structural characterization of sio2–csalo2 and sio2–rbalo2 glasses

Structural characterization of SiO2±CsAlO2 and SiO2±RbAlO2

glasses

Paul F. McMillan *, Andrzej Grzechnik 1, Harjot Chotalla

Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287 1604, USA

Received 7 July 1997; received in revised form 27 January 1998

Abstract

Glasses along the SiO2±CsAlO2 and SiO2±RbAlO2 joins were prepared by a sol±gel method and studied using 29Si

and 27Al nuclear magnetic resonance and by infrared and Raman spectroscopy. The 29Si and 27Al chemical shifts follow

the same trend with composition as found in previous studies, due to the formation of Q4(1Al) and Q4(2Al) species in

the glass, consistent with Raman spectroscopy. The Raman spectra indicate a larger proportion of 3-membered (Si, Al)-

containing rings than is present in other alkali aluminosilicate glasses with similar silica content. This e�ect occurs be-

cause the large alkali cations (Cs�, Rb�) require the formation of large cages in the glass structure, which in return re-

quires the simultaneous formation of additional small rings. Ó 1998 Elsevier Science B.V. All rights reserved.

1. Introduction

There have been extensive investigations of thestructure±property relations in aluminosilicateglasses using solid state NMR and vibrationalspectroscopy [1±5]. The present work comple-ments previous studies along `fully polymerized'aluminosilicate joins SiO2±MAlO2 (M�Li�,Na�, K�), in which all oxygens are shared be-tween tetrahedral SiO4 or AlO4 units. This studywas carried out to investigate the e�ect of thelarge alkali cations Rb� and Cs� on the glassstructure.

The primary applications of glasses with larg-er silica contents make use of their optical trans-

parency in several regions of the electromagneticspectrum. Fused silica has excellent transmissionin the visible to UV regions, and methods havebeen developed for synthesizing SiO2 of greatestpurity for glass ®bers. The melting point andsoftening temperature of pure silica are too highfor useful processing for many other applica-tions, and most commercial optical glasses arebased on Na±Ca silicate compositions. Aluminais added to improve chemical and mechanicalresistance, and to inhibit devitri®cation. Therehave been recent suggestions that adding smallamounts of metal aluminate (MAlO2) compo-nents to silica might also improve the transmis-sion in the near-IR region of the spectrum [5].As part of our study, we prepared glasses withhigh silica content (95±99 mol% SiO2), to in-vestigate their transmission in the near-IR re-gion.

The simplest method for preparing aluminosil-icate glasses for structural studies is to melt and

Journal of Non-Crystalline Solids 226 (1998) 239±248

* Corresponding author. Tel.: +1-602 965 6645; fax: +1-602

965 0474; e-mail: [email protected] Present address: Ecole Normale Sup�erieure de Lyon, 46 all�ee

d'Italie, 69364 Lyon cedex 7, France

0022-3093/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 4 1 6 - 5

quench a powder mixture; however, it is usuallynecessary to carry out several re-grinding andmelting steps to obtain homogeneous samples.In the case of alkali-containing silicates, this pro-cess often results in signi®cant loss of the volatilealkali component. The problem is avoided by useof a sol±gel method for preparation of chemicallyhomogeneous glass precursors. This method isused to obtain monolithic samples of pure SiO2

by removal of volatile components at low temper-ature [6±13]. It has proved more di�cult to ob-tain monolithic gels of aluminosilicates by thisroute [9,14]; however, even ®red gels in powderedform provide useful homogeneous precursors forpreparing glass samples via a thermal route forstructural studies. In the present study, we ap-plied the sol±gel method to obtain homogeneoussamples in the Cs2O±Al2O3±SiO2 and Rb2O±Al2O3±SiO2 systems. This study also permittedus to investigate the gelling characteristics ofthese compositions. For the purposes of the struc-tural study, thermal glasses were then preparedfrom the gel precursors.

2. Experimental

Gels with starting compositions along the SiO2±MAlO2 (M�Cs, Rb) joins ranging from 99% toapproximately 60% silica were prepared. Ethanolwas used as a solvent for tetraethylorthosilicate(TEOS), and Al and alkali metals were added tothe solution as nitrates. A major drawback in theuse of metal nitrate precursors is their tendencyto crystallize during dehydration. Such crystalliza-tion was avoided by evaporating a large fraction ofwater at low pH, before gelling sets in. Ethanol isadded both as a solvent for TEOS and to facilitatethe incorporation of water into the mixture, whichis necessary for hydrolysis. It is necessary to opti-mize the TEOS:ethanol:water ratio for successfulgel formation. The alcohol is required to producea single phase solution of TEOS and water to ob-tain a homogeneous sol. However, too large a con-centration of ethanol adversely a�ects the gelmorphology and increases the organic residue inthe gel and also the gelling time [6,8,11±13]. Inour study, the molar ratio of TEOS:ethanol:water

was optimized, with best results obtained at a ratioof 1:4:20.

Water and ethanol were mixed in a plastic beak-er and immersed in a water bath at 60°C. Aqueousaluminum nitrate and the respective alkali nitratewere then added dropwise while stirring. The cov-ered solution was then stirred at �50°C for 20 minand set aside to gel. The gelation time varied from1 to 24 h, depending on the composition of the so-lution. Some of the gels were yellow in color due toa slight excess of nitrates, but became colorlessduring ®ring. The gels were allowed to age for sev-en days, and were dried by making pinholes in theplastic cover. Increasing the number of holes eachday reduced the drying rate su�ciently that crack-ing was avoided. The gels with >�90 mol% SiO2

could be successfully dried as single opticallytransparent discs. However with smaller silica con-tent, the gels were increasingly di�cult to obtain ina single piece, although large clear fragments (sev-eral mm across) could usually be obtained.

All gels were then sintered in platinum cruciblesin air with an increase in temperature to about900°C to remove organics, nitrates, water etc. Ho-mogeneous thermal glasses were then obtainedfrom the gels by melting at �1850°C. The sampleswere sealed in pockets of Mo foil to minimize alka-li loss and melted in an induction furnace. All sam-ples quenched to clear fragments of glass.

Glass compositions were measured by electronmicroprobe (wavelength dispersive) analysis[JEOL JXA-8600], and are reported as analyzedwt% oxide components and in mol% in Table 1.The sample designation is given as Cs±X or Rb±X, where X represents the analyzed mol% SiO2

within the pseudo-binary SiO2±MAlO2 join. Theanalyzed glass compositions lay close to the 1:1M2O : Al2O3 join (Table 1; Fig. 1). Alkali losswas largest below �70 mol% SiO2, and the result-ing glasses lay far from the 1:1 join. These samplesare not described here. The analyzed wt% totalsdecrease systematically below 100% with increas-ing CsAlO2 content in the Cs-bearing glasses. Thisdecrease is partly due to the presence of OH andH2O groups (identi®ed by infrared spectroscopy)within the hygroscopic samples. However, the wa-ter content in these glasses indicated by IR spec-troscopy (�0.1%) is not as great as indicated by

240 P.F. McMillan et al. / Journal of Non-Crystalline Solids 226 (1998) 239±248

the analysis total, and does not di�er substantiallyfrom the OH/H2O content present in the corre-sponding SiO2±RbAlO2 glasses. The totals ob-tained for the Cs-glass analyses could then bedue to the Cs standards used, or to damage bythe electron beam during analysis [15]. The com-positions in mol% reported in Table 1 (andFig. 1) assume that all analyses summed to 100%.

Raman spectra were collected using a triplespectrograph system (ISA S3000) with a diode ar-ray detector (Princeton Instruments IY-750). Thespectra were ¯at-®eld corrected, and reduced to re-move the frequency and temperature dependenceof Stokes scattered intensity [4,16]. For FTIR mea-surements (Bio-Rad Digilab FTS-40) in the 40±700cmÿ1 region a 12.5 mm Mylar beam splitter, Hglamp source and DTGS detector were used. Forthe infrared region between 400 and 6500 cmÿ1,a ceramic globar source, KBr beam splitter andMCT detector were used. Near-IR spectra wereobtained using a quartz-halogen lamp source,quartz beamsplitter, and PbSe detector. Mid-IRtransmission spectra were obtained from ®nelyground samples mixed with CsI (400±4000 cmÿ1),and far-IR spectra from micropellets of pure sam-ple compressed between type IIA diamonds in aMerrill Bassett diamond anvil cell (40±700 cmÿ1).The mid infrared re¯ectance spectra were collectedfrom small polished glass pieces under the FTIR

Fig. 1. Ternary plot (mol% oxide) of glass compositions in the

Cs2O±Al2O3±SiO2 and Rb2O±Al2O3±SiO2 systems studied.

Tab

le1

Ele

ctro

nm

icro

pro

be

an

aly

ses

for

Cs±

Xa

nd

Rb

±X

gla

sses

Sa

mp

led

esig

na

tio

nS

iO2

(wt%

)A

l 2O

3(w

t%)

Alk

ali

oxid

e(w

t%)

To

tal

SiO

2(m

ol%

)A

l 2O

3(m

ol%

)C

s 2O

or

Rb

2O

(mo

l%)

Si/

(Si+

Al)

Cs9

99

4.8

�0

.87

0.9

1�

0.1

32.6

5�

0.3

598.4

�0.6

898.8

0.6

0.6

0.9

9

Cs9

58

4.9

�0

.33

3.8

6�

0.1

09.0

8�

0.1

598.0

�0.8

295

2.5

2.5

0.9

5

Cs9

17

3.1

�1

.36

.75

�0

.60

17.5

�1.3

97.3

�0.5

891

54

0.9

0

Cs8

76

7.4

�1

.49

.25

�0

.97

21.8

�1.1

98.5

�0.4

987

76

0.8

6

Cs8

15

6.7

�1

.61

3.4

�1

.325.8

�1.3

96.0

�0.7

281

11

80.7

8

Cs7

34

4.1

�1

.41

5.7

�1

.135.2

�0.6

095.5

�0.7

272

15

13

0.7

0

Rb

99

98

.10

�0

.27

0.9

8�

0.6

31.2

8�

0.4

5100.4

�0.1

899

0.6

0.4

0.9

9

Rb

97

93

.06

�0

.55

2.8

7�

0.1

04.6

1�

0.1

5101.0

�0.8

297

1.8

1.5

0.9

6

Rb

95

89

.5�

1.3

3.7

8�

0.6

7.6

6�

1.3

101.0

�0.5

895

23

0.9

5

Rb

89

78

.6�

1.4

7.8

�0

.97

15.1

�1.1

102.0

�0.4

989

56

0.9

0

Rb

83

68

.4�

1.6

11

.2�

1.3

20.3

�1.3

99.9

�0.7

284

88

0.8

4

Rb

78

60

.2�

1.6

13

.0�

1.1

28.0

�0.6

101.0

�0.7

478

10

12

0.8

0

Rb

72

53

.1�

1.1

18

.9�

0.9

630.3

�1.2

102.3

�1.4

472

15

13

0.7

0

P.F. McMillan et al. / Journal of Non-Crystalline Solids 226 (1998) 239±248 241

microscope (UMA-300). The far infrared re¯ec-tance spectra were measured from samples groundinto very ®ne powders and pressed to pellets usinga beam condenser in a main bench of the FTIRspectrometer. 29Si and 27Al magic angle spinning(MAS) NMR spectra were obtained at 9.4 T (Bru-ker MSL-400). 27Al spectra were obtained at spin-ning speeds up to 12 kHz, using a 0.6 ls pulselength (corresponding to a p/2 pulse), 7 ls dead-time and pulse delay of 300 ms. Shifts were refer-enced to external 1M [Al(H2O)6]3�. 29Si spectrawere acquired with a pulse length of 4.5 ms (p/6

pulse angle), 24 ls dead-time and pulse delay 120s. 29Si spectra were referenced to external tetrame-thylsilane (TMS).

3. Results

Raman spectra of Cs±X and Rb±X glass sam-ples are shown in Fig. 2. Consistent with previousstudies, the spectra show bands in the 450±600cmÿ1 and 900±1200 cmÿ1 regions, and a smallerasymmetric band near 800 cmÿ1 [2,4,18±22]. Thehighest frequency bands occur in the same regionas for binary silicate glasses and crystals [2,4]. PureAl±O stretching vibrations are at lower wavenum-ber [23±26], so that the highest frequency bands inthese glasses are best assigned as Si±O stretchingmodes, perturbed by the presence of Al atoms co-ordinated to the bridging oxygens [4,17].

In the high frequency (Si±O(Al) stretching) re-gion, initial addition of MAlO2 component resultsin appearance of a band at 1190 cmÿ1 [17,27]. Thisband is assigned to the Si±O(Al) stretching vibra-tion of Q4(1Al) units (fully polymerized SiO4 tetra-hedra, in which one of the linkages is to an AlO4

tetrahedron [28]) in the glass [4,17]. The appear-ance of this band is best seen in di�erence plots,obtained by subtracting the high frequency bandsof pure SiO2 from the spectrum of the alumino-

Fig. 2. (a) Raman spectra of Cs±X glass series. (b) Raman spec-

tra of Rb±X glass series.

Fig. 3. Di�erence Raman spectra in the high frequency region

for Cs±X glasses, obtained by subtracting the spectrum of SiO2

from each Cs±X glass spectrum.


silicate glass sample (Fig. 3). In samples with< �90% SiO2, this band is accompanied by a sec-ond one at 1020 cmÿ1, assigned to Q4(2Al) units(fully polymerized SiO4 tetrahedra joined to twoAlO4 tetrahedra). These two bands increase in in-tensity and move to lower frequency with decreas-ing silica content (Fig. 4).

In the low frequency TOT (T� Si, Al) bend-ing region [2,4], a band near 560 cmÿ1 growswith decreasing silica content, and dominates

the spectrum when there is �23 mol% MAlO2

component present (Fig. 2). The interpretationand signi®cance of this band is discussed in Sec-tion 4.

Infrared re¯ection and powder transmissionspectra for the Cs±X samples are shown inFig. 4. As in the case of the Raman spectra, theIR spectra of aluminosilicate glasses have threecharacteristic groups of bands near 1100, 800and 500 cmÿ1, which are assigned to (Si, Al)O4

stretching and bending vibrations of the fully poly-merized aluminosilicate framework [34±42]. In there¯ectance spectra, the re¯ectivity of the principlehigh frequency band diminished with increasing al-kali aluminate content, from �70% re¯ectivity forpure silica sample, to �35% re¯ectivity for compo-sitions with <80 mol% SiO2. This decrease indi-cates increasing anharmonicity of the Si±Ostretching vibrations as the silica content is de-creased [37±40]. The IR results are discussed inmore detail below.

The 29Si NMR peak positions and bandwidths for the Cs±X and Rb±X samples mea-sured in this study are plotted in Fig. 5 alongwith previous data for Li±X, Na±X, K±X andCa±X series from the literature [1,28,43±46]. Onlya single peak due to fully polymerized [4] coordi-nated silicon is observed, with a position and fullwidth at half height (FWHH) which can be relat-ed to the number and distribution of SiOAl link-ages [1,28]. For each replacement of Si by Al inthe next nearest neighbor (NNN) environment,a shift of �9 ppm occurs to less shielded (lessnegative ppm) chemical shifts for the tetrahedralSi site under investigation. This shift results in ashift in peak maximum and change in linewidthof the unresolved band in the spectra of alumino-silicate glass samples, which generally contain adistribution of Q4(nAl) sites. The 29Si peak posi-tions for the Cs±X and Rb±X samples measuredin this study lie along the same trend as those re-ported previously. The minimum value for theaverage 29Si NMR shift lies near )100 ppm,which corresponds to the replacement of between1 and 2 Si NNN atoms by Al, on average [28,45],in agreement with our observations from Ramanspectroscopy (i.e., that Q4(1Al) and Q4(2Al)groups are present).

Fig. 4. (a) IR re¯ectance spectra of Cs±X glasses. The spectra

above �500 cmÿ1 were obtained with ceramic source, KBr

beamsplitter and MCT detector. Spectra in the 50±500 cmÿ1

range were obtained with Hg lamp source, Mylar beamsplitter,

and DTGS detector. The vertical dashed line indicates the

match point between the two sets of spectra. (b) powder IR

transmission spectra of Cs±X glasses. The spectra below �500

cmÿ1 were obtained on ®lms of powder pressed between dia-

monds in a diamond anvil cell.


The full width at half height (FWHH) of the 29SiNMR peak gives an indication of the distributionof structural sites and species in the glass, and alsothe range in TOSi (T� Si or Al) and OSiO bondangles about the Si sites [1,3,28]. MeasuredFWHH for the Cs±X and Rb±X series are greaterthan those measured in previous studies

(Fig. 5(b)). The structural species present overthe compositional range (Q4(0Al), Q4(1Al) andQ4(2Al)) are the same as in previously studiedglasses at high silica content, so that this di�erenceis due to a larger TOT bond angle distribution inthe Cs- and Rb-bearing samples [1,28], associatedwith the presence of the large alkali cation.

Fig. 5. Plots of: (a) 29Si peak positions, (b) 29Si full width at half height (FWHH) and (c) 27Al peak maxima as a function of silica

content (expressed as the molar ratio Si/(Si+Al)). The points for the Cs±X and Rb±X series in this study are represented as black circles

and triangles, respectively. Values for the 29Si shift of SiO2 are shown as a black inverted triangle. The 29Si peak positions and FWHH)

for Cs±X and Rb±X glasses are compared with literature values on the plots (®lled symbols are grey): Oestrike et al. Li±X inverted

triangle; Na±X squares; K±X diamonds; Murdoch et al. Na±X crosses; K±X cross in a square; Ca±X circles; Engelhardt et al. Ca±

X triangles (see Refs. [28,43±46]).


The position of the 27Al NMR peak maximummeasured for these glass series is also consistentwith literature values [1,28,45] (Fig. 5; Table 2).Although the position of the 27Al peak maximumdoes not correspond to the isotropic chemicalshift, and it cannot be simply compared betweendi�erent studies because of changes in peakshape and position with magnetic ®eld, spinningrate and acquisition conditions for the quadrupo-lar nucleus [1,28,47], the 27Al peak position cangive useful information on the glass structure.Observation of a single peak is consistent withthe presence of only tetrahedrally coordinatedAl sites, as expected for these `fully polymerized'aluminosilicate glass structures. The trend in 27Alchemical shift with Si/(Si + Al) ratio is similar tothat reported in Ref. [44], although our peak po-sitions are shifted by �5 ppm to smaller chemicalshifts due to the lower ®eld (9.4 T) used in ourstudy.

As noted above, Lines [5] suggested that smalladditions of metal aluminate component to SiO2

glass might improve its transmission in the near-IR region. In our study, we investigated thenear-IR transmission of high silica (>90 mol%)samples. Although addition of the aluminate com-ponent did change the form of the overtone spec-trum of the lattice vibrations as expected, theposition of the near-IR edge remained essentiallyunchanged from vitreous silica. This invariance isexplained by the observation that the samples with>90 mol% silica contain substantial Q4(0Al)groups; i.e., pure SiO2-like regions of the glass

structure, with their characteristic IR signature(see below and Fig. 5).

4. Discussion

The IR absorption spectrum of SiO2 glass in theSi±O stretching region is dominated by a band at1060 cmÿ1, with a shoulder near 1200 cmÿ1. There¯ectance spectrum has its principal band maxi-mum near 1150 cmÿ1, corresponding to a trans-verse optical (TO) resonance at 1060 cmÿ1, and ahigher frequency shoulder near 1250 cmÿ1. Thesefeatures are due to asymmetric Si±O stretching vi-brations within the tetrahedral glass framework.The characteristic form of the silica spectrum isreadily identi®ed in the spectra of the aluminosili-cate glass samples with P 87 mol% SiO2, indicat-ing that substantial parts of the glass structureremain little changed by the addition of aluminatecomponent. In samples with smaller silica content,there is a shift in the position of the principal peakmaximum to lower frequency in both re¯ectionand absorption spectra (Fig. 4). In previous sys-tematic studies of similar aluminosilicate glasses,the high frequency peak of vitreous SiO2 near1100 cmÿ1 moved to lower wave number and be-came less intense as aluminate component is add-ed, and a low frequency shoulder near 950 cmÿ1

grew in intensity [34,35]. These bands are due toasymmetric stretching vibrations with a largeamount of Si±O character within the TOT linkages[41,46]. In the present glasses, no separate band

Table 2

NMR data for Cs±X and Rb±X glasses

Sample Si/(Si+Al) 29Si chemical shift (ppm) 29Si FWHH (ppm) 27Al peak maximum (ppm)

Cs99 0.99 ±110 � 2 a 18 � 2 a 44 � 2 a

Cs95 0.95 ±107 19 46

Cs91 0.90 ±106 20 47

Cs87 0.86 ±103 22 48

Cs81 0.78 ±100 23 50

Cs73 0.70 ±96 23 52

Rb95 0.95 ±107 20 45

Rb89 0.90 ±105 21 49

Rb83 0.84 ±101 22 50

Rb72 0.70 ±99 23 51

a Estimated measurement error in peak position [28,45].


near 950 cmÿ1 was detected, although an unre-solved feature near this position might be partlyresponsible for the shift in the band maximum tolower wave number.

For SiO2 glass, the IR band near 800 cmÿ1 isdue to asymmetric Si±O stretching vibrations withmainly Si displacements inside the tetrahedral`cage', related to the m3 stretching mode of isolatedSiO4 tetrahedra [4,40]. This band remains in thespectra until �80 mol% SiO2, at which composi-tion a second feature with a broad absorptionmaximum near 700 cmÿ1 is apparent (Fig. 4). Atthis composition, the 800 cmÿ1 band shifts to low-er wave number, and is not resolved from thebroad band at lower frequency. Similar changesare observed in this region in the Raman spectra(Fig. 2). The feature in the 700±800 cmÿ1 regionare mainly due to Al±O stretching vibrations[26], which may be coupled with tetrahedral Sicage motions [4,17].

The infrared band in the 400±500 cmÿ1 regionin assigned to tetrahedral deformation vibrations(OSiO and OAlO bending). Consistent with pre-vious studies, this band shifts to lower wave num-ber with decreasing silica content [26,34,35]. In thepresent sample series, the band becomes verybroad at silica content <90 mol%. The largerwidth of this band indicates substantial deforma-tion of the tetrahedral groups associated with thepresence of the larger alkali cations.

In the re¯ectivity spectra of samples with <90mol% silica, a broad feature is in the 100±200cmÿ1 region (Fig. 4(a)). From previous workon Li�- and Na�-containing aluminosilicateglasses, we assign this feature to vibrations ofthe alkali cation [37±40]. However, no peak isobserved at this position in the transmissionspectra (Fig. 5(b)). The most probable explana-tion is that the transverse optic (TO) mode fre-quency of the Cs� motions occurs below 40cmÿ1, and the feature observed in our re¯ectivityexperiment only corresponds to the higher fre-quency part of the band. The IR band due toNa� motions in other silicate glass compositionslies between 150 and 200 cmÿ1 [37±40]. Basedon the mass ratio ((mNa/mCs)

1=2� 0.42), we ex-pect that the Cs� vibration would lie between60 and 80 cmÿ1, and any weakening of the

Cs±O bond would decrease this wave numbereven further.

In Section 3, we noted that a band near 560cmÿ1, in the TOT bending region of the Ramanspectrum, grows with decreasing silica content,to dominate the spectrum by �23 mol% MAlO2

component (Fig. 2). Sykes and Kubicki [29,30]have proposed that this band is due to the pres-ence of 3-membered rings of (Si, Al)O4 tetrahedra.This interpretation is analogous to that proposedfor the band at 606 cmÿ1 in the Raman spectrumof pure SiO2 glass, which is assigned to the sym-metric oxygen `ring-breathing' mode of 3-member-ed siloxane ring `defects' in the glass [31].McMillan et al. (17) had earlier suggested a di�er-ent interpretation for the 560 cmÿ1 band in alum-inosilicate glasses, in a Raman spectroscopic studyof SiO2±Ca0:5AlO2 glass samples, which were ex-amined to much smaller silica content than thesamples studied here. These authors assigned theband to the symmetric bending vibration of AlOAllinkages in the glass structure (not necessarilywithin 3-membered rings), because it formed thedominant feature in the spectrum of the silica-freeend member. Recent multiple quantum MASNMR experiments have shown, however, that Al-OAl linkages are unlikely to be present at in alkalialuminosilicate glass samples with >75 mol% SiO2

[33] (although these structural units must be pres-ent for silica contents smaller than 50 mol% SiO2).We attribute the 560 cmÿ1 band to 3-membered(SiSiAl)-rings in the Cs±X and Rb±X glasses stud-ied here.

It is interesting that the 560 cmÿ1 band in theseCs- and Rb-aluminosilicate glasses is much moreintense relative to the TOT bending band with itsmaximum at �450 cmÿ1, compared with K-, Na-and Li-bearing aluminosilicate glasses with similarsilica contents [2,4,17±22]. This observation sug-gests that the 3-membered alumino-siloxane (SiS-iAl) rings are more abundant in the Cs- and Rb-containing glasses than in glasses with the other al-kalis. This di�erence can be ascribed to the largersites required to contain the larger alkali metalions, Cs� and Rb�, in the glass. These sites are as-sociated with a larger proportion of larger (Al, Si)rings. In crystalline polymerized tetrahedral net-work structures, it is well known that formation


of large rings results in simultaneous production ofsmall rings [48]. This principle is operating here,resulting in the greater dispersion in TOT bond an-gles suggested by the 29Si NMR linewidth data(Fig. 5(b)).

5. Conclusion

Compositions along the SiO2±CsAlO2 andSiO2±RbAlO2 joins with >90 mol% SiO2 couldbe obtained as monolithic samples via a sol±gelroute: compositions with <90 mol% SiO2 crackedon drying and aging. SiO2±CsAlO2 and SiO2±RbAlO2 glass samples were prepared by meltingthe gel precursors, and were examined by vibra-tional spectroscopy and solid state NMR spectros-copy. The variation in high frequency Ramanbands, along with the trend in 29Si and 27AlNMR chemical shifts, indicate that Q4(0Al),Q4(1Al) and Q4(2Al) units are present in propor-tions that are a function of silica content, consis-tent with previous studies on other alkalialuminosilicate glass systems. A major di�erencewith previous studies is the increased intensity ofthe Raman band at 560 cmÿ1, assigned to three-membered (SiSiAl) rings in the glass, comparedwith glasses containing Li�, Na� or K�, with sim-ilar silica content. This di�erence is attributed tothe e�ect of the larger ionic radius of Cs� andRb� in increasing the fraction of three-membered(SiSiAl) aluminosiloxane rings, as a result of theformation of large cages in the glass structure.

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structural characterization of sio2–csalo2 and sio2–rbalo2 glasses

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