Laplna et al. (1992)

Segel & Creel (1970), Gomostansky &

Stager (1968) Lapina et al. (1992)

Eckert & Wachs (1989) Eckert & Wachs (1989)

Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992)

Paulsen & Rehder (1982), Habayeb & Hileman (1980),

Allerhand (1970)

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646 Mul t inuc lear Solid-State N M R o f lnorganic Mater ia ls

Table 10.3. (Continued).

Compound ~iso (ppm)* XQ (MHz) xl Reference

V 2 0 5 - 610 0.8 0.04 Gornostansky & Stager (1967) K 3 V s O 1 4 ( o c t ) - 500 ND ND Lapina et al. (1992)

(tet) - 620 ND ND Lapina et al. (1992) R b 3 V 5 0 1 4 ( o c t ) - 496 ND ND Lapina et al. (1992)

(tet) - 619 ND ND Lapina et al. (1992) T13V5Oj4 (oct) - 500 ND ND Lapina et al. (1992)

(tet) - 594 ND ND Lapina et al. (1992) C s 2 V 4 0 1 1 ( o c t ) - 510 ND ND Lapina et al. (1992)

(tet) - 575 ND ND Lapina et al. (1992) Hg4V209 (oct) - 518 ND ND Lapina et al. (1992)

(tet) - 516 ND ND Lapina et al. (1992)

* Chemical shifts referenced to VOCI 3

A 51V and 2~ NMR study of single-crystal ferroelastic BiVO4 as a function of

temperature has shown that this material undergoes a second-order phase transition. The results have also identified the substitutional sites of Mn 2+ and Fe 3+ impurities in this compound as Bi 3+ and V 5+ respectively (Choh 1996). The magnetic and elec-

tronic properties of La•215 have also been investigated by 51V NMR (Mahajan

et al. 1991). At the composition about x = 0.7, this material undergoes a transition

from metallic behaviour to an antiferromagnetic insulator, reminiscent of the cuprate

superconductors such as La2-xSr• No evidence was found in lanthanum stron- tium vanadates for antiferromagnetic spin fluctuations or superconducting properties, and the 51V NMR data in the metallic composition range are typical of a narrow d-band

metal (Mahajan et al. 1991). 5~V NMR has been used to study a series of solid solutions in the system

ZrVz-xPxO7, some members of which show negative isotropic thermal expansion properties over a broad temperature range up to at least 950~ This unique thermal

expansion behaviour appears to be related to frustration in bending V-O-V (or P-O-P)

angles away from 180 ~ in a cooperative manner (Korthuis et al. 1995).

10.3.3 st V NMR of zeolites and catalysts Vanadium plays an important role in many industrial catalysts used extensively in a variety of applications including the production of SO3 from SO2, selective oxidation

of hydrocarbons, reduction of nitrogen oxides with ammonia, and in the manufacture

of many chemicals and chemical intermediates. Such catalysts typically consist of

vanadium compounds supported on oxides such as silica, alumina, titania, etc., and their activity depends on factors such as the chemical form and crystalline environment

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N M R of Other Quadrupolar Nuclei 647

of the vanadium. 51V NMR has been extensively used to provide information about the

surface species of oxide-supported vanadium catalysts, their interaction with the sup-

porting material and with the reacting molecules during the catalytic processes. The earlier literature on 51V NMR studies of catalysts has been extensively reviewed by Lapina et al. (1992).

Typical of such 51V NMR studies is an investigation of the surface state of vanadium

impregnated by a wet process on a TiO2-ZrO2 support (Reddy et al. 1992). At lower vanadium loadings, the 51V NMR spectra reveal the presence of 2 types of tetrahedral

vanadium units characterised by peaks at - 6 6 0 to - 6 8 0 ppm and - 4 5 0 ppm

(Figure 10.7A). At higher vanadium loadings in excess of a monolayer coverage, a third

species appears, characterised by 5~V NMR peaks at - 310 and - 1265 ppm. This arises

from vanadium in a distorted octahedral environment and reflects the presence of

crystalline V2Os (Reddy et al. 1992). 5~V static and MAS NMR have been used to

determine the quadrupole parameters and CSA tensors of strongly bonded vanadium species in VOx/TiO2 catalysts, in which a very large value of 5~V XQ was found (14-16

MHz), but with CSA tensor components similar to those of bulk V205 (Shubin et al.

1999).

A 51V NMR study of V205 supported on SnO2 and oL-Sb204 showed the presence at

A B

Oct Tet wt.% - , , Tet . ~. 1 3

t,~ t V 2 U 5

-310 Ii/~ : Oct t /: k : -1265 *

' 16 i

l, l i 10

I

! ! i

0 -5oo -1000

SlY shift (ppm) w.r.t. VOCI3

-350/"",-~

-350 Static ~ ed

j -508 -350

MAS / / ~ unheated

| , , = .

0 -500 -1000

SlV shift (ppm) w.r.t. VOCI3

Figure 10.7. A. 51V NMR spectra of a series of V20 5 catalysts supported on TiO2/ZrO2 after heating at 250~ for 2 h. showing the evolution of the distorted octahedral V-O species (V205) at

the higher vanadium concentrations. From Reddy et al. (1992), by permission of the American Chemical Society. B. 51V NMR spectra of a mixed V205-WO3 catalyst supported on a TiO2/A1203

substrate. Note the lack of narrowing under MAS conditions and the loss of the V2W40~94- resonance at - 508 ppm on heating at 200~ From Mastikhin et al. (1995), by permission of the

copyright owner.

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648 Multinuclear Solid-State NMR of lnorganic Materials

lower V concentrations of a dispersed oxide phase, with crystalline V205 appearing at higher V concentrations. The oxide phases in these catalysts were too dispersed to be detected by X-ray diffraction (Reddy and Mastikhin 1992). 51V spin echo and MAS NMR has been used to study the coordination and local order of V205 supported on SiO2 and indicates that the vanadia species on the silica surface is an (SiO)3V = O unit (Das et al. 1993).

Mixed oxide catalysts of V205 and WO3 supported on TiOx/Al203 have also been investigated by 5Iv NMR (Mastikhin et al. 1995). A sharp 51V line at about - 508 ppm in the as-prepared catalyst (Figure 10.7B) was identified as resulting from the anionic species V2W40194- in solution in the pores of the catalyst. Two broader lines were also found, 1 at - 550 ppm arising from tetrahedral vanadium and the other at -350 ppm being 1 branch of the octahedral V resonance typically found in these supported cata- lysts. Magic angle spinning has a minimal effect on the linewidth of the broad peaks (Figure 10.7B), indicating the presence of inhomogeneous broadening due either to a distribution of quadrupole and chemical shift parameters, or to dynamic processes in the second coordination sphere of the vanadium. Removal of the adsorbed water by evacuation at 200~ results in the loss of the peak from the vanadium-tungsten anion and the broadening of the entire spectrum, with further broadening occurring after heat treatment at 500~ (Figure 10.7B). By analogy with NMR results for V205 catalysts supported on ~/-A1203 and on TiO2, the spectra of the heated samples provided possi- ble evidence for surface interactions between mixed WO3-V205 species and the TiOx support, although direct detection of V-O-W fragments was not possible due to the broadness of the 51V spectra (Mastikhin et al. 1995).

The formation of A1VO4 at high vanadium concentrations on AlzO3-supported cat- alysts has been detected by 51V NMR which has also revealed the complex behaviour of this compound during subsequent calcination processes leading to fundamental changes in the structure of the catalyst surface (Sobalik et al. 1992). Changes in a series of vanadium-rhodium catalysts supported on silica substrates during calcination have also been monitored by 51V NMR which showed the presence of V205 at all tempera- tures, the appearance of a distorted VO4 species at 973 K and the formation of RhVO4 at 1173 K (Lapina et al. 1992a).

Sepiolite, a layered magnesium silicate mineral, is used to stabilise cracking catalysts against metal contaminants in crude oils. 5 ~V NMR has been used to study the interac- tion of sepiolite impregnated with vanadium from vanadyl napthenate solution (Occelli et al. 1992). Hydrothermal treatment of the V-impregnated sepiolite samples results in the formation of disordered microcrystalline ot-MgzV207 (or ~-MgzV207 at higher temperatures), together with an amorphous surface phase identified by 51V nutation NMR as having a structural environment similar to [~-MgzV207 (Occelli et al. 1992).

Zeolites are shape-selective catalysts with a silicate or aluminosilicate framework into which vanadium can be incorporated, producing materials especially useful in

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NMR of Other Quadrupolar Nuclei 649

reactions involving the partial oxidation of hydrocarbons under mild conditions. Such catalysts can have new and improved properties; the catalytic activity of V-substituted silicalite is quite different from that of V203 supported on a silica substrate.

Hydrothermal synthesis has been used to prepare a vanadium-substituted silicate with the structure of ZSM-12 (Moudrakovski et al. 1994). Although no 5~V NMR signal was detected in the as-prepared sample, calcination at 823 K followed by rehy- dration produced a spectrum characteristic of VO4 but with no indication of the pres- ence of any V203. The 5~V NMR data support a structural model in which the V is connected to the lattice by 3 bonds, with the fourth oxygen as a double bond, possibly hydrogen-bonded to an adjacent SiOH group (Moudrakovski et al. 1994).

10.4. 63Cu AND 6SCu NMR

63Cu and 65Cu are both spin - 3/2 nuclei with natural abundances of 69.09% and 30.91% respectively. The quadrupole moment eQ of 63CH is 1.07 times greater than for 65Cu, but the XQ values for both nuclei can be very large in copper compounds, mili- tating against the possibility of significantly narrowing the 63Cu or 65Cu spectra by magic angle spinning. Although the ~/-value for 63Cu is 0.934 of the value for 65Cu making the latter a more sensitive nucleus, this gain in sensitivity is not sufficient to compensate for the lower natural abundance of 65Cu. Furthermore, since the first-order quadrupolar broadening is proportional to the quadrupole moment eQ, and since both nuclei are spin - 3/2, the second-order quadrupolar broadening is proportional solely to (eQ)2/~/resulting in the NMR lines of 63CH being broader than those of 65Cu by a factor of 1.25. Nevertheless, 63Cu tends to be the preferred nucleus for most NMR stud- ies. The linewidths of the static spectra have often been observed to be due to a com- bination of quadrupole and CSA effects. The differences in eQ and the frequencies of each nucleus are such that the spectra acquired for the 2 nuclei at a single field can yield similar information to spectra acquired for a single nucleus at 2 fields, allowing the deconvolution of the quadrupolar and CSA broadenings. CuC1 is a useful secondary chemical shift standard for these nuclei.

63CH NMR has provided useful structural information in a series of mixed copper halide crystals in which the copper ion is located in a suitably symmetrical site. The 63Cu MAS NMR spectra of a series of solid solution within the system CuBrxll-x are broadened due to the overlap of resonances from the 5 species CuBr4, CuBr3I, CuBr212, CuBrI3 and CuI4 (Endo et al. 1993). The spectra could be resolved by fitting Gaussian peakshapes, providing information about the various atomic structures present in the solid solutions. Similar 63Cu NMR experiments have also been carried out in the system CuClxBrl_x (Endo et al. 1993a).

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650 Multinuclear Solid-State NMR of Inorganic Materials

The temperature dependences of the 63Cu chemical shifts in a series of cuprous halides have been determined by Becker (1978). These results suggest that CuBr is the most ionic of these compounds and CuI is the most covalent. The 63Cu shifts of the low temperature (cubic) phases increase with temperature due to increasing vibrational overlap. At higher temperatures the 63Cu shifts become increasingly diamagnetic with temperature, reflecting the highly disordered state of the Cu + under these conditions (Becker 1978).

10.4.1 63Cu NMR of superconductors and superfast ionic conductors 63Cu NMR and nuclear quadrupole resonance (NQR) have been extensively applied to studies of the electronic phenomena in high-temperature superconducting oxides. These studies have exploited the unique ability of NMR and NQR to distin- guish the behaviour of different atoms at different crystallographic sites. A full dis- cussion of the extensive 63Cu NMR literature on the superconducting compound YBa2Cu307_y and related materials is beyond the scope of this chapter, but a few examples are presented which give an idea of the range of NMR results for these compounds.

The effect of oxygen stoichiometry on the temperature dependence of the Knight shift has been demonstrated by 63Cu NMR measurements on well-characterised YBa2Cu306.63 magnetically aligned to orient the powder grains and ensure that the lines from the powder are as narrow as possible (Takigawa et al. 1991). The 63Cu NMR spectrum of such a sample (Figure 10.8A) shows a relatively sharp central resonance corresponding to the planar Cu(2) sites. A broad downfield feature corresponds to Cu(1) chain sites with 4 oxygen nearest neighbours, while a sharp upfield feature which shows a small magnetic shift and is easily saturated by fast pulse repetition is associ- ated with Cu(1) chain sites with no O nearest neighbours (Takigawa et al. 1991). These 2 copper sites show sharply contrasting temperature (T) dependences of their 63Cu nuclear relaxation rates (Walstedt et al. 1988); in the normal state the chain sites exhibit a roughly linear temperature dependence as predicted by the simple Korringa model, while the planar sites relax with an approximately T ~/2 dependence. The anisotropy of the Cu relaxation in the planar sites has been examined in detail using the 63CI1 NMR shift to partition the susceptibility and estimate the density of states (Walstedt et al. 1988). Comparison of the relaxation rates of 63Cu with those of ~70 for the planar sites in the same YBa2Cu307 sample (Figure 10.8B) has revealed the exis- tence of a characteristic temperature range (between 20 K and 110 K) in which both relaxation rates exhibit identical temperature dependences over almost 3 orders of magnitude (Hammel et al. 1989). These results have led to the conclusion that the cop- per relaxation is enhanced by antiferromagnetic spin fluctuations which are undiminished in the superconducting state. Comparison of the 63Cu NMR relaxation rate data for

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N M R of Other Quadrupolar Nuclei 651

A YBa2CuaO6.63

planar

chain chain

54 56

planar

80 84

planar

82.0 82.8

H (kOe)

100

10

O r - .

0.1

YBa2Cu307

I I I I

f

1 10 100 i000 63Cu l/T1 (s -1)

Figure 10.8. A. 63CH NMR spectra of magnetically aligned YBa2Cu306.63. Upper: Central transition, T = 80 K, alignment parallel to c-axis. Centre: High-field quadrupole satellite (1/2, 3/2) transition,

T = 50 K, alignment parallel to c-axis. Lower: Central transition, alignment perpendicular to c-axis, showing similar second-order effects in the distribution of the EFGs as in the satellite transition

spectrum. From Takigawa et al. (1991). B. Relationship between the 170 relaxation rate and the 63Cu relaxation rate for the planar sites of YBazCu307 with temperature as an implicit parameter. The solid line of unity slope indicates the relationship 63Cu T1-1/170 T1-1 -- 19.3. The data deviate from this

relationship above 110 K. From Hammel et al. (1989). Both figures used by permission of the copyright owners.

YBa2Cu307 in the normal state with inelastic neutron scattering results for the same

material has revealed similar very short correlation lengths for the antiferromagnetic

fluctuations (Gillet et al. 1994). Measurements of both the spin lattice relaxation and

the Gaussian spin-spin relaxation times have been made on YBazCu408 by both 63Cu

NMR and NQR methods (Corey et al. 1996). Excellent agreement was found between

the NMR and NQR-der ived Gaussian spin-spin relaxation times, which indicated a

temperature regime just above Tc in which the Gaussian relaxation becomes indepen-

dent of temperature, similar to behaviour found by Takigawa et al. (1994) for

YBazCu306.63. The data for the normal and superconducting states could be explained

in terms of appropriate theory (Corey et al. 1996).

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652 Multinuclear Solid-State NMR of Inorganic Materials

In addition to the much-studied Y-Ba cuprate series of high-Tc superconductors, several other types of cuprate superconductors have been investigated by 63CH NMR. The relaxation behaviour of one such compound, Bi2Sr2CaCu208, which has been found to be proportional to the spin paramagnetic shift at temperatures <100 K as in other cuprate superconductors, was interpreted in terms of spin-gap effects (Walstedt et

al. 1991). 63Cu NMR has also been used to investigate the superconducting properties of HgBa2Ca2Cu308+~, a compound with a Tc of 133 K (Magishi et al. 1996). These studies suggest that the temperature dependence of the relaxation measured for both the planar (fourfold) and pyramidal (fivefold) CuO2 layers in this compound arises from combined relaxation channels to vortex cores, as well as from a residual density of states at the Fermi level associated with the gapless superconductivity (Magishi et al. 1996). The first anion-doped high-Tc cuprate superconductor to be dis- covered, Nd2CuO4-xF• with a Tc as high as 27 K, has been studied by 63Cu NMR (Dai et al. 1994). Measurements of the orientation dependence of magnetically aligned samples suggest that the electric quadrupole interaction at the copper site is surprisingly small for a structure in which the Cu is located in the centre of a square of oxygen atoms. A similar conclusion was reached for Nd2CuO3.sFo.2 by Sugiyama et al. (1993) on the basis of 63Cu and 19F NMR measurements.

A 63Cu NMR study of the diffusion mechanism of Cu + in the superionic conductor CuA1Br4 as a function of temperature (Tomita et al. 1999) has shown a linear change in the 63Cu ~iso values with temperature, attributed to thermal expansion of the crystal lattice. Changes in the central transition of the 63Cu powder NMR spectrum with tem- perature were simulated (Figure 10.9A), allowing the CSA to be determined. No change was found in the CSA with temperature, even at higher temperatures where the diffusion of Cu + is sufficiently rapid to average out the CSA, suggesting that the dif- fusion process proceeds via tetrahedral sites, since although larger octahedral intersti- tial sites are also available, their occupancy by Cu + would change the 63Cu CSA lineshape (Tomita et al. 1999). Analysis of the 63Cu satellite peak positions allowed the temperature dependence of the nuclear quadrupole coupling constant • to be estab- lished (Figure 10.9B). This behaviour suggests that in apparent contradiction to the conclusions from the CSA measurements, the Cu site approaches spherical symmetry at 420 K (Tomita et al. 1999). A similar effect was not observed in the 27A1 XQ values of the same samples (Figure 10.9B), indicating that the environments of these Cu and

A1 sites are very different. The compound CuGeO3 is a one-dimensional Heisenberg antiferromagnetic material

which undergoes a spin-Peierls (SP) transition at about 14 K, at which spin and lattice dimerisations may simultaneously occur. 63Cu and 65Cu NMR and NQR measurements have provided information about the Cu 2 + electronic state and the spin dynamics in this material (Itoh et al. 1995). An anisotropic Knight shift with axial symmetry was observed and analysis of its temperature dependence suggested a decrease in the spin

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NMR of Other Quadrupolar Nuclei 653

A

526 K

' ..... , ...... s~mu.!ated_

500

300

294 K 100

ved

................... ': " ---s_!mulated (ii)

i ' i i I -

71.87 71 88 71.89

Frequency (MHz)

~7AI

! _

300 400 500

Temperature K

Figure 10.9. A. Observed and simulated 63Cu NMR spectra of CuA1Br4 at two different temperatures. B. Temperature dependence of the 63Cu XQ values (solid circles) and e7A1 XQ values

(open circles) for CuA1Br4. From Tomita et al. (1999), by permission of Elsevier Science.

susceptibility below the temperature of the SP transition. The data suggest that the Cu 2+ can be described by a single-ion model in an octahedral crystal field with tetrag- onal symmetry. The temperature dependence of the nuclear spin-lattice relaxation rate shows the appearance of a gap in the magnetic excitation spectrum below the SP tran- sition temperature. These experiments showed that supertransferred hyperfine interac- tion, a characteristic of the planar Cu 2+ in cuprate superconductors, does not play a

significant role in CuGeO3 (Itoh et al. 1995).

10.5. 69Ga AND 71Ga NMR

10.5.1 Generalcons iderat ions

The 2 NMR-active nuclei of gallium both have spin I - 3/2 but different quadrupole moments (Q = 1.71 • 10 -29 m 2 for 69Ga and Q = 1.07 • 10 -29 m 2 for 71Ga). Gallium

is chemically similar to aluminium, forming analogous compounds, but despite the sim- ilarity in site distortion between A1 and Ga compounds and the quadrupole moment of 27A1 being intermediate between those of 69Ga and 71Ga, well-resolved Ga NMR spectra

are more difficult to obtain because the second-order quadrupole broadening of the central transitions of 69Ga and 71Ga for a given EFG is greater than for 2:A1 by a factor

of about 11. However, both nuclei have good sensitivity, and the effect of going from 69Ga to 71Ga at 7 T produces similar benefits in terms of second-order quadrupolar

effects equivalent to obtaining the same 69Ga spectrum at 17.5 T (Massiot et al. 1995).

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654 M u l t i n u c l e a r So l id -S ta te N M R o f I norgan i c M a t e r i a l s

The 71Ga chemical shifts of a number of gallium compounds are given in Table 10.4.

From the known crystal structures of these compounds, 4-coordinated Ga which falls in

the chemical shift range 107 to 222 ppm (with respect to Ga(H20)63+) can readily be

distinguished from 6-coordinated Ga at - 80 to - 42 ppm. The 71aa shifts bear a linear

relationship to the 27A1 shifts of the analogous compounds (Figure 10.10), allowing, in

principle, the 71Ga shift of any unknown compound to be inferred from the 27A1 shift of

the A1 analogue (Bradley et al. 1993) via the relationship

671Ga (ppm) = 2.83(627A1) (ppm) - 4.50 (10.8)

A more correct version of this relationship, in which all the points are true isotropic

chemical shift positions and which includes a number of additional data points has

been given by Massiot et al. (1999) as

671Ga (ppm) = 2.84(627A1) (ppm) - 1 (10.9)

Table 10.4. 71Ga NMR parameters for gallium compounds.

Compound ~iso (ppm)* • (MHz) Xl Reference

[3---Ga203 (tet) 200-220, 198 11.0, 1.1 0.85 Massiot et al. (1995), (1999) (oct) 40-50, 25 8.3, 8.34 0.08, 0.01 Massiot et al. (1995), (1999)

oL-GaOOH.H20 42 ND ND Bradley et al. (1993) NaGaO2 223 ND ND Bradley et al. (1993)

[3-LiGaO2 83 ND ND Miyaji et al. (1992) Y3Ga5Ol2 (tet) 219 13.1 0.05 Massiot et al. (1999)

(oct) 5.6 4.1 0.03 Massiot et al. (1999) MgGa204 (tet) 171 7.6 ND Massiot et al. (1999)

(oct) 74 7.6 ND Massiot et al. (1999) LaGaGe207 (Ga v) 75.8 15 0.7 Massiot et al. (1999)

Ga(PO3)3 - 40 ND ND Bradley et al. (1993) GaPO4 (quartz struct.) 100.3 8.6 0.51 Massiot et al. (1999) GaPO4 (crist. struct.) 118 4.7 0.45 Massiot et al. (1999)

NHn[Ga(SOn)2].12H20 - 1.9 ND ND Timken & Oldfield (1987) Ga-ZSM-6 150-160 ND ND Bayense et al. (1989),

Chen et al. ( 1991), Kentgens et al. (1991)

Ga-erionite 157 ND ND Bradley et al. (1993) Ga-natrolite 169 ND ND Timken & Oldfield (1987) Ga-zeolite Y 172 ND ND Timken & Oldfield (1987) Ga-zeolite X 174 ND ND Timken & Oldfield (1987) Ga-sodalite 183.5 ND ND Thomas et al. (1983)

Ga-gehlenite (tet) 250 ND ND Massiot et al. (1999) (oct) 233 > 13.5 ND Massiot et al. (1999)

* chemical shifts relative to Ga(H20)63+

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NMR of Other Quadrupolar Nuclei 655

�9 3oo- r

-~ 150 " ~ -

.p . . .~

~ -150 -50

o

~ CN=6

| l ,

0 50 100

27A| shift (ppm) w.r.t. AI(H20)63§

Figure 10.10. Plot of the 71Ga chemical shifts of gallium compounds with only oxygen in the first coordination sphere vs. the 27A1 chemical shifts of the analogous aluminium compounds. The

scatter is attributed to the fact that not all the shift values may be the isotropic shifts. From Bradley et al. (1993), by permission of John Wiley and Sons Ltd.

The 71Ga NMR parameters of the well-defined 5-coordinated Ga site in the compound LaGaGe207 have been determined from its static spectrum (Massiot et al. 1999). The giso value (75.8 ppm) falls between the typical shift range for Ga (~v~ and Ga (vI~, with a XQ value of 15 MHz and an estimated CSA of --~ 100 ppm (Massiot et al. 1999).

10.5.2 69,71Ga N M R o f crystalline compounds Both the 69Ga and 71Ga MAS NMR spectra of ~-Ga203 have been reported (Massiot et al. 1995). One-half of the Ga atoms in this oxide are in tetrahedral sites and the other half are in octahedral sites. The spectral widths extend over at least 200 kHz and are thus too broad to be usefully narrowed by MAS. However, the static spectra from both 69Ga and 71Ga show similar features (Figure 10.11) and can be simulated with 2 second-order quadrupolar lineshapes corresponding to the tetrahedral and octahedral sites, although the site population ratio derived from the simulations show a systematic underestimation of the wider contribution. The resulting XQ values for the 2 sites (Table 10.4) indicate that the tetrahedral site is more distorted than the octahedral, con- sistent with the known crystal structure of this compound (Massiot et al. 1995).

GaPO4 is similar to A1PO4 in being able to crystallise in structural forms related to the silica polymorphs quartz and cristobalite. The 71Ga NMR spectra of both GaPO4 structures have been recorded, the NMR parameters (Table 10.4) being consistent with the more symmetrical tetrahedral Ga environment in the cristobalite form (Massiot et al. 1999).

Very fast spinning MAS speeds (up to 35 kHz) have been used to distinguish the tetrahedral and octahedral Ga sites in the 71Ga NMR spectra of a series of Ga-rich fluoro-amphiboles NaCazMg4Ga3Si6Oz2F2, since more conventional spinning

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656 Multinuclear Solid-State NMR of Inorganic Materials

A 69Ga B 71Ga

~ oct

F --~] ~. tet

' 10000 ' o ' - i o o o 0 '

~J tet ~ _ c ~

~-------k simulated

oct

2000 0 -2000

Ga shift (ppm) w.r.t. Ga(H20)63+

Figure 10.11. A. Observed and simulated 7 T static 69Ga NMR spectrum of [3-Ga203 showing the octahedral and tetrahedral contributions to the simulation. B. The corresponding 7 T static

71Ga NMR spectrum and simulation. From Massiot et al. (1995), by permission of the copyright owner.

speeds (14 kHz) resulted in an overlap of the spinning sidebands of the tetrahedral and octahedral resonances (Sherriff et al. 1999). The resulting 7~Ga MAS NMR spectra (Figure 10.12A) showed a single tetrahedral peak at 230 ppm and 2 low-intensity peaks attributed to an octahedral quadrupolar doublet at about 40 ppm. The crystal structure of this mineral indicates that the occupation of the octahedral sites should be about one- third of the tetrahedral sites; the NMR measurement therefore significantly underesti- mates the intensity of the octahedral Ga site, possibly due to large quadrupolar effects associated with this distorted site (Sherriff et al. 1999).

The formation of germanate phases with the mullite structure (Ga6Ge2013, (Ga,A1)6Ge20~3) from sol-gel precursors has been studied by multinuclear solid state NMR, including 71Ga MAS NMR (Meinhold and MacKenzie 2000). Many of the 71Ga spectra were dominated by the quadrupolar lineshape of 13-Ga203 from the structural units of the intermediate phase a-Ga4GeO8 or from unreacted starting material. The 71Ga NMR spectrum of crystalline Ga6GeeO13 was found to be broad and featureless, as were the 69Ga MAS NMR spectra of these samples. Static 69Ga NMR spectra obtained for quenched and annealed samples of the clinopyroxene LiGaSi206 have been interpreted as indicating the presence of 2 different electronic states of the octa- hedral Ga ions (Ohashi et al. 1995)

A series of crystalline gallosilicate molecular sieves with the zeolite [3 structure synthesised by a rapid method from alkali-free hydrogels have been studied by NMR methods, including 7~Ga MAS NMR (Occelli et al. 1999). The 71Ga spectra (Figure 10.12B) show that most samples contain only tetrahedral Ga in framework

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NMR of Other Quadrupolar Nuclei 657

A

L ! f

400

Oct

r t F - P

200 0

B B

et ~ O c t

I i I p I

300 100 -100

71Ga shift (ppm) w.r.t. Ga(H20)63+

Figure 10.12. A. 71Ga MAS NMR spectra of 2 gallium-fluoro amphiboles spun at 28 kHz. The most Ga-rich sample (upper) shows a small octahedral Ga feature which may arise from a gallium

sapphirine impurity. From Sherriff et al. (1999), by permission of the Mineralogical Society of America. B. 71Ga MAS NMR spectra of 2 galliosilicate molecular sieves with the ~ zeolite structure.

The tetrahedral Ga spectrum (upper) is typical of gallium in framework sites. The additional octahedral Ga resonance (lower spectrum) arises from extra framework Ga generated during thermal

treatment of the sample. From Occelli et al. (1999), by permission of Elsevier Science.

sites, but one sample clearly showed the presence of extra-framework octahedral Ga, due in part to the thermal treatment of the sample after synthesis. Gallium has also been incorporated into the structure of a microporous titanosilicate ETS-10 which unlike other zeolite-type materials contains framework atoms in octahedral coordination. 71Ga MAS NMR of the gallium-substituted sample ETGS-10 shows the presence of only tetrahedral Ga, indicating that its isomorphous substitution occurs only on the silicon sites to avoid the neighbouring titanium (Rocha et al. 1995). In this respect the behaviour of gallium is similar to that of aluminium in the analogous titanoalumi- nosilicate ETAS-10. A 69Ga and 71Ga MAS NMR study of a series of gallium ana-

logues of the zeolite ZSM-5 at 2 applied magnetic fields has allowed the value of XQ (1.9-2.2 MHz) to be determined from the difference of the peak position of the 69Ga and 71Ga resonances (Kentgens et al. 1991). Changes in the linewidth as a function of the magnetic field revealed the presence of both second-order quadrupolar broadening, and broadening due to a chemical shift distribution.

10.5.3 69"71Ga N M R of other compounds

A series of caesium gallate glasses xCs20. (1- x)Ga203 where x = 0.3 to 0.7 have been studied by 69'71Ga NMR (Zhong and Bray 1987). Both static and MAS conditions were

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658 Multinuclear Solid-State NMR of Inorganic Materials

used to establish the presence of tetrahedral and octahedral coordination in these glasses. The results indicate that in glasses with Ga:Cs ratios of less than 3:7, the Ga is in solely tetrahedral network-forming sites, but as the gallium content increases, the excess Ga enters octahedral sites.

The 71Ga NMR spectra of the semiconductors GaAs and InGaAs show a single res- onance in GaAs but the spectrum of InGaAs consists of a sharp intense peak as in GaAs, but with an underlying broad, weak resonance. Under MAS conditions, the InGaAs spectrum shows an extensive and complex sideband structure, by comparison with the simpler MAS spectrum of GaAs (Kushibiki and Tsukamoto 1986).

10.6. 87Rb NMR

10.6.1 General considerations Rubidium compounds are important in a number of areas of materials science, as cat- alysts for ammonia synthesis and oxidation of methane, as a component of some glasses and as a dopant metal in buckminsterfullerene (C6o) causing it to become superconducting at 28 K.

There are 2 NMR-active rubidium isotopes, 85Rb (I = 5/2, natural abundance 72.8%) and 87Rb (I = 3/2, natural abundance 27.2%). The sensitivity of 87Rb is greater than that of 85Rb, but its residual homonuclear dipolar broadening is larger and its relaxation time tends to be longer (100-300 ms for simple Rb salts) which is an advantage for acquir- ing DAS spectra. Most of the published solid state rubidium NMR uses 87Rb as the nucleus of choice, although the larger quadrupole moment of 85Rb can be useful in pro- viding an indication of the number of chemically different Rb sites present in a salt (Cheng et al. 1990).

The simple rubidium salts such as the halides and nitrate have small • values giving rise to narrow resonances, whereas the chromate, acetate, sulphate and hydroxide have larger XQ values giving wider central transition lineshapes (Cheng et al. 1990). The NMR interaction parameters of a number of rubidium compounds are collected in Table 10.5.

10.6.2 S7Rb NMR of crystalline compounds Dynamic Angle Spinning (DAS) has been used to obtain the 87Rb NMR spectra of several rubidium salts (Baltisberger et al. 1992), including RbNO3 which contains 3 inequivalent Rb sites and could not be resolved in the static NMR spectrum (Cheng et al.

1990). DAS NMR was found to narrow the 87Rb spectral lines significantly more than MAS or VAS (variable angle spinning), except in the case of RbC1 (Figure 10.13A), in which the nucleus is in a cubic environment with no second-order quadrupolar

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N M R o f O t h e r Q u a d r u p o l a r N u c l e i

Table 10.5. 87Rb NMR interaction parameters for rubidium compounds.

659

Compound ~iso XQ "q Reference (ppm)* (MHz)

RbF 50 0 0 Cheng et al. (1990) RbCI 128, 0, 0 0, 0 Cheng et al. (1990),

127 Baltisberger et al. (1992) RbBr 155 0 0 Cheng et al. (1990) RbI 183 0 0 Cheng et al. (1990)

Rb2SO4 site 1 46.4, 42 2.6 0.89 Cheng et al. (1990), site 2 3.0, 16 3.2 0 . 1 3 Baltisberger et al. (1992)

RbzCO3 site 1 18.9 5.0 0.75 Cheng et al. (1990) site 2 - 7.0 3.2 1.0 Cheng et al. (1990)

RbzCrO4 site 1 - 47.4, - 11 5.2 0.48 Cheng et al. (1990), site 2 52.8 11.5 0 . 7 5 Baltisberger et al. (1992)

Cheng et al. (1990) RbC104 3.8, 3.2, 0.16, Cheng et al. (1990),

- 16.2 3.2 0.10 Baltisberger et al. (1992) RbOH.H20 30.5 4.3 0.77 Cheng et al. (1990)

RbNO3 site 1 - 26.2 1.83 0 . 1 2 Baltisberger et al. (1992) site 2 - 26.8 2.07 1 . 0 0 Baltisberger et al. (1992) site 3 - 30.9 1.85 0 . 4 8 Baltisberger et al. (1992)

RbOOCH.H20 0 2.8 0.3 Cheng et al. (1990) Rb acetate.H20 7.6 6.9 0.47 Cheng et al. (1990)

* chemical shifts referred to RbNO3 solution

broadening. MAS is able to average the homonuclear dipolar interaction present in this compound but DAS will not, resulting in a broader DAS spectrum. DAS allowed the MAS powder patterns of RbNO3 to be separated (Figure 10.13B) and the quadrupolar parameters for each site to be determined by single-site simulation (Baltisberger et al.

1992). The isotropic shifts derived by DAS for a number of rubidium salts other than RbC1 are considerably different from those from the static 87Rb spectra (Table 10.5);

the DAS parameters are claimed to be more reliable since they do not depend on sim- ulations requiting a number of adjustable parameters (Baltisberger et al. 1992). A triple-

quantum MAS NMR approach has also been used to separate the 3 Rb sites in RbNO3, demonstrating the usefulness of this technique for such closely overlapping sites (Fer- nandez and Amoureux 1996). By simultaneously selecting the 2 mirror coherence transfer pathways (0)(_+ 3 ) ( - 1) the pure-phase 2D 87Rb spectrum of RbNO3 was obtained (Figure 10.14) with a significant gain in sensitivity. The resulting isotropic chemical shifts and quadrupolar products are in agreement with those determined by DAS measurements (Baltisberger et al. 1992).

87Rb NMR has been used to measure the temperature dependence of the second- order shifts of the central transition of RbSCN, a compound which undergoes an

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660 Multinuclear Solid-State NMR of Inorganic Materials

A

RbCI

A I I ! , , |

145 125

MAS

DAS

I t

105

RbNO3 / ~ ~

JAM _ _ L , ,, t 11 I

-25 -30 -35 -40

S7Rb shift (ppm) w.r.t RbC! soln.

-10 -

-30

-50

observed simulated

_L_ _L_

\ ' S ' -20 -33 -45 -20 3 -45

S7Rb shift (ppm) w.r.t. RbCI soln.

Figure 10.13. A. 87Rb MAS and DAS NMR spectra of RbC1 (upper spectra) and RbNO3 (lower spectra) acquired at 11.7 T. B. 87Rb powder pattern cross-sections through the F2 dimension of a pure-phase MAS- detected DAS spectrum of RbNO3 acquired at 11.7 T with simulations of the lineshapes from the 3 sites.

From Baltisberger et al. (1992) by permission of the American Chemical Society.

~ g e e t l o n 3

s ~ e c t i o n 2

section 1

30

25 ~,

20

-20 30 -40 v2 (ppm)

Figure 10.14. 87Rb two-dimensional triple-quantum MAS NMR spectrum of RbNO3 showing the three Rb sites and their corresponding anisotropic sections. From Fernandez and Amoureux (1996), by

permission of the copyright owner.

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NMR of Other Quadrupolar Nuclei 661

antiferroelectric phase transition at 435 K from a high-temperature paraelectric tetragonal phase to a low-temperature antiferroelectric orthorhombic phase (Blinc et al. 1995). The 87Rb NMR results provide a physical picture of the structural transformation in terms of a disordering process connected with the formation of dynamic clusters or microdomains which are embedded in a long-range-ordered matrix below the transition temperature and which become random above the transition temperature (Blinc et al.

1995). 8VRb NMR has also been used to study the low-temperature phase of RbzZnC14, a one-dimensionally modulated incommensurate crystal which exhibits successive phase changes through 4 phases. Single-crystal measurements of the change in the 87Rb NMR lineshape as a function of temperature in the vicinity of the low temper- ature commensurate phase change at 74.6 K have confirmed the crystal structure of this phase, while an anomaly in the temperature dependence of the spin-lattice relaxation time was interpreted in terms of the condensation of the soft mode inducing this trans- formation (Apih et al. 1992).

Alkali metals such as rubidium are added to the surface of alumina catalysts to improve their efficiency in facilitating the partial oxidation of ethylene to ethylene oxide. 87Rb NMR has been used to gain an understanding of the interactions between Rb salts and the reactive sites of ~/-alumina (Cheng and Ellis 1989). When the alumina was impregnated with solutions of RbI, RbC1, RbNO3 and RbzSO4 at concentrations giving a submono- layer coverage, 4 rubidium species were identified after oven drying. Two of these species, described as surface salts, have 87Rb chemical shifts which depend on the nature of the impregnating anion, while the other species (described as surface species) are only weakly bonded to their oxo-anion and have chemical shifts which are essentially inde- pendent of the anion. The 87Rb relaxation times suggest that both the surface salts and sur- face species can exist in a disordered form containing interstitial vacancies which provide a mechanism for migration of Rb + from site to site (Cheng and Ellis 1989).

Alkalides and electrides are stoichiometric salts containing alkali metal cations complexed by crown ethers. Charge balance is provided by the alkali metal anions (alkalides) or trapped electrons (electrides). 87Rb and 85Rb NMR has been used to study a number of rubidium alkalides, electrides and related compounds (Kim et al.

1996). Spin-echo NMR measurements were used to obtain reliable values of giso, Xo and qq for these compounds.

10.6.3 87Rb NMR of rubidium fuUerides. Solid C6o (buckminsterfullerene) can be intercalated by alkali metal atoms to form MxC6o compounds where x = 1--~ 6. The discovery that M3C6o displays superconduc- tivity, with transition temperatures as high as 33 K, has promoted considerable research interest, including a number of solid state NMR studies of both the structure and superconductivity mechanisms in these compounds.

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662 Multinuclear Solid-State NMR of Inorganic Materials

Above 450 K, the 87Rb NMR spectrum of Rb3C6o contains 2 sharp resonances arising from rubidium in non-equivalent octahedral and tetrahedral sites (Walstedt et al.

1993). The octahedral peak appears at about 52 ppm and the tetrahedral peak is at about 195 ppm with an octahedral:tetrahedral intensity ratio of 1:2, consistent with the known crystal structure. As the temperature is lowered these resonances broaden and shift slightly and a third tetrahedral resonance appears; at 200 K the 3 resonances occur at 40 ppm (octahedral), 165 ppm (tetrahedral) and 270 ppm (new tetrahedral). The forma- tion of the second tetrahedral site has been explained in terms of alkali-metal vacancies which occur only in the tetrahedral positions (Apostol et al. 1996). Measurements of the 87Rb and 85Rb relaxation rates indicate a quadrupole relaxation mechanism involving phonons, and no change in either the NMR spectrum or the relaxation rates was found in the vicinity of Tc for this compound (Corti 1993).

High-temperature superconductivity has also been demonstrated in mixed alkali metal fullerides, including the ternary alkali compound KRbCsC6o which has a Tc of 28-29 K (Maniwa et al. 1993). The 87Rb NMR spectra of this and the related binary alkali compound K2RbC6o (Figure 10.15A) show that whereas the rubidium ions occupy predominantly octahedral sites ( - 1 5 0 ppm) in the latter, they are located primarily in tetrahedral sites (about 0 ppm) in KRbCsC6o. These results, together with 133Cs NMR, provide evidence that the alkali metal atoms are site-selectively interca- lated into the face-centred-cubic C6o lattice according to their size, with the largest ion (Cs) preferentially occupying the octahedral sites and the smaller K and Rb ions occupying mainly tetrahedral sites (Maniwa et al. 1993).

87Rb NMR has been used to study the electronic properties and phase transitions in another rubidium fulleride, RbC6o, in which the rubidium is located in octahedral sites of the NaC1 structure (Tycko et al. 1993). The 87Rb NMR spectra (Figure 10.15B) indi- cate a phase transition in this compound at about 300 K, the low-temperature phase containing a broad octahedral Rb resonance at - 120 ppm, being replaced above the transition temperature by a narrower tetrahedral line with a chemical shift decreasing strongly from 615 ppm at 353 K to 410 ppm at 473 K. The NMR data indicate that the high-temperature phase is a paramagnet in which the electronic dynamics are domi- nated by electron-electron effects. The electron-spin susceptibility is greatly reduced in the low-temperature phase in which most of the unpaired electron spins have become paired (Tycko et al. 1993).

10.7. 93Nb NMR

93Nb is a nucleus with spin = 9/2, a relatively large quadrupole moment (Table 1.2, Chapter 1) and 100% natural abundance. 93Nb NMR is difficult since, in the solid state, electric field gradients arising from the electronic cloud at the nucleus can interact

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NMR of Other Quadrupolar Nuclei 663

A

Oct

K2RbC6 o /

K R b C s C 6 ~

I I I | . . . .

200 0 -200

87Rb shift (ppm) w.r.t. RbCl soln.

B RbC6o

T(K) / ~

Tet

313

323 ~.

800 400 0 -400

a7Rb shift (ppm) w.r.t. RbCI soln.

Figure 10.15. A. 87Rb NMR spectra of the rubidium fullerides K2RbC6o (upper) and KRbCsC6o (lower), from Maniwa et al. (1993). B. 87Rb NMR spectra of the rubidium fulleride RbC6o at various temperatures. Note the broad octahedral Rb resonance in the low-temperature phase

progressively replaced by the narrower tetrahedral resonance above the phase transition temperature. From Tycko et al. (1993). Both diagrams used by permission of the copyright owners.

with the nuclear electric quadrupole, giving rise to considerable spectral broadening. However, the technical importance of a number of niobates as piezoelectric and opto- electric ceramic materials has provided the stimulus for several recent 93Nb NMR studies using techniques such as high-speed MAS, DAS and MQMAS to overcome broadening problems. Chemical shifts have been quoted in the literature with respect to solid Nb205 or a saturated solution of NbC15 in wet acetonitrile, the latter being the more commonly used reference substance.

The 93Nb NMR spectra of a number of alkali and lead niobates have been acquired using MAS, DAS, MQMAS and two-dimensional nutation NMR to improve the resolution and determine values of the quadrupolar products PQ for these materials (Prasad et al. 2001). The 9.4 T 93Nb MAS NMR spectrum of LiNbO3 spun at 25 kHz (Figure 10.16A) shows a lineshape dominated by the second-order quadrupolar inter- action, with only marginal improvement in resolution at a field of 14.1 T. For most of the metal niobates, the second-order interaction is not removed by MAS alone, even at high magnetic fields and fast spinning speeds. The expected improvement in 93Nb resolution provided by DAS spectroscopy is offset by significant homonuclear

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664 Multinuclear Solid-State NMR of Inorganic Materials

A M A S

14.1 T

-750 -1000 -1250

93Nb shif t ( p p m )

B D A S C M Q M A S D 2 D n u t a t i o n

.1 00 11,ti, i! ~ o -1000"

,,I ~ i, I ~ ......... , ......... , , ....... ......... , ........ , ..... lu 'r ......... i ......... , ......... w . . . . . . . . . . . I ......... t ........ ~ ......... r ........

-1000 -1400 ~ 300 100 ~ 1 3 5

Frequency (ppm) Frequency (ppm) Frequency (vm~) w.r.t. NbCIs in acetonitrile

Figure 10.16. 93Nb NMR spectra of LiNbO3. A. MAS NMR spectra acquired at 14.1 T (spinning speed 18 kHz) and 9.4 T (spinning speed 25 kHz). B. DAS NMR spectrum also showing the 1D

projection of the isotropic dimension. C. Triple-quantum MAS NMR spectrum also showing the 1D projection of the isotropic dimension. D. Pure-phase 2D nutation spectrum also showing the 1D

projection of the nutation dimension. From Prasad et al. (2001) by permission of the copyright owner.

Nb-Nb dipolar interactions which dominate the centreband in the isotropic dimension of the DAS NMR spectrum of LiNbO3 (Figure 10.16B). Although the use of MQMAS NMR to improve the resolution is hampered by the large quadrupole interactions of 93Nb, making multiple-quantum excitation and conversion less efficient, the triple- quantum 93Nb spectrum of LiNbO3 (Figure 10.16C) shows an improvement in resolu- tion of approximately an order of magnitude over the DAS spectrum (Prasad et al.

2001). The numerous spinning sidebands in the isotropic dimension of the MQMAS NMR spectrum arise typically from rotor modulation of the anisotropic chemical shift and quadrupolar interactions in the conversion period being different from that of the excitation period. Since the 93Nb quadrupolar products PQ of LiNbO3 and the related alkali niobates determined by MQMAS NMR are large (22.1-22.7 MHz), two- dimensional nutation spectroscopy has been used to provide complementary informa- tion. The 2D nutation spectrum of LiNbO3 (Figure 10.16D) shows a single Nb site with its centre of gravity at 4VRF corresponding to a XQ value of 20 MHz (Prasad et al. 2001).

Lead magnesium niobate, Pb(Mgo.33Nbo.66)O3, a relaxor ferroelectric material with a high dielectric constant and useful electrorestrictive properties, occurs in both perovskite and pyrochlore structures. The perovskite contains Nb(V) in the multiple B-sites of the structure, with a 93Nb MAS NMR spectrum showing 2 distinct Nb environments for which the quadrupolar parameters were determined from the triple- quantum MAS NMR spectrum (Cruz et al. 1999). The broader of these 2 resonances, centred at - 980 to - 1000 ppm with respect to NbC15 in acetonitrile, has been assigned to a range of axial or rhombic Nb(ONb)6_•215 B-site configurations occurring in the Nb-rich regions (Fitzgerald et al. 2000). The shift of the other sharper

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N M R o f O t h e r Q u a d r u p o l a r N u c l e i

Table 10.6. 93Nb NMR interaction parameters for niobium compounds.

665

Compound giso(ppm)* XQ (MHz) x I Reference

LiNbO3 - 1004, 22.1t, 0 Prasad et al. (2001), - 1009 22.1 Kind et al. (1968)

NaNbO3 - 1073 22.7 t, 0.82 Prasad et al. (2001), 19.7 Kind et al. (1968)

KNbO3 - 1050 23.1 0.80 Fitzgerald et al. (2000), Kind et al. (1968)

PbNb206 - 1113, - 1090 16.8 t, 19 0.5 Prasad et al. (2001), Prasad et al. (1999)

Pb2Nb207 - 1003 13.C - Prasad et al. (2001) " - 978 17.0 t - Prasad et al. (2001)

Pb3Nb4013 - 995 13.7* - Prasad et al. (2001) PbsNb4015 - 1013 16.6 t - Prasad et al. (2001)

" - 975 17.9 t - Prasad et al. (2001) Pb3Nb208 - 999 18.9 t - Prasad et al. (2001)

" - 951 20.6 ~ - Prasad et al. (2001) Pbl.g3Nb1.vlMgo.2906.39 - 995 13.7 t - Prasad et al. (2001)

(perovskite) Pb1.83Nbl.71Mg0.2906.39 - 1014 26.8 t - Prasad et al. (2001)

(pyrochlore)

* chemical shifts relative to NbC15 in acetonitrile t quadrupolar product values PQ

resonance at - 902 p p m is unusual for B-site Nb(ONb)6 configurat ions, and has been

tentat ively expla ined in terms of 1" 1 M g / N b order ing in Mg-r ich regions where the

local symmet ry is near ly cubic (Fi tzgerald et al. 2000). The 93Nb M A S N M R and

M Q M A S N M R spectrum of the pyrochlore form contains a single resonance suggesting

a d i s t r ibu t ion of i sot ropic chemica l shifts and quad rupo le coup l ings a t t r ibuted to

disorder in the Nb env i ronmen t (Cruz et al. 1999). Structural details of both the per-

ovskite form of lead t i tanium niobate and a series of lead niobate pyrochlores have also

been s tudied by 93Nb D A S N M R (Prasad et al. 2001) and by 2D nutat ion spect roscopy

(Prasad et al. 1999, Fi tzgerald et al. 2000, Prasad et al. 2001).

1 0 . 8 . 133Cs N M R

10 .8 .1 G e n e r a l c o n s i d e r a t i o n s

133Cs is a spin I - 7/2 nucleus of 100% natural abundance and a very small quadrupole

m o m e n t of - 3.4 x 10 -31 m 2. 133Cs in inorganic caes ium compounds (Table 10.7)

shows a modera te chemical shift range (about 300 ppm). The chemical shifts are nor-

mal ly referenced to aqueous CsC1 solution.

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666 M u l t i n u c l e a r So l i d -S ta t e N M R o f I n o r g a n i c M a t e r i a l s

Table 10.7. 133Cs NMR interaction parameters of caesium compounds.

Compound ~iso (ppm)* XQ (kHz) xl Reference

CsOH.H20 ND 98.2 "~0 Amm & Segel (1986) CsBr 258.2, 0 0 Mooibroek et al. (1986),

260.3 Haase et al. (1977) CsC1 223.2, 0 0 Mooibroek et al. (1986),

228.1 Haase et al. (1977) CsI 275 ND ND Haase et al. (1977)

CsC104 0 135, 0.09, Mooibroek et al. (1986), 126 0.08 Tarasov et al. ( 1991)

CsBrO4 ND 115 0 Tarasov et al. (1990a) CsIO4 ND 168 0.45 Tarasov et al. (1990) CsCN 135.3 95 0 Mooibroek et al. (1986) CsNO3 - 9.2, 140 ND Mooibroek et al. (1986),

- 14.9 Haase et al. (1977) CszSO 4 100, ND, ND Mooibroek et al. (1986),

91.7 38.0 Haase et al. (1977) Cs2CO3 152.7 ND ND Haase et al. (1977) Cs2CrO4 - 61.3 ND ND Haase et al. (1977)

CsTcO4 site 1 - 83** 370 --~0 Tarasov et al. (1992) site 2 - 115"* 270 --~0 Tarasov et al. (1992)

CsAuC13 128 272 0 Mooibroek et al. (1986) CsSD ( - 81~ 257 134 0 Mooibroek et al. (1986) CsSeD ( - 61 ~ 276 134 0 Mooibroek et al. (1986)

CsA1TiO4 66.0 ND ND Hartman et al. (1998) Cs2CdSi5012 77.1, 25.7 ND ND Kohn et al. (1994) Cs2ZnSi5012 64.4, 8.0 ND ND Kohn et al. (1994) CszMgSisOl2 62.9, - 4.3 ND ND Kohn et al. (1994) CsCd(SCN)3 94.4 148 0.98 Kroeker et al. (1997)

LiCsSO4 - 181 0.92 Lim et al. (1999) CsMnC13 site 1 - 153 0 Lim et al. (1997b)

site 2 - 212 0 Lim et al. (1997b) CsPbC13 - 207 0.38 Lim & Jeong (1999a)

Cs3Sb 620 105 ND Dupree et al. (1982) CsAu 375 ND ND Dupree et al. (1980)

* chemical shifts quoted with respect to aqueous CsC1 solution ** chemical shifts quoted with respect to aqueous CsBr solution

1 0 . 8 . 2 133C$ N M R o f c r y s t a l l i n e caesium compounds A comprehensive single-crystal 133Cs NMR study of Cs2CrO4 has yielded the magni-

tude and orientations of the 133Cs chemical shielding and quadrupolar tensors for the 2

crystallographically distinct Cs sites in this compound, indicating that the chemical

shielding and quadrupolar interactions are not coincident for the 2 distinct caesium posi-

tions (Power et al. 1994).

The temperature dependence of the 133Cs N M R interaction parameters of solid

CsC104, CsBrO4 and CsIO4 have been determined as a function of temperature. In

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NMR of Other Quadrupolar Nuclei 667

CsC104 the temperature dependence of XQ for both 133Cs and 35C1 is linear, with a nega- tive temperature coefficient, but the temperature dependence of the 133Cs -q-value shows an anomaly at 342 K (Figure 10.17A) which is not related to any abrupt structural change but to a change in the relative values of the tensor components of the electric field gradient (Tarasov et al. 1991). 133Cs and 79'81Br NMR of CsBrO4 show lineshapes dominated by quadrupolar effects. An increase in 133Cs • and a decrease in ~q with increasing temperature (Figure 10.17B) is thought to be associated with anisotropic behaviour of the lattice a-parameter (Tarasov et al. 1990a). The behaviour of CsiO4 is more complex, being shown by 133Cs and 127I NMR to involve 2 phase transitions, one at 243-300 K and the other at 420M40 K. The first phase transition shows temperature hysteresis effects, and the 2 phases can coexist over a finite temperature range. The 127I NMR spectra also suggest the samples are showing reverse piezomagnetism which is unusual in non-mag- netic crystals (Tarasov et al. 1990).

133Cs NMR measurements have been used to study the nature of phase transitions

occurring in several caesium compounds. CsSCN undergoes a first-order structural phase transition at 470 K from a low-temperature orthorhombic antiferroelectric phase to a high-temperature cubic paraelectric phase. Measurements of the ~33Cs spin-lattice relaxation time of this compound suggest that the high-temperature cubic phase is a rotationally-disordered plastic phase (Furukawa et al. 1991). A single crystal study of the relaxation rates of this compound in the vicinity of the phase transition indicates the onset of large amplitude reorientations of the thiocyanate groups, with no evidence of orthorhombic microdomains above the transition temperature (Blinc et al. 1995a), by

A CsCIO 4 B CsBrO4

200 N

100

rJ~ 0

I t I ' I

~.2 -. q

, I . . . . . . 0 100 200 300 400

T e m p e r a t u r e K

- 0.4

t20

11o N

~, 100

e~ "~ 90

80

ZQ " ~ " " "'--"-'~"

| , , I , , I , , J

100 160 220 280

T e m p e r a t u r e K

0.4 q

0.2

Figure 10.17. A. Temperature dependence of the 133Cs NMR parameters XQ and ~1 for polycrystalline CsC104. From Tarasov et al. (1991). B. Temperature dependence of the 133Cs NMR

parameters Re and "q for polycrystalline CsBrO4. Adapted from Tarasov et al. (1990a).

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668 Multinuclear Solid-State NMR of Inorganic Materials

contrast with KSCN and RbSCN. 133Cs NMR has also been used to study the phase transitions occurring in CsSnC13 and CsPbBr3 (Sharma et al. 1991).

Single-crystal 133Cs NMR studies have been reported of CsCd(SCN)3 allowing the relative orientations of the EFG tensors to be determined (Kroeker et al. 1997), and of ferroelastic CsPbC13 in which the NMR measurements revealed the presence of a twinned crystal structure (Lim and Jeong 1999a). The temperature dependences of the 133Cs quadrupolar parameters have also been determined for single crystal LiCsSO4

(Lim et al. 1999) and single crystal CsMnC13 (Lim et al. 1997b). A 133Cs NMR study of CsTcO4 has revealed the presence of 2 crystallographically

inequivalent caesium sites of which the relative populations vary with temperature. The results provide information about changes in the crystal field potential in the vicinity of the cations accompanying a first-order phase transition from an orthorhombic to a tetrago- nal form at 389 K (Tarasov et al. 1992).

An incommensurate phase existing in single-crystal Cs2HgBr4 over a narrow tem- perature range has been studied by 133Cs NMR. Below the onset temperature of the incommensurate phase the compound occurs as a paraelectric phase, changing to an antiferroelectric phase at higher temperatures. Changes in the 133Cs resonance inten- sity and lineshape at 243 K provide evidence of the existence of the incommensurate phase, and unequivocally indicate a soliton lattice in this phase (Boguslavskii et al.

1990). 133Cs NMR has been used to investigate an unusual phase transition which occurs

in the compound CsCuC13 at 4.2 K under the influence of a magnetic field applied par- allel to the c-axis. At a critical value of the magnetic field (11.19 T) the 133Cs reso- nance abruptly disappears, with no hysteresis in the sweep direction of the magnetic field. Analysis of the spectral lineshapes above and below this critical field value suggests that the anomalous change in the magnetisation occurs as a result of the quantum spin fluctuations of a one-dimensional ferromagnet with s - 1 / 2 (Chiba et al.

1993). The compound CsOH.H20 exists as a hexagonal [3-phase at room temperature,

undergoing a slight modification to a hexagonal a-phase above 340 K. Below 232 K the Cs and O atoms of the hexagonal [3-phase undergo a further small change resulting in the structure becoming monoclinic. A ~33Cs NMR study of "pseudosingle crystal" CsOH.H20 shows at room temperature all 7 expected energy level transitions, from which the XQ value are derived (Amm and Segel 1986). The 133Cs XQ values show an almost linear decrease with increasing temperature throughout the hexagonal [3 to hexagonal c~ transition (Figure 10.18), but a discontinuity of about 13 kHz ocurs at the hexagonal [3 to monoclinic transition at 236 K, indicating that the monoclinic phase has a longer longitudinal relaxation time than the hexagonal [3-phase. The 133Cs NMR results, complemented by ~H NMR, indicate that the hexagonal [3-phase is affected by an additional relaxation mechanism such as atomic diffusion, but the

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NMR of Other Quadrupolar Nuclei 669

N

rj~

160

120

80

40

monoclinic

~ o

~176 1

hexagonal h e x a g o n a l

(a)

! I I I

2 0 0 3 0 0 4 0 0 5 0 0

Temperature K

Figure 10.18. Change in the 133Cs quadrupole coupling constant XQ with temperature for pseudosingle crystal CsOH.H20. Note the 13 kHz discontinuity at the monoclinic to hexagonal

13 transition at 236 K. Adapted from Amm and Segel (1986).

change in ~33Cs XQ, which continues down to 180~ must involve a different mecha- nism (Amm and Segel 1986).

10.8.3 133Cs NMR of minerals and zeolites 133Cs and 295i MAS NMR has been used to study 3 caesium compounds with the struc- ture of leucite (Kohn et al. 1994). The 133Cs NMR spectra of Cs2CdSisO12, Cs2ZnSisO12 and Cs2MgSisO12 all show 2 narrow resonances of approximately equal area, consistent with the expected occurrence of 2 alkali sites in leucite structures with 6 tetrahedral T-sites. This result for Cs2ZnSisO12 is not consistent with the proposed structure which predicts 3 Cs sites with relative occupancies of 2:1:1, suggesting a need to reassess the structural space group in the light of the NMR data. The ~33Cs shifts are influenced by the framework cation, becoming more negative from Cd to Zn to Mg (Table 10.7) (Kohn et al. 1994).

Barium hollandite, Bal.4(A1,Ti)2.28Ti6016, is an important component of Synroc, a synthetic material developed for the immobilisation of high-level waste from nuclear reactor fuel. The hollandite component of Synroc takes up alkali metal ions such as radio- active Cs + by substitution for Ba 2+ in the structural channels. This uptake has been studied by 133Cs MAS NMR which shows a single resonance at 211 ppm from Cs in the channel sites in the absence of paramagnetic ions (Hartman et al. 1998). Replacement of A13+ by Ti 3+ in the channel walls causes the 133Cs NMR peak to broaden and shift to

640 ppm, and also provides a sensitive means of monitoring the formation of water-sol- uble CsA1TiO4 which, if present, would compromise the aqueous durability of Synroc.

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670 Multinuclear Solid-State NMR of lnorganic Materials

Cation adsorption onto phyllosilicate minerals is an important process with practical consequences for soil/water systems, sediments, natural hydrothermal processes, meta- morphic environments and waste disposal sites. The adsorption of Cs + by a number of clay minerals has been studied by 133Cs NMR which provides information about the number and nature of the adsorption sites, and the hydration state of the cation. 133Cs MAS NMR at various temperatures shows that adsorption of Cs on the clay mineral hec- torite, (Mg,Li,A1)3Si4Olo(OH)2.Cs+o.33, occurs in several distinctly different chemical sites between which motional averaging occurs at about - 40~ if interlayer water is present (Weiss et al. 1990). Below about - 60~ motional averaging of the adsorbed Cs is sufficiently slow for 2 Cs resonances to be resolved, 1, at about - 30 ppm, arising from Cs relatively tightly bound to the basal oxygens, the other, at about - 8 to + 30 ppm, arising from Cs in a region of compositional gradient (called the Gouy dif- fuse layer). After dehydration of the hectorite at 500~ the NMR spectra indicate that the adsorbed Cs remains in the interlayer in 2 new sites giving rise to 133Cs resonances at about 30 and - 120 ppm. The latter peak corresponds to more highly coordinated Cs (CN --~ 12) located in the hexagonal holes formed by oxygen atoms on both sides of the interlayer, and the former peak corresponds to less highly coordinated Cs (CN ---9) associated with the hexagonal hole on only one side of the interlayer and interacting with fewer oxygen atoms on the opposite side (Weiss et al. 1990).

~33Cs NMR has been used to study the hydration state of the cation in Cs-exchanged vermiculite, a swelling mica mineral with a typical formula (Mg,Ti,Fe,A1)3 (Si,A1)4Olo(OH)2X2+o.45 (Laperche et al. 1990). The results indicate that the 133Cs isotropic chemical shift is directly related to the hydration state of the mineral and depends on the configuration of the oxygens from the lattice and water ligands and the back-donation from the ligand to the cation. The large • value found for 133Cs in the vermiculite interlayer (about 6.7 MHz) results from an appreciable degree of stacking disorder due to the large radius of Cs + which prevents its engagement with the pseu- dohexagonal lattice oxygen network. The corresponding value of ~1 is close to unity (Laperche et al. 1990).

133Cs MAS NMR has been used to examine the structural sites occupied by Cs adsorbed on a variety of phyllosilicate minerals with a view to determining possible relationships between the 133Cs chemical shift and the chemical and structural param- eters of the clays (Weiss et al. 1990a). Significant differences are found between the 133Cs NMR spectra of samples in the form of an aqueous slurry and those fully dehy- drated by heating at 450~ consistent with an increase in the direct bonding of the exchanged Cs + to the basal oxygen atoms with increasing dehydration. For the hydrated slurry samples, a reasonable correlation was found between the ~33Cs chemical shift and the ratio of tetrahedral A1 to total tetrahedral atoms of the clay mineral, with the data for dioctahedral and trioctahedral minerals falling on different lines (Figure 10.19A). The 133Cs peak becomes less shielded as the content of tetrahedral A1

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NMR of Other Quadrupolar Nuclei 671

increases, since although motional averaging occurs in these fully hydrated samples, the position of the Cs resonance is the weighted average of the peaks from the various hydrated sites. Only very poor correlations were found with the degree of tetrahedral distortion and with the total layer charge of the hydrated clay minerals. The 133Cs NMR

spectra of the fully dehydrated samples contain 2 resonances, each of which shows a reasonable correlation with the degree of tetrahedral A1 substitution (Figure 10.19B),

but with no separate trends apparent for dioctahedral and trioctahedral minerals. Some- what similar correlations were also found with the total layer charge of the fully dehy- drated minerals (Weiss et al. 1990a).

Calcium silicate hydrates are nanocrystalline porous materials of variable composition and poor crystallinity analogous to the compounds occurring in hydrated cements. The behaviour of these materials is of practical interest in determining the possible performance and long-term durability of storage facilities for nuclear waste and other hazardous substances. In a study aimed at improving the understanding of the surface chemistry of calcium silicate hydrate compounds, their interaction with CsC1 and NaC1 has been studied by 133Cs and 23Na NMR (Viallis et al. 1999). The NMR results

indicate that both Cs and Na have an affinity for the calcium silicate hydrate surface, on which they are located in a diffuse ion swarm. Freeze-drying changes the environ- ment of the adsorbed cations, reflected in the 133Cs chemical shift (200-250 ppm)

A B

20

10

r ~

r ~

-10

-20

[ ]

dioctahedral ~

trioctahedral

0 0.06 0.12

AI(IV)/(AI (W) + Si)

{ 0.18

4 o ~ ~ ~ dioctahedral + t trioctahedraltrioctahedral

0

.40 ~-~

"~ J ~ dioctahedral + - 1 2 0 ~ - - -- ~ trioctahedral

- 1

i [ , - t

0 0.12 0.24

Al~ 0v) + Si)

Figure 11).19. A. Relationship between the 8.45 T 133Cs MAS NMR room temperature chemical shifts of fully hydrated Cs-exchanged clay minerals and their degree of tetrahedral A1 substitution. Open squares denote the dioctahedral minerals, open circles denote the trioctahedral minerals. Note that due to motional averaging in these samples, only one caesium resonance is observed. B. The

same relationship for samples fully dehydrated at 450~ The 2 lines correspond to the 2 133Cs resonances observed in these samples. Note the similar behaviour of the dioctahedral and trioctahedral

minerals when dehydrated. From Weiss et al. (1990a) by permission of the Mineralogical Society of America.

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672 Multinuclear Solid-State NMR of Inorganic Materials

arising from inner-sphere surface complexes formed by interaction of the dehydrated

Cs cations with the oxygen atoms of the bridging Si units. The Cs involved in these

inner-sphere complexes occurs in 2 distinct environments, with and without chloride in the coordination sphere (Viallis et al. 1999).

~33Cs NMR has been used to monitor the dehydration of the Cs-exchanged zeolite mordenite (Chu et al. 1987). Both the static and MAS 133Cs NMR spectra of the fully

hydrated material show a single resonance at - 64 ppm arising from motional averag-

ing of the fully hydrated Cs + (Figure 10.20). Progressive dehydration results in the

migration of the Cs into zeolite lattice sites characterised by 133Cs resonances resolved

by MAS as peaks at - 157 and - 2 4 ppm (Figure 10.20B). The more intense and broader resonance can be fitted by 2 peaks, at - 157 and - 186 ppm with XQ = 3.1

MHz and ~1 = 0.6. The quadrupolar fitting parameters of the - 24 ppm resonance are very similar. The 2 Cs sites with similar chemical shifts have been identified as being

near the centre of an 8-membered oxygen ring in the mordenite structure, with the other site located off-centre of a 6-membered oxygen ring (Chu et al. 1987).

Binary caesium-lanthanum oxides supported on the mesoporous molecular

sieve MCM-41 have potential catalytic applications for base-catalysed reactions. A 133Cs MAS NMR study of this system revealed shorter Cs-O bond lengths in the

A Static

<___

<._..

200 -200 -600

fully anhydrous

1' increasing

dehydration

fully hydrated

B MAS

200 -200 -600

133Cs shift (ppm) w.r.t. CsCI soln.

Figure 10.20. A. 133Cs static and B. MAS NMR spectra of caesium mordenite in various stages of dehydration. Asterisks denote spinning side bands in the MAS NMR spectra. From Chu et al.

(1987), by permission of the American Chemical Society.

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NMR of Other Quadrupolar Nuclei 673

MCM--41-supported system by comparison with the unsupported mixed oxides, while a small difference observed in the 133Cs chemical shifts of the hydrated and dehydrated materials has been taken to indicate a weak interaction between water and the Cs + (Kloetstra et al. 1997).

Changes with temperature have been recorded in the ~33Cs NMR spectrum of fully hydrated zeolite A containing a mixture of Cs, Li and Na as the exchange cations (Ahn and Iton 1991). Below 268 K the single Lorentzian 133Cs peak is resolved into 3 resonances corresponding to zeolite sites associated with the single 8-membered ring, the single 6-membered ring and an a-cage site near the 4-membered ring. Simulation of the experimental lineshapes has provided information about the chemical exchange kinetics between the 3 sites, indicating that the rate-defining step is the exchange of Cs + between the 6 and 8-membered ring sites (Ahn and Iton 1991).

10.8.4 t33Cs NMR of fullerides, superionic conductors and semiconductors The sites occupied by Cs in the mixed alkali fullerides CsRb2C6o and Cs2RbC6o have been identified by 133Cs NMR (Maniwa et al. 1992). The 133Cs NMR spectrum of CsRb2C6o (Figure 10.21A) shows a single resonance at - 370 ppm with respect to aqueous CsC1, indicating that the caesium in this compound is located solely in octa- hedral sites. By contrast, the 133Cs NMR spectrum of Cs2RbC6o (Figure 10.21A) contains both this octahedral resonance and a tetrahedral resonance at - 120 ppm. The chemical shifts of both the octahedral and tetrahedral sites show a very small but similar temperature dependence (Figure 10.21B). The room-temperature spin-lattice relaxation time of the octahedral site was found to be longer than that of the tetrahedral site, suggesting that the latter experiences a larger electric field gradient consistent with its smaller cavity size and lower symmetry (Maniwa et al. 1992).

CsHSO4 is a proton superionic conductor displaying a liquid-like proton self-diffusion constant above 417 K. The room-temperature monoclinic structure undergoes recon- structive first-order phase transitions at 318 K and 417 K. The nature of the transla- tional disorder in the superionic plastic phase formed above 417 K has been investigated by a 133Cs NMR single crystal study of the deuterated compound CsDSO4 (Dolin~ek et al. 1986). The 133Cs NMR spectrum of the room-temperature phase sug- gests the presence of 2 physically non-equivalent (but chemically equivalent) Cs sites. The formation of the superionic phase is accompanied by a decrease in the ~33Cs quadrupole coupling which, however, remains non-zero for both ~33Cs and 2H, indi- cating that the translational diffusion between these 2 nuclei involves well-defined lat- tice sites and is not completely random (Dolin~ek et al. 1986).

The alkali antimonide compounds such as Cs3Sb are of technical importance as semiconductors with high photoelectric quantum efficiency in the visible region. The room-temperature ~33Cs chemical shift of Cs3Sb has been determined to be 620 ppm

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674 Multinuclear Solid-State NMR of Inorganic Materials

A

400 . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . .

0

~ ~,,,lq

C ~ ~ 2"r") ~'~ -4oo

. , i , , i , i I , L

300 -300 -900 133Cs shift (ppm) w.r.t. CsCl soln.

Cs2RbC6o --~ ",a

|

C s R b 2 C 6 o

tetrahedral site

1

o f oetahedral site

-800 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 100 200 300

Temperature K

Figure 10.21. A. 1 3 3 C s NMR spectra of the fullerides Cs2RbC60 and CsRb2C60. The resonance at - 370 ppm arises from Cs in the octahedral site and that at about - 120 ppm is from Cs in the

tetrahedral site. B. Temperature dependence of the octahedral 133Cs shifts in CsRb2C60 and Cs2RbC6o and of the tetrahedral shift in Cs2RbC6o, from Maniwa et al. (1992), by permission

of Elsevier Science.

(Dupree et al. 1982), indicating its much higher p-character than CsI (275 ppm) and CsAu (375 ppm). Furthermore, this shift decreases markedly with temperature up to 500~ leading to the conclusion that a previously proposed ionic structural model is incorrect (Dupree et al. 1986).

10.9. 139La NMR

139La is a spin I - 7/2 nucleus with a 99.9% natural abundance and good NMR sensi-

tivity, but its utility for solid-state NMR studies is limited by its quadrupole broaden- ing which is greater by a factor of about 30 than for 27A1 in sites with the same structural distortion and in the same magnetic field. The chemical shifts of 139La are normally quoted with respect to aqueous LaC13 solution.

The broad 139La NMR spectrum of the single La site in polycrystalline La203

(Figure 10.22A) has been determined by the frequency-swept spin-echo method (Bastow 1994), yielding an isotropic chemical shift of 424.8 ppm and XQ = 58.52 MHz. The lanthanum site in this compound has axial symmetry, making x I = 0.

The frequency-swept technique has also been used to determine the 139La NMR

spectra of the lanthanum perovskite compounds LaCrO3, LaMnO3 and LaCoO3 (Bastow 1994). LaCrO3 has an orthorhombic structure with a 139La NMR spectrum

showing a well-defined second-order quadrupolar lineshape (Figure 10.22B). By con- trast, the 139La NMR spectrum of orthorhombic LaMnO3 (Figure 10.22C) shows an

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NMR of Other Quadrupolar Nuclei 675

A

o

o

o o

o

o o

LazO3 C

o

o

o

o

o

o o o o

o oo%r ~ o o o o o : 15

%

0 .4 0 -0 .4

MHz D

LaCrO 3 g g

o 0 0 o o ~ 1 7 6

o o oOoo o

o o o

o o

o

LaMnO3

o O ~ 1 7 6

o o

o o

o o

o

o

o

5 MHz

LaCo03 o

o

%

o

o

o

c ~ o O~o 0% 0 o ooO o ~ o o

o o oOoO O o o o o o o ooo ooo o o oO 0%0

. . . . . ~ ,

0.5 0 -0 .5 1 .0 0

MHz MHz

Figure 10.22. A selection of 139La NMR spectra of La203 and perovskite-type La oxide compounds, determined by the frequency-swept spin-echo technique. From Bastow (1994) by

permission of the copyright owner.

approximately symmetric lineshape with no visible quadrupolar structure. The shift of this compound (7 MHz) indicates a transferred hyperfine field from the Mn to the La which apparently swamps the quadrupolar contribution to the linewidth (estimated to be no more than 0.75 MHz) (Bastow 1994). The 139La NMR spectrum of LaCoO3 (Figure 10.22D) has sharply-defined quadrupolar structure arising from its smaller value of XQ compared with the other compounds of this type. The spectrum is suffi- ciently narrow for the singularities of the (1/2, 3/2) and ( - 3/2, - ~/2) satellite transitions to be clearly visible. 139La NMR has also been used to study the low-spin to high-spin transition in LaCoO3 (Itoh and Natori 1995).

The lanthanum environment in LaA103 is a symmetric LaO12 unit, giving rise to a static 139La NMR resonance sufficiently narrow to produce a recognisable quadrupo- lar lineshape without the use of an echo pulse sequence which is narrowed further by MAS (Dupree et al. 1989). The highly symmetric La site in LaB6 has also allowed the 139La NMR spectrum of this compound to be obtained readily (Lutz and Oehler 1980).

The 139La NMR interaction parameters of some lanthanum-containing compounds are collected in Table 10.8.

Doping the lanthanum perovskites with a divalent metal cation such as Sr 2 + produces

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676 Multinuclear Solid-State NMR of lnorganic Materials

Table 10.8. 139La NMR interaction parameters of lanthanum compounds.

Compound 8iso(ppm)* XQ (MHz) ~1 Reference

La203 424.8 58.52 0 Bastow (1994) LaCrO3 442.5 48 0.15 Bastow (1994) LaCoO3 4230 23.8 0 Bastow (1994) LaAIO3 375 6 0 Dupree et al. 1989

LaAll ~Ols 46** ND ND MacKenzie et al. 1999 Lao.ssSro. 15CrO3 442 33 0 Bastow (1994) La(NO3)3.6H20 - 100 21.5 0.85 Thompson & Oldfield (1987)

La acetate - 30 11 0.65 Thompson & Oldfield (1987) LaB6 - 128 ND ND Lutz & Oehler (1980)

* chemical shift with respect to aqueous LaCI3 solution

** central position of a broad featureless resonance

compounds with interesting magnetic and electronic properties, some of which may find useful applications as high-temperature fuel cell electrodes. The frequency-swept 139La

NMR spectrum of Lao.85Sr0.15CrO3 consists of a featureless peak in which the second- order quadrupolar lineshape is blurred by disorder arising from Sr substitution. The shift of the peak is similar to that of LaCrO3 but the mean quadrupole interaction is lower than in LaCrO3, having been driven by the presence of the Sr to assume a structure closer to a cubic perovskite with La in a more regular octahedral site (Bastow 1994). The compound Lao.8Sro.2MnO3 is ferromagnetic at room temperature and also exhibits metallic conduction properties. As in LaMnO3, the 139La NMR lineshape is broad and featureless but skewed towards the high frequency side. The shift of this peak indicates a transferred hyperfine field from the Mn to the La, with the broad distribution of mag- netic hyperfine fields swamping the quadrupolar contribution to the linewidth (Bastow

1994). 139La NMR has been used in combination with 27A1 MAS NMR to study the for-

mation of crystalline LaAl11018 by heat-treatment of a precursor gel (MacKenzie et al.

1999). The spectra, acquired using a Hahn spin-echo pulse sequence, are very broad and featureless (Figure 10.23A), but as the solvent and by-products are progressively removed from the gel samples by heating at increasingly high temperatures, the centre-of-gravity (cog) of the 139La resonance progressively moves to less-shielded

values (Figure 10.23B). On crystallisation of LaAl11018 at about 1000~ the peak cog abruptly becomes narrower and adopts an even more deshielded value, gradually settling down to its final value as the La moves into its lattice position in the mirror plane of the hexaluminate. Although La occurs in a 12-coordinated oxygen polyhedron in both LaA103 and LaAl110~8 the 139La NMR spectrum of the latter is much broader due to the lower symmetry of its La site in which 6 of the 12 coordinating oxygens are

located significantly further away (MacKenzie et al. 1999). Lanthanum-exchanged zeolite-Y is an important catalyst used for fluid cracking in

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NMR of Other Quadrupolar Nuclei 677

A

nab

250-

0

100 -250

1200oc 6 ~., -500

~. -750

i i i ! i ,

4000 0 -4000

139La shift (ppm) w.r.t. LaCI 3 soln.

o ~

crystallisation

~ o

/

~ , ! . !

500 1000

Temperature (~ 1500

Figure 10.23. A. 139La NMR spectra of LaAlllOlo gel precursor during the thermal evolution of the crystalline hexaluminate phase. B. Change in the 139La NMR peak position of LaAlllOls gel

during heating. Note the discontinuity in the peak position at the point of crystallisation. From MacKenzie et al. (1999).

the petroleum industry. The presence of lanthanum in zeolite-Y catalysts is thought to significantly improve their thermal stability due to the presence of oxygen-bridged La polynuclear cations in the sodalite cages. The static 139La NMR spectra of calcined La-exchanged zeolite-Y contain 2 signals (Herreros et al. 1992); a broad underlying resonance is ascribed to La ions which have migrated into the small cages in the struc- ture during calcination while a superimposed sharp symmetric signal at - 34 ppm with X Q - - 8.2 MHz is attributed to La cations located in the supercages. 139La NMR has also been used to monitor the migration of lanthanum cations from the large cavities to the SI' position in the sodalite cages in a study of La-exchanged zeolite sodium-Y. The lanthanum migration was found to cause the Si-O-T and A1-O-T angles to become strained, a result confirmed by 27A1 and 29Si MAS NMR (Hunger et al. 1995).

Lanthanum compounds, particularly the cuprates and related phases, show interest- ing and potentially useful electronic properties ranging from superconductivity (as in the lanthanum strontium and lanthanum barium cuprates) to metallic behaviour (as in lanthanum cuprate). 139La NMR has been used to elucidate structural details of the metallic conductor LazCuO4+~ (Hammel et al. 1993). The electric field gradient (EFG) at the lanthanum site in a single crystal sample has provided information about the strongly temperature-dependent distribution of lateral displacements of the oxygens forming the apices of the oxygen octahedra in this structure. These effects

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678 Multinuclear Solid-State NMR of lnorganic Materials

are considered to be an intrinsic response of the structure to hole doping, and are remarkable for the marked influence of a rather low concentration of holes (Hammel et al. 1993). 139La NMR has also shed light on the structural changes occurring in this compound below 220 K in which the oxygen octahedra develop a significant tilt upon the appearance of the highly disordered low-temperature structure (Hammel et al.

1991). ~39La NMR has been used to examine the possibility that substitution of Sr for La in LaNiO4 can bring about a change from an antiferromagnetic state to a metallic state in La2-xSrxNiO4+~ where x --~ 1 (Furukawa and Wada 1992). Strontium substitution of the antiferromagnetic phase produces a monotonic decrease in the internal magnetic field at the La sites up to x --~ 1, whereupon a transition to a non-magnetic phase occurs. The 139La Knight shift and spin-lattice relaxation behaviour of this phase is typical of a normal metallic state, confirming the nature of the phase transition (Furukawa and Wada 1992).

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