EXAFS study of alkali galliosilicate glasses

Journal of Nt,n-Crystalline Solids 105 (1988) 139-148 North-Holland, Amsterdam

139

EXAFS STUDY OF ALKALI GALLIOSILICATE GLASSES * Paige L. HIGBY Sachs~Freeman Associates, Landover, MD 20375, USA

James E. SHELBY New York State College of Ceramics at Alfred University, Alfred, N Y 14802, USA

James C. PHILLIPS Chemistry Department, SUNY, Buffalo and S U N Y X21 Beamline, NSLS, Building 725, Brookhaven Nat. Lab., Upton, N Y 11973, USA

Alan D. LEGRAND SUNY )(21 Beamline NSLS, Building 725, Brookhaven National Laboratory, Upton, N Y 11973, USA Received 13 August 1986 Revised manuscript received 2 February 1988

The extended X-ray absorption fine structure (EXAFS) of NazO-Ga3-SiO 2 glasses of 3 compositional series containing either a constant Na20 or SiO 2 content, or a constant G a / N a ratio of 1.0, analogous crystalline compounds and 4 potassium-containing glasses were measured on the SUNY X21 beamline of the National Synchrotron Light Source at Brookhaven National Laboratory. The data were analyzed to extract Ga K-edge position, coordination numbers and Ga-O bond lengths for each glass composition. This study indicates that the G a - O bond length is 1.83 ]k, and that the coordination number of oxygen about gallium is constant at 4, regardless of glass composition.

1. Introduction

The physical properties of alkali silicate glasses are greatly affected by the addition of Ga203 [1-4]. For example, Lapp and Shelby found that the glass transformation temperature increases by 200 °C in glasses containing a constant 20 mol% Na20 as Ga203 replaces SiO2 up to a G a / N a ratio of 1.0-1.2. The glass transformation temperature then decreases as additional Ga 203 is added to the glass. A similar behavior was also observed for aluminosilicate glasses [5,6]. The property/ composition dependence of alkali galliosilicate

* Supported by NSF contract DMR-830445 and DOE contract DEA C0280ER10759 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

glasses was found to be similar to that of alkali aluminosilicate glasses in a study by Piguet and Shelby [3,4] in which gallium was substituted for aluminum in glasses of otherwise identical composition. Physical property results exhibited additive behavior between galliosilicate and aluminosilicate glasses, indicating that gallium and aluminum occupy comparable sites in the glass structure. These results demonstrated that it is valid for structural models proposed for sodium aluminosilicate glasses [5,6] to be applied to alkali galliosilicate glasses [1-4] in order to explain the physical property data discussed above. Structural models for alkali gallio/aluminosilicate glasses agree that, for glasses with M / R less than 1.0-1.2 (where M is either Ga or A1 and R denotes any alkali ion), the M 3+ ion enters the

140

P.L. Higby et al. / EXAFS study of alkali galliosilicate glasses

alkali silicate glass structure in predominantly 4fold oxygen coordination as a network former. The formation of 4-coordinated M 3+ tetrahedra causes an alkali ion to become localized near the MO 4 unit in order for charge balance to be maintained. However, above an M / R ratio of 1.0-1.2, the proposed models disagree. The decrease in network connectivity above M / R = 1.0-1.2 suggests that M 3+ acts as a network modifier in this compositional range instead of as a network former. The environment of the M 3 + ion in this modifying role is the subject of some controversy. The proposed modifier environments include six-coordinated M 3÷ ions [1,5,6] and M - S i tetrahedral triclusters [7]. Oscillations on the high-energy side of K-edge absorption spectra are the source of extended X-ray absorption fine structure, better known as EXAFS. These oscillations are caused by the interaction of a K-shell photoelectron with surrounding atoms as it is being ejected from the photo-absorbing atom. The theoretical background and explanation of EXAFS has been the subject of many articles [8-13], and hence need not be repeated here. Generally, EXAFS is sensitive to the immediate local environment of a specific atom in a material. Since structural models for alkali galliosilicate glasses center on the gallium ion environment, it is expected that gallium K-edge EXAFS will shed light on the overall structure of these glasses. EXAFS studies in material science have been reviewed by Gurman [14]. Application of EXAFS to glass systems has been achieved in a few cases [15-19]. Of particular interest is the study by McKeown et al. [15a] on the A1-EXAFS of sodium aluminosilicate glasses, since it would be expected that their results would parallel those for galliosilicate presented in this study. McKeown et al. found that the A1/Na ratio did not significantly affect the A1-O bond length of - 0.177 nm. As will be discussed later, the EXAFS spectra for a certain environment can be calculated and compared with experimental data. Also, EXAFS data for materials with known structures whose gallium environments are close tO those expected for the unknown samples can be measured and used a guide for determining the structure of the

unknown materials. In this study, EXAFS of crystals whose gallium environments were known to be 4-coordinated by oxygen were measured and analyzed in the same manner as the glasses.

2. Experimental procedures 2.1. Sample preparation Glasses used in this study were prepared by conventional melting of reagent grade alkali carbonates and SiO 2 and 99.999% pure Ga203. A detailed discussion of the melting and annealing procedure can be found elsewhere [1]. Glass samples were powdered and weighed amounts were thoroughly mixed with boron nitride to optimize the Ga K-edge absorption and provide a large volume of powder for sample homogeneity. The sodium galliosilicate glass compositions studied were arranged in three series: (1) Constant 20 mol% N a 2 0 , replacing SiO z with Ga203, varying both the G a / N a ratio and SiO z content. (2) Constant 60 mol% SiO2, varying the G a / N a ratio. (3) Constant G a / N a = 1.0, varying the SiO 2 content. The EXAFS of three potassium galliosilicate glasses were also measured to check for any alkali-identity dependence, along with 3 crystalline compounds: fl-Ga203, NaGaSiO4, and KGaSiO 4. The two galliosilicate crystals were made by devitrification of stoichiometric glass composition. The crystal identities were verified by X-ray diffraction.

2.2. Data collection Room temperature X-ray absorption spectra were recorded at the SUNY X21A1 beam port at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory [19]. NSLS was operating at 2.4 GeV for the first 7 sample, and at 2.5 GeV for the remainder of the data set. Data were recorded at storage ring currents of between 30 and 90 mA at 2.4 GeV and between 33 and 65 mA at 2.5 GeV. A Si(220) double crystal mono-

P.L. Higby et aL / EXAFS study of alkali galliosilicate glasses

chromator with a gold-coated bent toroidal mirror was used for most of the spectra. The mirror collects 3 mrad of beam intensity and focusses it to a - 1 ram= spot (providing a 25-fold improvement of the intensity per unit area and rejecting monochromator harmonics). Spectra of four samples were re-collected at a later date with a Si(ll 1) monochromator (2.523 GeV). No significant differences were found upon comparison with the earlier data. All spectra were recorded in the transmission mode with ion chambers detecting the incident and transmitted signals. The monochromator was first calibrated with copper foil, setting the first inflection point to 8990.3 eV [20], then with Ga 203 before each sample. Three to five scans of 20 rain and approximately 400 points at 2 s / p o i n t were collected and averaged for each sample.

2.3. Background subtraction The background subtraction method used in this study has been described elsewhere [21]. The absorption due to gallium was isolated from the total absorption by the standard procedure of fitting a polynomial in the pre-edge region and extrapolating the polynomial through the entire spectrum. The EXAFS oscillations were then extracted from the total gallium absorption by a three-step process. First the oscillations are smoothed above the edge by making each point of the spectrum equal to the mean of its nearest neighbors, with iteration to convergence. This gives a first approximation to the atomic absorption background. Next, a cubic spline, fitted to the smoothed data, gives a better approximation to atomic absorption, and is subtracted from the original spectrum. Finally, in ref. [21], the EXAFS oscillations thus obtained are normalized to account for the falloff of atomic absorption with increasing energy by using a Victoreen function with the coefficients in table 3.2.2C of ref. [22].

2.4. Spectrum fitting The experimental spectra were fitted with the theoretical expression:

x(k)=s~_, {(N,F~(k))/kR2)}

exp(-2R,/X)

× e x p ( - 2o/2k 2 ) sin(2kR i + dPi(k)),

141

where x ( k ) = X-ray absorption fine structure, k = photoelectron wave vector, s = overall scale factor (accounts for background subtraction scaling errors, sample inhomogeneity, etc.), and for sphere i: N, = number of atoms (or occupancy), o;= Debye-Waller factor (accounting for thermal vibration of the atoms interacting with the photoelectron), R; = radius, ~ ; ( k ) = phase shift, taken from ref. [23] and fitted to a polynomial expression [24], ~ = backscattering amplitude from each of the N atoms. The backscattering amplitudes were from ref. [25], and the electron mean free path (Xe) was fixed at 0.8 nm [26]. The fitting of calculated to experimental data was accomplished in two steps: first, experimental and calculated spectra were compared in the range k = 4.0 to - 1 3 reciprocal angstroms, and EOs and then shell parameters (coordination number, bond length, Debye-Waller factor, etc.) varied until good agreement was obtained as discerned by inspection and by reduced X 2. X= is a measure of residual error between theoretical (T) and experimental (E) spectra and is calculated by

XZ={1/(n-2-v)}

~_, {(Ea-Tj)/%} 2, (2) j=l,n

where n = the number of data points, v = the number of variables. Second, these parameters were refined with a least-squares grid search algorithm over a parameter range which was shown to be adequate by the first segment of the search. For example, spectra are calculated for various R i. The R; range for refinement is taken from a minimum where the theory oscillations are clearly shifted to the k side of the experimental spectrum, maximum R; is indicated by a shift to lower k. Error estimates used in the least-squares analysis were obtained by fitting a cubic spline (S) to the experimental spectrum (E) which was then subtracted to get an initial value for the error for data points ~j. Then, five neighboring cjs were averaged to get the final estimate of the error for each point: =

E

(Ej-=-j

- sj

j=l,5

During fitting, phase and amplitude functions were not refined, nor was any use of the model

142

P.L. Higby et aL / EXAFS study of alkali galliosilicate glasses

compounds made to attempt to extract amplitude or phase functions. Note also that, for single shell fits, the parameters s and N, are interchangeable. In practice N was fixed at 4 (see below) and s was varied.

event for a parallel effect to be expected if a gross coordination change was present in the glasses in this study.

3. Results and discussion

All glass spectra are clearly dominated by a single coordination sphere which can be fit by inputting the values corresponding to a sphere consisting of 4 oxygens at an average distance of 0.183 nm (within 0.001 nm) from the central gallium atom. In some cases, addition of a second oxygen sphere at a distance of 0.2 nm slightly decreased the residual error. Figure 2 shows the experimental and calculated EXAFS spectra for 2 0 N a 2 0 - 1 5 G a 2 0 3 - 6 5 S i O 2 glass. The excellent agreement between the two curves is easily seen in this figure. The parameters resulting from the least squares refinement of all spectra are given in table 1. Listed parameters which have not previously been explained are: kmax: The k value (in reciprocal angstroms) for the individual spectrum where the calcula-

3.1. Near-edge structure Figure 1 shows the measured Ga K edge spectrum for a 20NazO-20Ga203-60SiO 2 glass. It was found that all glasses gave the same near K-edge spectra within experimental errors from shifting monochromator calibration and background subtraction. A coordination change which involved a large percentage of the species in question would be expected to appear as a K-edge shift [28]. A shift in the energy of the A1 Kc~ peak as measured by X-ray fluorescence was found by Day [29] and attributed to a change in A1 coordination from 4 to 6. The interaction leading to a K a peak is close enough to that for a K edge 1200

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J

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~.0i 40 , PHOTOELECTRON ENERGY {keY)

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Fig. 1. Example of K absorption edge for a Na20-Ga203-SIO2 glass.

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I 7.5

90

j~

12.0

13. 5

15.0

ELECTRON k VECTOR (angsLroms-i) Fig. 2. Comparison of measured (solid) and calculated (dashed) EXAFS spectra for a 20Na 20-15Ga

Eo:

tion was truncated. This value was determined by the signal-to-noise ratio. Binding energy of photoejected electron g i v e n h e r e as t h e d i f f e r e n c e f r o m t h e i n f l e c -

203-65SIO2

glass,

t i o n p o i n t t o t h e p o i n t o f t h e e d g e (10.361 eV). T h e v a l u e s o f X 2 v a r y f r o m 1.72 t o 11.2. I t s h o u l d b e n o t e d t h a t a v a l u e o f X 2 = 1,0 w o u l d i n d i c a t e a

Table 1 Refined parameters for EXAFS spectra Composition

R (nm)

02

X2

kmax (,~ 1)

E 0 (eV)

s

20Na 2 0 - 2 G a 203 -78SIO 2 20Na20-5Ga203 -75SIO 2 20Na 20-10Ga 203-70SIO 2 20N a 20-15Ga 203 -65SIO 2 20Na20-20Ga203-60SIO 2 20Na20-25Ga203-55SIO 2 20Na 20-30Ga 203-50SIO 2

0.184 0.183 0.183 0.184 0,183 0.184 0.183

0.0006 0.0006 0.0006 0.0002 0.0004 0.0008 0.0010

1.57 1.63 3.02 1.74 4.82 4.52 3.76

13.3 13.3 13.4 13.3 13.3 16.0 12.4

16.5 19.7 20.5 21.8 16.0 16.5 17.5

0.692 0.694 0.614 0.512 0.542 0.658 0.480

35Na 20-5Ga 203-60SIO 2 30Na20-10Ga 203-60SIO 2 25Na20-15Ga203-60SiO 2 20Na20-20Ga203-60SIO2 16Na20-24Ga203-60SiO 2

0,183 0.183 0.183 0,183 0.183

0.0007 0.0009 0.0003 0.0004 0.0013

8.06 7.70 3.64 4.82 5.33

13.3 13.4 13.4 13.3 15.3

19,0 17,0 18.5 16.0 16.0

0,663 0.604 0,595 0.542 0.586

10Na 20-10Ga203-80SiO 2 15Na 2O-15Ga 203-70SIO 2 20Na 20-20Ga 203-60SIO 2 25Na20-25Ga 203-50SIO 2 30N a 2O - 30Ga 203 - 4 0 S i O 2 20KeO-20Ga 203-60SIO 2 25NazO-25Ga203-50SIO 2

0.183 0.183 0.183 0.183 0.183 0.184 0.183

0.001 0.001 0.0004 0.0007 0.0034 0.0006 0.0015

6.16 9.42 4.82 6.64 10.7 4.40 5.70

15.5 15.5 13.3 13.5 13.3 13.3 12.3

19.8 17.5 16.0 18.2 17.5 16.0 22.0

0.637 0,626 0,542 0,629 0.687 0.542 0.605

0.184 0.183

0.0003 0.0017

2.52 4.75

12,3 13.3

19.2 20.0

0.368 0.699

Crystals NaGaSiO 4 KGaSiO 4

144

P.L. Higby et al. / EXAFS study of alkali galliosilicate glasses

Table 2 Results of second sphere calculations Composition

X~

0cc(2)

X~ 1.70

0 c c ( 2 ) rain

0 c c ( 2 ) m~x

0.08 -

0.9 1.1

20Na20-2Ga203-75SIO 2 20Na 20-5Ga 203-75SIO 2

2.42 1.72

0.11

2 0 N a 2° - 1 0 G a 203 - 7 0 S i O 2

3.09

0.22

2.94

-

1.2

2 0 N a 2° - 1 5 G a 203 - 6 5 SiO 2

1.83

-

-

0.01

0.8

20Na20-20Ga203-60SiO 2 20Na 20-25Ga203-55SiO2

4.94 3.81

-

-

0.01 -

0.8 0.7

20Na20-30Ga203-50SIO 2

4.47

0.21

3.81

-

0.7

35Na 20-5Ga 203-60SIO 2

8.38

0.07

8.36

-

1.5

3 0 N a 2O - 1 0 G a 203 - 6 0 S i O 2 25Na20-15Ga203-60SIO 2

6.24 3.82

0.40 0.17

5.70 3.71

-

1.6 1.2

2 0 N a 2 0 - 2 0 G a 203 - 6 0 S i O 2

4.94

-

-

1 6 N a 2 0 - 2 4 G a 203 - 6 0 S i O 2

5.37

-

1 0 N a 2 O - 1 0 G a 2 0 3 -80SIO2 1 5 N a 2O - 1 5 G a 203-70SIO 2

6.16 9.42

0.11 -

2 0 N a 2 0 - 2 0 G a 203 - 6 0 S i O 2 2 5 N a 2 0 - 2 5 G a 203 - 5 0 S i O 2

4.94 6.64

0.48

30Na 20-30Ga 203-40SIO 2

10.2

0.8 0.7

0.01

1.t 0.9

0.01 -

0.8 1.2

-

0.15

0.9

6.65 9.20

0.20

8.45

0.04 -

0.7 0.9

NaGaSiO4

2.51

-

-

0.01

1.6

KGaSiO4

4.75

-

-

0.01

0.8

2 0 K 2 0 - 2 0 G a 203 - 6 0 S I O 2 2 5 K 2 0 - 2 5 G a 203 -50SiO 2

11.2

6.07

0.01 0.01

Crystals

perfect correlation between experimental and calculated data. Determination of an unacceptable value of X 2 varies for individual spectra, depending on the signal-to-noise relationship in each case. For this reason, inspection remains an integral step in the fitting process. The error in first-sphere radius was defined as that interval which would cause the X z value to double, and was found to be approximately 0.001 nm. Therefore, within this error, the first sphere radii and occupancy values are constant for the spectra of all of the samples listed in table 1. Also, the value of 0.183 nm for G a - O bond lengths in 4-fold coordinated Ga 3+ is equal to values reported for NaGaSiO 4 glass [29] and 4-fold Ga 3+ in/3-Ga203 crystals [27]. With " N " set at 4, the " s " parameter varies from 0.694 to 0.480 with a mean of 0.602. The difficulties of obtaining agreement between measured and calculated EXAFS amplitudes is well known [9]. Difficulties with measurements (inhomogeneous samples, harmonic contamination) tend to reduce the observed ampli-

tudes. The theoretical model used in this study also does not take into account multiple scattering or mean free path effects completely - effects which lower amplitudes. That " s " is less than 1.0 and varies - + 20% is to be expected. There is no apparent trend of " s " within a composition series. It was concluded that the first shell Ga coordination number is not varying with glass composition within a + 20% range. These values of " s " are consistent with 4-fold coordination and known limitations of the measurement. Fitting the first shell as 6-fold coordination would yield an " s " range of 1.04-0.72 with a mean of 0.903, which is unacceptably high. Introduction of a second oxygen sphere at 0.2 nm, the published G a - O bond length for six-coordinated gallium [27], either left the X 2 value unchanged at very low concentrations of 6-fold coordination and increased X 2 a s the concentration was increased, or actually decreased the X 2 value. Table 2 contains the results of second oxygen sphere calculations, assuming that the first-sphere

145

P.L, Higby et al. / EXAFS study of alkali galliosilicate glasses 2000 ~

"'

20Na20-30GB20s-50S i O~

i500

t000

I

o

20Na2 O- J . 5 G a 2 0 s - 6 5 S i O ~

~c

500

fJ 20Na20-5Ga20s-75SiOa

o

I J

-500

___a___ 4

B

B

10

12

i

14

.-- _

R .... _~

:16

IB

20

ELECTRON k VECTOR (angstroms-~)

Fig. 3. Experimental (solid) and calculated (dashed) EXAFS spectra of glasses containing a constant 20 mol% Na20.

parameters remain unchanged. Similar attempts at reducing X2 by changing the first-sphere parameters and adding a second sphere were much less successful than those accomplished by keeping the first-sphere parameters constant. In table 2, "X~" is the value calculated from the first-sphere parameters only. Occ(2) is the second-sphere oci/~.

2000

cupancy value which m i n i m i z e s the X2 value, tabulated in the column labelled "X~"- For spectra in which the addition of a second sphere did not improve the error value, OCC(2)max is the value for the second-sphere occupancy which causes a doubling of the X2 value calculated from the first-sphere parameters only. The second-

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ELECTRON

lo

k VECTOR

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te

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(angstroms -I)

Fig. 4. Expefiment~ (sofid) and calculated (dashed) EXAFS spectra of glasses containing a constant 60 mol% SiO2

146

P.L. Higby et al. / EXAFS study of alkali galliosilicate glasses

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~2

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16

18

20

ELECTRON k VECTOR ( a n g s t r o m s - t ) Fig. 5. Experimental (solid) and calculated (dashed) EXAFS spectra of glasses with a constant G a / R ratio of 1.0.

sphere occupancy can be converted to the proportion of six-coordinated gallium atoms by dividing the occupancy value given by 6 and multiplying by 100%. For example, Occ(2)min=0.06 would t300

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indicate that less than 1% of the gallium ions in the sample measured were in six-fold oxygen coordination. There is no apparent trend in the second-shell occupancy values, or even in the com-

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o ×

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400

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-200

-500

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t0

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ELECTRON k VECTOR ( a n g s t r 0 m s - t )

Fig. 6. Experimental (solid) and calculated (dashed) EXAFS spectra of sodium and potassium galliosilicate glasses.

P.L. Higby et al. / EXAFS stud)' of alkali galliosilicate glasses

positions whose EXAFS calculations are affected by the addition of a second sphere. Therefore, it is proposed that the decrease in X2 due to the addition of a second sphere into the EXAFS calculation is not conclusive evidence that an actual second sphere is affecting the photoelectron as it is ejected from the gallium atom. Figures 3, 4, and 5 show the experimental and calculated EXAFS data for representative glass compositions in each of the three compositional series studied. Again, not only is there good agreement between experimental and calculated spectra, but there is a striking similarity between the EXAFS for glasses of very different G a / N a ratios a n d / o r SiO 2 contents. The consistency of the results for sodium galliosilicate glasses strongly indicates that the gallium environment does not change with glass composition. EXAFS studies of certain other glass systems [17,18] have concluded that the atom of interest (Ge and Ti, respectively) could occupy two different coordination states, the ratio of which depends on the composition of the glass. In both cases, the spectra exhibited no large perturbation which could be ascribed to the presence

|

2500

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|

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147

of two distinct environments of the species being studied. In the case of lithium germanate glasses [17], the EXAFS spectra appear to be almost identical. However, addition of a six-fold oxygen sphere to the standard four-fold oxygen sphere about the Ge 4+ ion caused a systematic decrease in the error due to the mismatch between the theoretical and experimental spectra. In the case of the present study, the fact that the addition of a second coordination sphere does not systematically decrease the error value lends credence tot the assertion that Ga 3 + remains in four-fold coordination throughout the compositional ranges studied. Figure 6 illustrates the lack of effect of alkali identity on the EXAFS of glasses whose compositions are otherwise identical. The similarity between the two spectra indicates that the alkali identity does not affect the immediate Ga 3+ environment. EXAFS for NaGaSiO 4 and KGaSiO 4 crystals and their corresponding glass compositions are shown in fig. 7. X-ray diffraction analysis confirmed the crystalline samples as having structures

|

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KGaSi04 GLASS

1500

KGaSi04 CRYSTAL o

iO00

×

NaGaSi04 GLASS ¢_J

500

>k:

o~

NaGaSi04 CRYSTAL

,~ -500

K/ I 8

I I 8

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tlO

t2

!

4

|

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L

tB

20

ELECTRON k VECTOR (angstroms- ~) Fig. 7. Comparison of EXAFS spectra of potassium and sodium galliosilicate glasses and crystals.

148

P.L. Higby et al. / EXAFS study of alkali galliosilicate glasses

analogous to aluminosilicate structures which contain 4-fold coordinated A13÷ [31]. While the quality of the EXAFS spectra of the crystalline materials is not as good as that of the glasses, calculated spectra with parameters almost identical to those for the comparable glass compositions in each case (see table 1) result in the best fit for the crystal spectra. The constancy of the immediate gallium environment in the alkali galliosilicate glasses with changing composition leaves unresolved the question of the cause for the decrease in connectivity of the glass network as the G a / R ratio increases beyond 1.2. The information presented in this study can do no more than contend that no matter what structural role the Ga 3+ ion plays, it does not change oxygen coordination to the extent previously proposed [5,6]. The models which do not advocate a coordination change, such as the tricluster model [7] gain a measure of credence. However, no significant evidence has been found to substantiate this model specifically. A modifier structure in which an oxygen is missing from the tetrahedral configuration but the basic tetrahedral bond length and angles are maintained has also been suggested. The latter seems a plausible argument, but the data in this study cannot address this possibility directly.

4. Conclusions (1) The average G a - O bond distance in the alkali galliosilicate glass system is (0.183 _+ 0.001) nm, independent of glass composition and consistent with previously published work. (2) Ga first shell oxygen coordination is independent of glass composition within the 20% error of the measurement described in this study. (3) Ga coordination is 4-fold Ga 3÷ within the error of this measurement. The authors would like to acknowledge Josef C. Lapp for glass samples and helpful discussion, James Walker for computer aid, and Robert L. Snyder for arranging this collaboration. Part of this work was performed at the NSLS, Brookhaven.

References [1] J.C. Lapp and J.E. Shelby, J. Am. Ceram. Soc. 69 (1986) 126. [2] J.C. Lapp and J.E. Shelby, Adv. Ceram. Mat. 1 (1986) 174. [3] J.L. Piguet and J.E. Shelby, Adv. Ceram. Mat. 1 (1986) 192. [4] J.L. Piguet and J.E. Shelby, J. Am. Cerarn. Soc. 68 (1985) C232. [5] D.E. Day and G.E. Rindone, J. Am. Ceram. Soc. 45 (1962) 489. [6] V.K. Hunold and R. Briickner, Glastechn. Ber. 53 (1980) 149. [7] E.D. Lacy, Phys. Chem Glasses 4 (1963) 234. [8] A.E. Stern, Sci. Am. 234 (1976) 96. [9] P.A. Lee, P.H. Citrin, P. Eisenberger and B.M. Kincaid, Rev. Mod Phys. 53 (1981) 769. [10] E.A. Stern, Phys. Rev. B 10 (1974) 3027. [11] F.W. Lytle, D.E. Sayers and E.A. Stern, Phys. Rev. B 11 (1975) 4825. [12] E.A. Stern, D.E. Sayers and F.W. Lytle, Phys. Rev. B 11 (1975) 4836. [13] B. Lengeler and P. Eisenberger, Phys. Rev. B 21 (1980) 4507. [14] S.J. Gurman, J. Mat. Sci. 17 (1982) 1541. [15] D.A. McKeown, G.A. Waychunas and G.E. Brown, J. Non-Cryst. Solids (a) 74 (1985) 325; (b) 74 (1985) 349. [16] G.N. Greaves, J. Non-Cryst. Solids 71 (1985) 203. [17] A.D. Cox and P.W. MacMillan, J. Non-Cryst. Solids 44 (1981) 257. [18] D.R. Sandstrom, F.W. Lytle, P.S.P. Wei, R.B. Greegor, J. Wong and P. Schultz, J. Non-Cryst. Solids 41 (1980) 201. [19] J.C. Phillips, K.J. Baldwin, W.F. Lehnert, A.D. LeGrand and C.T. Prewitt, Nucl. Instr. and Meth. A 246 (1986) 182. [20] J.A. Bearden, Rev. Mod. Phys. 39 (1967) 78. [21] J.C. Phillips, J. Bordas, A.M. Foote, M.H.J. Kock and M.F. Moody, Biochem. 21 (1982) 830. [22] International Table for X-ray Crystallography, Macgillavry and Rieck, 1968. [23] B.K. Teo and P.A. Lee, J. Am. Chem. Soc. 101 (1979) 2815. [24] B.K. Teo, P.K. Lee and A.L. Simons, J. Am. Chem. Soc. 99 (1977) 3856. [25] B.K. Teo, P.A. Lee, A.L. Simons, P. Eisenberger and B.M. Kincaid, J. Am. Chem. Soc. 99 (1977) 3854. [26] J.C. Phillips, R. Bauer, J. Dunbar and J.T. Johnson, J. Inorg. Biochem. 22 (1984) 179. [27] V.A. Kolesova, Izv. Akad. Nauk SSSR, Ser. Khem. 4 (1966) 669. [28] M.E. Fleet, C.T. Herzberg, G.S. Henderson, E.D. Crozier, M.D. Osborne and C.M. Scarfe, Geochim. Cosmochim. Acta 48 (1984) 1455. [29] D.E. Day and G.E. Rindone, J. Am. Cer. Soc. 45 (1962) 579. [30] W.B. Simmons and D.R. Peacor, Am. Min. 57 (1972) 1711.