Journal of Non-Crystalline Solids 94 (1987) 101-121 North-Holland. Amsterdam

101

SHORT RANGE ORDER IN THE NETWORK OF THE BOROPHOSPHATE GLASSES: A “P NMR-MAS (MAGIC ANGLE SPINNING) STUDY

Marco VILLA Dipartimento di Fisica “A. Volta” e CISM-GSNM, Via Bassi 6. 27100 Pavia. Italy

Mauro SCAGLIOTII ’ Laboraroire de Physique des Solides, Unit& AssociPe au CNRS, CJniversire P. et M. Curie, UA 154, 4, pl. Jussieu, 75231 Paris Cedex 05, France

Gaetano CHIODELLI Centro di Studio per la Termodinamica er Eletrrochimica dei Sisremi Salini Fusi e Solidi del C.N.R.. c/o Dipartimento di Chimica Fisica. Viale Taramelli 16, 27100 Pavia, Italy

Received 22 December 1986 Revised manuscript received 9 July 1987

We analyze the “P NMR-MAS (Magic Angle Spinning) spectra in a number of borophosphate glasses. These spectra consist of several, relatively broad, Gaussian lines which are due to different types of PO,, tetrahedra. In addition to the phosphate units occurring in the M,O: P,Os systems (i.e., branching, middle, end, and monomeric units) structural units typical of borophosphates have been identified in the MAS spectra: the BPO, unit, two types of boron-bonded middle units (MB1 and MB2). and a boron-bonded end unit (EB). The oxygens introduced with the metal oxide M,O are preferentially taken by the phosphate units. It is also shown that, in contrast with the behavior of borates and phosphates, substantial changes are induced into the borophosphate glass network by addition of a doping salt (AgI or LiCl). The doping salt appears to favor the transfer of negative charge from the borate to the phosphate units. In the conclusions, we discuss the connections of the “P NMR-MAS spectra with optical basicity data and “B NMR findings.

1. Introduction

Nuclear magnetic resonance is a technique suited to investigate the short range order in amorphous solids since it provides information about coordina- tion and bonding of the resonating species. For example, the resonance of boron nuclei (“B and iiB) in the borate glasses is affected by the electric field gradients at the nuclear position, which are mainly determined by the arrange- ment of the B-O bonds [l]. Although less exploited, the resonance of 27A1 provides, for the aluminate glasses, a similar type of information [2]. More

’ Present address: Cise - Tecnologie Innovative, Via Reggio Emilia 39, 20090 Segrate (Mi), Italy.

0022-3093/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

102 M. Villa er al. / -“P NMR-MAS in borophosphate glasses

recently, it has been shown that *%i and 3’P NMR spectra in solid silicates [3] and phosphates [4], obtained while spinning the sample at the magic angle [MAS] reveal the presence of SiO, and PO, tetrahedra with different coordina- tions. In a NMR-MAS experiment the sample is rotated in order to average out, or to reduce, the angular-dependent part of the local interaction.

While information about electronic site symmetry and neighbouring mag- netic dipoles is lost, the NMR-MAS technique gives spectra which are much easier to interpret than those obtained with stationary samples. Furthermore, a correlative analysis of stationary and MAS spectra often allows full determina- tion of the chemical shift tensor. With the help of high resolution solid-state techniques, 31P NMR has eme g r ed as a powerful and, by now, established technique for structural investigations of crystalline and amorphous phos- phates, i.e., substances where phosphorus is tetrahedrally coordinated to four oxygens [5]. Several authors [6-91 have obtained the 31P chemical shift tensors of a number of phosphate units (see below). While the isotropic chemical shift is primarily affected by the number of non-bridging oxygens (NBOs) bonded to a phosphorus, it has also been shown that changes in type and coordination of the cation have a noticeable effect upon the 3’P MAS spectra [lO,ll]. In this paper, we will exploit the sensitivity of the 3’P MAS technique to detect changes in bonding and coordination of the PO, units. We will discuss the structure of glasses with composition MX : M,O : B,O, : PZO, (M = Li, Ag; X = Cl, I). A companion Raman investigation of these borophosphates has been presented before (ref. [12], paper I in the following).

Our main purpose is to understand how the modifier oxide M,O is distributed among borate and phosphate units. We will anticipate our main results by listing the units that we have identified in the 31P NMR-MAS spectra: - “BPO,” unit, with four oxygens briding towards B-atoms, and a nominal

+ 1 charge. This unit is found in crystalline BPO,, which is made by corner-sharing PO: and BOY tetrahedra arranged in a silica-like structure.

- Branching unit, with three oxygens briding towards P-atoms and one doubly bonded oxygen, designated as BP.

- Middle units, M+POi, with two briding oxygens and the double P = 0 bound resonating among the other oxygens. In addition to the middle unit bonded to two P-atoms, indicated with MP, we have tentatively assigned two well-defined spectral features to a middle unit bonded to one boron and one phosphorus (MBl), and a middle unit bonded to two borons (MB2).

- End units, 2M+POi2, with one briding oxygen. This oxygen may bridge with another phosphorus atom (EP unit), or with a boron atom (EB).

- Monomeric unit 3M+POy3, indicated by MO. Symbols and structural formulas of these units are summarized in table 1, where the double P = 0 bond has been represented as localized, while it actually resonates among the NBOs of the unit. Identification of MBl, MB2 and EB units has been possible only with the AgC-based borophosphates, and

M. Villa e! ul. / “P NMR-MAS in borophosphate glasses 103

Table 1 Phosphate units of the borophosphates

Name

BPO,

Symbol

BPO,

Charge

+1

#NBOs

0

Structure

B-O O-B

B-O=p’,-B

0 O-P Branching BP

Middle MP -1 2

\\P’ P-O’ ‘O-P

0 O--M+ \\P’

P-O’ “0-p

MB1 -1 2 0 O--M+

%\p’ P-O’ ‘O-B

MB2 -1 2 0 O--M+ \\P’

B-O’ \‘O-B

0 O--M+ End EP -2 3

EB -2 3

\\P’ P-O’ ‘O--M+

0 O--M+ \\P’

B-O’ -‘O--M+

Monomer MO -3 4 0 O--M+

\\P’ M+-0’ ‘O--M+

it has required computer-assisted decomposition of multicomponent 3’P MAS spectra of dozens of glasses, analysis of the stationary NMR spectra, and supporting Raman results (see paper I). On the other hand, spectral contribu- tions due to branching units bonded to B-atoms have not been identified. This may be due to an overlapping of these contributions with the BP and BPO, signals or, more likely, to the fact that formation of B-bonded branching units is energetically unfavored.

2. Experimental methods

The 3’P NMR spectra have been collected at room temperature on powders obtained by crushing bulky ingots (- 1 cm3) a few minutes before the beginning of the experiment. We used Nicolet pulse spectrometers with two Nalorac superconducting magnets, energized at 3.6 and 8.5 T, respectively. The home-built spinner, of the Andrew-Beams type, was made of Delrin. In the MAS experiments, the sidebands where identified by comparing spectra with different spinning rates, between 1500 and 5000 Hz. The spectrometers were calibrated daily by running a MAS spectrum of bis(diphenylophos- phino)-butane. All 31P spectra will be referenced to 85% H,PO,, which was

104 M. Villa er al. / “P NMR-MAS in borophosphate glasses

occasionally added to the powders to provide an internal reference. Care was taken to avoid saturation effects, which may unevently affect different por- tions of the spectrum, thus making meaningless a quantitative analysis of the peak intensities.

Most MAS spectra have been decomposed in several Gaussian lines, with the help of the software that comes with the Nicolet spectrometers. In practice, we approximately subtracted from a spectrum what appeared to be its main component, then the second largest component, and so on. Then, position, width and intensity of each component were iteratively refined by minimizing the difference between experimental and simulated spectra. The root-mean- square deviation between the experimental lineshape and the sum of its Gaussian components was, typically, 5%. Most often, the major causes of such a deviation are baseline distortions and inaccurate phasing of the experimental spectrum. In the following, we will represent the experimental spectra with full curves, and their Gaussian components with dotted curves, or with vertical bars having heights proportional to the intensities.

The overall intensity of the 31P NMR signal is proportional to the number of P-atoms of the sample, and, after proper calibration, MNR could have been used to determine the phosphorus content of our glasses. However, in this paper such an analytical application has not been attempted. In the following, we will only talk about the relative intensities of the Gaussian components of the MAS spectra and we will address the question of how these intensities are related to the weights of the corresponding units. In a first set of experiments, we added small amounts (< 1%) of crystalline BPO, to rotors filled with silver metaphosphate. This situation is very favorable, since the signals are well separated, and differ greatly in width. The NMR intensities of the BPO, signal agreed well with the intensities predicted from weight measurements, and the NMR determinations of BPO, were actually more accurate than those ob- tained by weighing the entire sample. We found experimentally that, by mixing ingredients with similar weights and with well separated spectral features (e.g. a lithium metaphosphate and a lithium oligophosphate), the NMR reproduces within - 1% the expected intensities. A less favorable situation occurs when two peaks can be distinguished, but their overlapping is substantial (see the case of the MP and MB1 units discussed in the next section). In few situations, where a spectral feature can be described by a small peak on the shoulder of a major peak, we can exchange the roles of major and minor peaks, without degrading the accuracy of the decomposition. In these cases, the choice of the “correct” intensities is an educated guess, made on the basis of the Raman results, the evolution of the spectral feature with composi- tion, and/or information from the stationary spectra. However, if we guessed right, the decomposition procedure yields the intensities with an uncertainty of a few percent. An even less favorable situation arises when the MAS spectra are nearly featureless, in which case a qualitative estimate of the weights of the different contributions can be made only if we know the spectral regions where these contributions should occur. When a comparison between the MAS

M. Villa et al. / mTrlP NMR-MAS in horophosphare glasses 105

spectral intensities and results of chemical analysis has been made (see the discussion of the sodium metaphosphate presented below) a reasonable agree- ment has been obtained. However, for the borophosphate glasses, support for our interpretation comes only from spectroscopic evidence. A procedure similar to that used by Duncan and Douglass [7] has been applied to the analysis of the stationary spectra. When the MAS spectrum had a single component the following steps were performed: - Rough average values for the chemical shift tensor principal components a,,

a,, 6, were evaluated from the peaks and shoulder of the stationary spectrum and from the MAS spectrum.

- Gaussian distributions for a,, a,, and 6,. subjected to the condition that the experimental distribution of 6, (i.e., the MAS spectrum) is obtained when the fluctuations of the principal components are uncorrelated.

- The theoretical spectrum was numerically calculated, convoluted with a Gaussian broadening function and compared with the stationary spectrum on the computer screen.

- The parameters of the simulation were iteratively changed until a fair agreement between theoretical and experimental spectra was obtained.

- The average anisotropic shift, the average asymmetry parameter and, as a check, the 6, distribution were then computed. When the MAS spectrum consisted of two peaks, these steps were preceded

by subtraction of the contribution due to one of the units. The subtraction was usually carried out by comparing stationary spectra of samples having differ- ent populations of the two units, as determined from the MAS data.

Powders of crystalline BPO, have been produced by baking overnight at 13OO’C a mixture of BzO, and H,NH,PO,. For details of preparation and characterization of the other samples we refer to another paper [12] and references therein. The compositions reported for the pure phosphate have been determined through the NMR-MAS spectra. We believe them to be correct to within a few percent. since the MAS spectra are rather simple, the peaks are well separated, and our assignments agree with the literature (6-8). For the boron-containing samples, we can give only the nominal composition, i.e. the composition one would obtain if all water and ammonia had evaporated from the batch mixture while the glass formers, the halide salt and the modifier (M,O) had been retained.

3. Pure phosphate glasses

The “P-MAS spectra of four silver phosphates glasses with compositions ranging from a nominal n = [P]/[M] = 2 (upper spectrum) to n = 0.75 (bottom spectrum) are represented in fig. 1. A major feature of these spectra is a peak near -20 ppm with a full width at half height (FWHH) of about 10 ppm, which is assigned to the middle units (MP). The ultraphosphate glasses (n > 1) have also a peak upfield, which is assigned to branching units (BP) and, in

I 1

I I

I I

20

O si

(;;m

) -4

0

Fig.

1. “P

M

AS

spec

tra

of the

silv

er

phos

phate

gla

sses

Ag

,O

. n P

,O,.

A n=

O.9

7

lJ&,

Li

i Li-M

P

n=0.

69

20

0 -2

0 -4

0 -6

0 # 3i(

PPm

)

Fig.

2. Th

e top

tra

ce

is the

“P

M

AS

spec

trum

of a

comm

ercia

l so

dium

“meta

phos

phate

” (se

e tex

t). Th

e oth

er

trace

s ar

e the

sp

ectra

of

Li ,O

: P

,O,

glass

es.

The

value

s of

n ha

ve

been

ob

taine

d fro

m the

M

AS

spec

tra.

M. Villa el al. / “P NMR-MAS in borophosphare glasses 107

silver phosphates, occurs at - - 36 ppm. The oligophosphate glasses (n < 1) have a peak downfield from the middle peak, which is due to end units (EP), and is substantially narrower than the BP and MP contributions (FWHH = 5.7 ppm). The values of n identifying the spectra in fig. 1 have been calculated from these intensities, and are systematically smaller than the nominal n-val- ues. This is certainly due to evaporation of phosphorus oxide from the melt and, maybe, to intake of moisture for n > 1. Notice that end and branching units do not coexist, and that position of the MP peak is somewhat dependent upon composition. It shifts - 5 ppm downfield, over a small composition range near the metaphosphate composition (n = 1).

The upper part of fig. 2 reports the spectrum of a commercial sodium “metaphosphate” powder (a Graham salt by Fisher), which consists of three Gaussian components. The main peak accounts for 80% of the total intensity, and occurs in the same spectral region where peaks of silver metaphosphate are found. The small (10% intensity) peak to the left (+ 1.4 ppm) is due to end units charge-compensated by sodium ions. These assignments agree with chemical shift data of sodium phosphates reported in the literature [6-81. The central component at - 5.4 ppm is intermediate between the - 6.9 ppm peak of the end units we found in hydrated HPO, powders (n = 0.72) and the peak at + 1.4 ppm of the Na+ coordinated end units (sodium pyrophosphate). For this reason, we tentatively assign this feature to end units coordinated to both Na+ and HC. If this is the case, the actual composition of the Fisher “metaphosphate” would be Na20-H20-(20)NaPO,, while according to the manufacturer the composition, as calculated from the phosphate chain length, is Na?O-(13)NaPO,. In other words, we find a - 30% excess (in mol) of the modifier, which may be due to water contamination. According to the NMR result, the water content of our sample is 0.8 wt%, compared with the 0.4 wt% of water found by Gray and Klein [13] in the dryest form of sodium metaphosphate they could obtain.

The next trace of fig. 2 shows the spectrum of a lithium ultraphosphate with a nominal [P]/[Li] ratio of 2, and its decomposition in Gaussian components. The major features of this spectrum are due, as expected, to branching units (- 41.7 ppm, with a 15.4 ppm FWHH) and middle units (- 28.3 ppm, with a 11.6 ppm width). The bump before the peak at - 28.3 ppm is due to a small (3.7%) component at - 23.8 ppm, which is assigned to middle units formed by hydration, being very close to the MP peak we observed in a hydrated metaphosphoric acid (- 22.3 ppm). It follows that the actual composition of this lithium ultraphosphate is (0.6)Li,O-(O.O37)H,O-PzO,, or n = 1.56. As with silver phosphate glasses, the resonance of the middle unit of the lithium phosphates shifts - 5 ppm in the paramagnetic direction when going from an ultraphosphate to an oligophosphate composition, and the EP signal is about half as wide as the BP and MP peaks (lower traces of fig. 2).

Our isotropic chemical shift data in phosphate glasses are collected in table 2. We hadded also the chemical shift of the monomeric unit, MO, as determined in a crystalline phase of silver orthophosphate (see below), and

108 M. Villa er al. / “P NMR-MAS in borophosphare glasses

Table 2 Isotropic chemical shifts in phosphate glasses

Composition Cation(s) unit

(ppm) 4 (ppm)

FWHH

n =1.56 Li (&H) n>l A8 BPO, (crystalline) n = 1.56 Li (&H) n = 1.56 Li (&H) nz1 Ag n<l Li n = 0.72 H n = 0.83 Na (&H) n-Cl A8 n = 0.72 H II<1 Li n = 0.83 Na (&H) n = 0.83 Na (&H) Li borophosphate glass Crystallized Ag,PO,

BP -42 15 BP -36 12 BPO, -30 3 Li-MP -28 12 H-MP -24 5 Ag-MP -22 12 Li-MP -22 12 H-MP -22 13 Na-MP -19 10 Ag-MP - 17 11 H-EP -7 10 Li-EP -6, -4 I NaH-EP -5 8 Na-EP 1 5 Li-MO 8 4 Ag-MO 28 (-1kHz)

that of crystalline BPO,. In the M,O : P,O, glasses, signals from different units occur in well-separated spectral regions, although the shift of middle and branching units depends somewhat upon composition. The shift appears to be primarily correlated with the state of oxidation of the phosphate unit, but the position of the BPO, signal is definitely out of line. Furthermore, the para- magnetic shift roughly correlates with the cationic radius of the metal ion, rather than with the electronegativity of M.

4. Borophosphate glasses

As stated before, in this section we will give only the nominal composition of each glass, i.e. the n and y = [B]/[B] + [P] values of the original batch. It is well known that the branching unit is relatively unstable and, for n > 1, PzO, evaporation and water intake occur, as we have noted for the pure phosphates. However, substitution with boron causes a rapid disappearance of branching units, and formation of more durable glasses. Differences between actual and nominal compositions are believed to be smaller than 10% for glasses with y > 0.2.

4.1. n>l

The bars at the top of fig. 3 summarize the results of the previous section for silver phosphates (see table 2). The spectra of fig. 3 refer to silver borophosphates with nominal composition n = 2; i.e., Ag,O-2[( y)B,O,-(1 -

“9 yv

p ‘\

MP+M

Bl +B

P4

K

Y=O.

2

. .,,

Y=

O.5

f-l

Fig.

3. M

AS

spec

tra

of Ag

,O

. n(

yB,O

, (1

-y)P,

O,)

with

n

= 2.

-30

z n -20

G -1

0

100

a?

p 50

: z 0

I I

1 I

I

0.0

0.2

0.4

0.6

0.8

1.0

Y

BP04

Fig.

4. Po

sition

(do

ts),

widt

h (ba

rs)

and

inten

sity

(trian

gles)

of the

“

midd

le”

line

in the

M

AS

spec

tra

in the

Ag

,O.2(

yB,O

,.(l

- y)P

,O,)

syste

m.

We

reca

ll tha

t the

ac

tual

comp

ositio

n of

the

samp

le wi

th

y =

0 co

rresp

onds

to

n =

1.21

(see

fig.

1).

5

110 M. Villa er al. / “P NMR-MAS in borophosphare glasses

y)P,O,]. When y < 0.6, a peak in the middle unit region (- -20 ppm) accounts for - 80% of the total intensity. The upper portion of fig. 4 gives, as a function of the boron fraction y, the center of the peak in the middle region (circles) and its width (bars), while the lower part gives the peak intensity. The changes with composition of the spectral feature in the middle region may be explained as follows, When we begin adding boron, BP units are converted into BPO, units (top trace of fig. 3) and a new unit is formed which is slightly more paramagnetic than the MP unit. All these signals are sufficiently broad and close to the MP peak that the primary effect of boron addition is a substantial broadening of the “middle” line (fig. 4, top). For boron contents higher than y = 0.3, the “middle” line shifts in the paramagnetic direction and narrows, due to the progressive conversion of BPO, and MP units into the a new unit (see fig. 4). This conversion is essentially completed at y = 0.6 and can be followed more clearly by examining the shape of the stationary (non-spinning) spectra (see fig. 5). Since for glasses with 0.2 <y -C 0.6 the “middle” line accounts for 80% of the signal, it follows that the changes of the stationary spectra are essentially due to formation of the new unit at the expense of MP units. The upper spectrum of fig. 5 is described by an anisotropy AS = - 155 ppm and an asymmetry parameter q = 0.5, which are values typical of middle units we found in the pure phosphates [7]. With increasing the boron content, the high field shoulder in the spectra of fig. 5 progressively disappears. While it may be tempting to suggest that the nearly symmetrical peak of the y = 0.6 glass is due to a distribution of isotropic chemical shifts, the fact that the stationary spectrum is nearly ten times broader than the corresponding MAS spectrum (see fig. 4) rules out this interpretation. It follows that the width of the stationary spectrum of the y = 0.6 glass is essentially due to the chemical shift anisotropy, and that the disappearance of the high field shoulder, observed in all the spectra of metaphosphate [7], is due to an asymmetry parameter close to 1. In fact, in a powder pattern due to chemical shift anisotropy, the singularity moves to- wards the center, and the shoulders decrease, when n -+ 1. By applying the simulation procedure outlined in the experimental section we found, for the glasses with y = 0.5 and y = 0.6, values of AS = - 100 ppm and n = 0.9. These are average values, which are believed to be representative of the anisotropy and asymmetry of the new unit. The position of this unit (= -15 ppm) suggests that it is of the middle type. Since this unit is created upon addition of boron, we tentatively identify it with the MB1 unit (see table 1).

Interpretation of the other peaks of fig. 3 is simpler. Formation of MB2 units is tentatively linked with the increase of a peak near 0 ppm, with increasing y from 0.5 to 0.8. Both position and width of this peak depend little upon composition, which is the behavior expected from a spectral feature due to a well-defined structural unit. We rule out that this peak is due to some type of end unit because the EP signal in pure silver phosphates is shifted - 7 ppm in the paramagnetic direction, and we expect the end signal to be further moved downfield by addition of boron. As a matter of fact, for y > 0.7 a peak

1 I

I I

I

IO

100

0 -1

00

-200

5 (p

pm)

Fig.

5. St

ation

ary

“P

NMR

spec

tra

of sil

ver

metap

hosp

hate

and

of sil

ver

boro

phos

phate

s wi

th

n =

2 an

d dif

feren

t y-v

alues

.

20

O c

.-20,

-4

0 -6

0

\ Y=

O.6

3i(P

Pm)

Fig.

6. M

AS

spec

tra

of sil

ver

boro

phos

phate

s wi

th

n =

3.

112 M. Villa er al. / -“P NMR-MAS rn borophosphare glasse.~

is observed which occurs at - 12 ppm, i.e., 5 ppm downfield relative to the EP signal. We assign this spectral feature, which has a well-defined position and width (FWHM - 3 ppm), to the EB units. We rule out the presence of a significant amount of EP units, because in other samples (see below) we found their presence signaled by a peak at - 7 ppm as in pure silver phosphates. We stress that we used a “blind” decompositions procedure, without assumptions about position and width of the components.

At y = 0.9, the X-ray analysis shows the presence of crystalline Ag,PO,. Therefore, the narrow peak in the paramagnetic side (28 ppm) is assigned to MO units in a crystalline phase. In the corresponding stationary spectrum (fig. 5. bottom), the MO units cause a 1 KHz wide peak, the width of which is field-independent. This means that the dipolar interaction, rather than a distribution of the isotropic chemical shifts, causes the broadening of this peak. A small fraction of crystalline Ag,PO, (- 3%) is observed also in the glass with y = 0.8, although no evidence for a crystalline orthophosphate is provided by X-rays in this sample.

Figure 6 illustrates the effects of the P-B substitution in the MAS spectra of another family of silver ultraphosphates (n = 3). In this figure, the intensity and position of the Gaussian components are represented with vertical bars, and the markers at the top summarize the preceding discussion. At low boron contents (from y = 0.2 to y = 0.6) a wide peak is observed, covering a spectral region typical of branching, BPO,, and middle (MP + MBl) units, and we have no way of quantitatively evaluating the fractions of these units. Analysis of the stationary spectrum for y = 0.7, n = 3 suggests that the main peak of the corresponding MAS spectrum (fig. 6) is essentially due to MBl. For y = 0.7, small components at the BPO, and MBZ positions can be isolated, and we may rule out the presence of branching units. However, it is likely that the BP units have disappeared at a much lower boron content since the Raman spectra of lithium borophosphates with n = 3 and y > 0.3 do not reveal the presence of branching units [12]. A further increase of boron brings about formation of MB2 and EB units, as with the n = 2 silver borophosphates, but no monomeric units are observed in the n = 3 series.

The “P MAS spectra of several lithium borophosphate glasses with n = 2 have been discussed in a previous paper (fig. 3 of ref. [ll]) and will not be reported here. When increasing the boron fraction, these spectra undergo changes similar to those observed in the silver borophosphate; i.e., the peak in the middle unit region broadens and then shifts in the paramagnetic direction. However, with these lithium glasses we are unable to resolve to contributions of MBl, MB2, and EB units, since their separation is smaller, and, possibly, their width larger, than in silver phosphates. The most interesting information obtained from these spectra is the evidence for a phase separation phenome- non near y = 0.6, which is confirmed by analysis of the Raman spectra [ll].

Si (P

Pm)

EP 1

M

3i(P

Pm)

Fig.

7. M

AS

spec

tra

of the

sil

ver

boro

phos

phate

s Ag

zO

yB,O

, (1

-

Y )P

*O,.

Fig.

8. M

AS

spec

tra

of the

lith

ium

boro

phos

phate

s Liz

0 .yB

,O,.

(1 -y)

P*O,

. z w

114 M. Villa er al. / -“P NMR-MAS in borophosphare glasses

4.2. n=l

The simple MAS spectrum of a silver metaphosphate (fig. 1) takes the complex shapes reported in fig. 7 when phosphorus is progressively substituted by boron. However, we can decompose these spectra into a few Gaussian components (bars in fig. 7), which are readily assigned to BPO,, MP, MBl, MB2, EP, EB and MO units with the help of our previous results. For y = 0.6, a sharp peak due to crystalline silver orthophosphate (clipped in fig. 7) is observed. As in the n = 2 silver borophosphates, the non-spinning spectra show that MP units are rapidly converted into MB1 units, when we begin substituting P,O, with B 0,. Notice that, in the y = 0.3 and y = 0.5 samples, two peaks are resolved in the end unit region, which approximately occur where the EP and EB contributions are expected.

The MAS spectra of several lithium borophosphates with n = 1 are repre- sented in fig. 8. Again, we cannot discriminate among the different types of middle (M) and end (E) units. An analogy with the silver phosphates would suggest that the main peak for y = 0.2 is predominantly due to MBl, while that for y = 0.4 is essentially due to MB2. On the other hand, the amount of lithium orthophosphate (MO units) is accurately given by the intensity of the peak at 9 ppm in the y = 0.6 spectrum of fig. 8.

4.3. n<l

Fig. 9 reports the spectra of two silver borophosphates with n = 0.75, and represents (top) the Gaussian components expected in the phosphate Ag,O . 0.75P,O, (see fig. 1). Again, the simultaneous presence of EB and EP units is evidenced by the decomposition procedure. Notice that MO units are formed within a vitreous phase, as revealed by an MO peak substantially broader than that of crystalline Ag,PO, (see figs. 3 and 7). With increasing the boron content, the fractions of monomer and end units increase. For y = 0.23, no evidence is found of phosphorus-coordinated middle units (MP).

Although with lesser detail, similar trends can be followed in the lithium borophosphate glasses with n = 0.66 (see fig. 10). Again, the bars at the top represent the spectral components of a pure phosphate with composition equal to the nominal n = 0.66. The fraction of the end units increases from 50%, when y = 0 and y = 0.1, to 73%, when y = 0.2. As with the silver oligophosphates, the most dramatic effect occurring when we begin adding boron is the conversion of MP units into B-bonded middle units (MB, in fig. 10). Within the limited composition range which allows glass formation no MO units are observed.

4.4. Effects of doping

We begin discussing the silver glasses, where the changes induced by the dopant (AgI) can be followed with greater detail in comparison with Li-con-

M. Villa er al. / “P NMR-MAS in borophosphare glasses 115

.” L

116 M. Villa er al. / “P NMR-MAS in horophosphare glasses

M. Villa et al. / -“P NMR-MA.9 in borophosphoie glasses 117

118 M. Villa et al. / “P NMR-MAS in borophosphare glasses

taining glasses. As in paper I we indicate with x the ratio [MX]/[M,O] + [MX]. The main phenomenon that occurs upon addition of AgI to the n = 2, y = 0.8 glass (see fig. 11) is the increase of monomeric and end units at the expense of middle units (see fig. 11). When we begin adding the dopant (x = 0.2) boron-bonded middle units (MB1 anb MB2) are transformed into EB and EP units while the MO signal acquires a width characteristic of a vitreous phase. It seems that the EP fraction, which is not detected in the undoped glass, increases at the expense of the MB2 fraction. Notice that also the intensity of the MO signal increases with doping, and that it progressively shifts in the diamagnetic direction.

Similar trends are observed when doping the silver borophosphate of composition n = 1, y = 0.6 (see fig. 12). Notice, in particular, that the mono- mer peak in the glass with x = 0.75 is found - 8 ppm upfield with respect to the position of crystalline Ag,PO,.

The NMR spectra of the doped lithium glasses with n = 2, y = 0.4 have already been discussed (fig. 5 of ref. [ll]). They display a broad, nearly featureless peak in the middle region, with a shoulder at the BPO, position. The intensity of the BPO, contribution decreases from 28 to 7% in going from x = 0.2 to x = 0.7 while the middle B peak shifts in the paramagnetic direc- tion. The latter fact is believed to reflect the transformation of MP units into MB1 and MB2. In the n = 1, y = 0.2 lithium glasses the doping causes a progressive increase of the end units at the expenses of the middle ones (see fig. 13)). Notice that, to the difference of the AgI-doped silver borophosphates, no monomeric units are formed. This finding can explain the different behavior of the glass transition temperature in silver and lithium glasses as a function of the dopant content (see paper I and references therein).

5. Discussion and conclusions

While different factors, such as P-O bond lengths and P-O-P bond angles, possibly contribute to the 31P isotropic chemical shift in the phosphates, a theoretical study [14] has indicated that variation of the paramagnetic contri- bution to the shielding tensor is dominant in determining 6,. Intuitively, we expect that the paramagnetic contribution is associated with the covalent character of the P-O bonds, and that this character increases with increasing the NBOs fraction and the electron-donor power of O*-. If this is the case, a quantitative correlation should exist between the optical basicity of the phos- phate oxygens and the 3’P paramagnetic shift. Experimentally, this fact is suggested by the optical basicities reported for sodium phosphates [15], where the electron-donor character of the oxygen increases monotonically with increasing the Na,O molar fraction, but makes an unexpected jump near the metaphosphate composition. This phenomenon is probably related to the - 5 ppm paramagnetic shift observed in the 3’P MAS signal due to the middle units when we go from an ultraphosphate to an oligophosphate composition

M. Villa er al. / “P NMR-MAS in borophosphare glasses 119

(see figs. 1, 2 and related comments). It would be interesting to see whether the optical basicity of a phosphate increases, as we expect, upon partial substitu- tion of P,O, with B,O,, and to compare the properties of optically active ions in silver phosphates and in alkali phosphates with equal M,O content.

Analysis of the Raman data [12] has already indicated that, for a constant Ml0 content, substitution of phosphorus with boron increases the average number of NBOs in the phosphate units. As long as the MAS spectra can provide an estimate for the weights P, of the different units, we may evaluate quantitatively this charge transfer by defining the negative charge associated, on average, with the phosphate units as

3

average charge of PO, = c iP,, i=-1

where i = -1 for BPO,, i = 0 for branching units, i = 1 for middle units (MP + MB), i = 2 for end units, and i = 3 for MO units. In fig. 14 we have represented with open circles the average charge of silver borophosphate glasses, with closed circles the charge in partially crystallized samples, and with crosses the phosphate charge in AgI-doped glasses. The large error bars of the phosphorus-rich samples with n = 2 and n = 3 reflect the uncertainty of estimating the contributions of middle units, branching, and BPO, units in the MAS spectra of figs. 3 and 6. The dashed curves in fig. 14 represent the behavior expected if all the modifier oxide was taken by the phosphate units until these units are all converted into PO:- monomers (maximum charge model for the phosphate). The dotted-dashed lines represent the y-dependence of the phosphate charge expected if borates and phosphates had the same affinity for the modifier oxide (equal partition model). Figure 14 shows that, for y < 0.5, all the available negative charge is taken by the phosphate units. For the glasses with n > 1 containing more boron the phosphorus (y > 0.5) the negative charge remains approximately the same with changing the boron fraction. However, the doping with AgI has the effect of bringing the average charge of the phosphate close to the values predicted by the maximum charge model (see fig. 14).

Our data are related with the findings of Beekenkamp and Hardeman [16], and those of Yun and Bray [17] who studied sodium and potassium borophosphates with “B NMR. These Authors call for the preferential forma- tion of a “BPO, network” (please note the quotes), in which BOY is charge-compensated by a PO,f unit. In particular, according to Yun and Bray, the modifier oxygens are equally divided between the “BPO,“s and a pure borate network when [B] > [PI, while the modifier is partitioned between the BPO, and the excess P,O, when [P] > [B]. Such a description is in partial agreement with our results, since it implies that the average charge of PO, follows the maximum charge model for y < 0.5, and stay constant when y -5 (see fig. 14). Furthermore, the fact that, in all systems studies, addition of a small B,O, fraction induces great changes into the 3’P MAS spectra, with

120 M. Villa et al. / “P NMR-MAS in borophosphare glasses

creation of boron-bonded phosphate species, certainly shows that formation of a borophosphate network is favored.

However, in contrast with Yun and Bray, we should rule out that this borophosphate network is made of the same type of phosphate unit we found in our X-ray controlled BPO, samples, and which consists of a PO, with the four oxygens briding towards BO; units. While we have found some evidence of BPO, units in glasses with low B,O, and M,O contents, the main effect of boron addition to a phosphate is the formation of MBl, MB2 and EB units, which cannot be confused with the PO,‘s found in crystalline BPO, (see table 2). Therefore, the reference of Yun and Bray to “formation of a BPO, network” should not be taken too literally.

The 31P data do not directly say whether the PO,s in the borophosphates are preferentially linked to BO, or BO, units. On the other hand, the results of Yun and Bray demonstrate that, when y < 0.5, the borons are essentially four-coordinated. However, it should not be concluded that the modifier oxygen is preferentially used to convert BO, into BO, units, since the 31P NMR spectra clearly show that middle-types and end-types of phosphate units are formed, which also requires that negative charges are transferred to the phosphate network. Therefore, the combined evidence from “B and “P NMR spectroscopies suggests the following picture. For y < 0.5, and in the absence of the modifier oxide, the BPO, structure exists; upon addition of M20, some P-(NBO)-M+ bonds replace the P-O-B motifs, but most of the borons remain four-coordinated. This implies that, as suggested by Beekenkamp on the basis of “B evidence [16], the four-coordinated borons found in the borophosphates are different from those found in the borate glasses. We suggest that such a borate unit is not negatively charged, or that the use of a formal charge model to describe the borophosphate should be replaced by a more sophisticated model of charge distribution among phosphate and borate units.

In boron-rich borophosphates, the average charge of the phosphates is approximately independent of y, in agreement with the model of Yun and Bray which prescribes a partition of the modifier between the modified “BPO,” network and the remaining borate units. According to these authors, the number of three-coordinated BO, units increases with increasing the M,O content and the boron content above y > 0.5 [17]. Correspondingly, we observe an increase of MB2 units at the expense of MBls. If these phenomena are related, it may be suggested that MB2 units are mostly connected with three-coordinated borons, while MB1 units are preferentially bonded to BO, units. While all this suggests that the spectral features assigned to MB1 and MB2 units are due to middle units in well-defined environments, we have to acknowledge that information from 3’P MAS and Raman spectroscopies provide an incomplete description of the borophosphate network.

This research has been supported by the international Affair Division and the project “Energetica II” of the Italian CNR. The NMR-MAS data have

M. Villa et al. / “P NMR-MAS in borophosphare glasses 121

been collected at the Colorado State University, Regional NMR Center, funded by the NSF (Grant CHE-820881).

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