Ga K-edge EXAFS analysis on the coordination of gallium in PbO–Ga2O3 glasses

Journal of Non-Crystalline Solids 221 Ž1997. 199–207

Ga K-edge EXAFS analysis on the coordination of gallium in PbO–Ga 2 O 3 glasses Yong Gyu Choi a , Jong Heo a

a,)

, V.A. Chernov

b

Non-Crystalline Materials Laboratory, Department of Materials Science and Engineering, Pohang UniÕersity of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk, 790-784, South Korea b Siberian Synchrotron Radiation Center at Budker INP, 630090 NoÕosibirsk, Russia Received 7 April 1997; revised 2 July 1997

Abstract Ga K-edge extended X-ray absorption fine structure spectra of Ž X .PbO– Ž1 y X .Ga 2 O 3 glasses, where X is 0.7, 0.75 and 0.80 in mole fraction were recorded at room temperature to understand the coordination scheme of gallium in glasses. Analyses of the spectra indicate that most of the gallium forms GaO4 tetrahedra at the Ga–O bond distance between 0.1856 and 0.1861 nm with a negligible amount Žless than 5% of total Ga-polyhedra. of GaO6 octahedra. The longer Ga–O bond distance in glasses compared to that in b-Ga 2 O 3 crystal composed of similar GaO4 tetrahedra indicated the presence of oxygens bonded to three independent cations. q 1997 Elsevier Science B.V.

1. Introduction Heavy metal oxide glasses in the binary PbO– Ga 2 O 3 system can easily be formed via the conventional melt-quenching within the compositional range of 69 to 81 mol% PbO w1x. They are also stable against devitrification upon heating with a wide infrared transmittance up to the wavelength of 8 mm w2x. According to Shelby w2x, there seems to be a structural change at ; 75 mol% of PbO based on a maximum in the infrared transmittance at this composition. However, there was no evidence of such structural modification in the glass-transition temperatures although slight changes in the densities and molar volumes were observed w2x. Several other re-

)

Corresponding author. Tel: q82-562 279 2147; fax: q82-562 279 2399; e-mail: [email protected].

ports on the physical properties of both binary and ternary PbO–Bi 2 O 3 –Ga 2 O 3 glasses w1,3x suggested that the concentration of gallium in the glasses is one of the primary factors affecting the properties of such glasses. Therefore, the fundamental understanding on the short-range order structure and a coordination scheme of the gallium in the binary glasses is essential. Since both oxides in glasses are not traditional glass formers, the structural roles of gallium and lead have been a subject of many research works. Dumbaugh originally predicted that the gallium ions are surrounded by six oxygens at a distance of 0.197 nm w3x. More recent investigations using the IR, Raman and nuclear magnetic resonance ŽNMR. spectroscopies suggested that most gallium ions have fourfold coordination oxygens w4–6x. Quantitative analyses on the gallium coordination using X-ray and neutron diffraction suggested that less than 10% of

0022-3093r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 4 1 8 - 3

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Y.G. Choi et al.r Journal of Non-Crystalline Solids 221 (1997) 199–207

total gallium in glasses are surrounded by six oxygens w7x. Network-forming ability of Pb 2q originates from its high polarizability which allows distortion of polyhedral angles w8,9x. In fact, most of the research on PbO–Ga 2 O 3 glasses suggested a network-forming role for Pb 2q by forming PbO 3 and Žor. PbO4 polyhedra w4,5,10–12x. Hannon et al. w13x assigned Pb 2q as charge compensators and network-modifiers of gallium polyhedra. Formation of these types of polyhedra, on the other hand, causes shortage of oxygen if there are oxygens bridging only two cations. Miyaji et al. w7x suggested the presence of the three-coordinated oxygens in the glass with 50PbO–50GaO1.5 Žmol%. composition from the analyses of their neutron and X-ray diffraction study. The other reports w4–6,8,10,11x, however, did not comment on the formation of three-coordinated oxygens. The present study aims at the quantitative analysis of the coordination of gallium and oxygens in these binary PbO–Ga 2 O 3 glasses by analyzing the Ga K-edge EXAFS spectra. Debye–Waller factors and bond lengths of the Ga–O bonds were also obtained from three glasses with different compositions.

2. Experimental procedure 2.1. Sample preparation Starting materials are 99.9% yellow PbO and b-Ga 2 O 3 . 10 g batches with a composition of Ž X .PbO– Ž1 y X .Ga 2 O 3 , where X is 0.7, 0.75 and 0.8 Žmole fraction. were mixed and melted at 10008C for 15 min in air using a platinum crucible. Melting time was short to minimize the volatilization of lead oxide. A short melting time of 15 min after sufficient mixing of the starting materials seemed to be appropriate to obtain homogeneous samples due to the low viscosity of the melts. Yellow–orange colored samples were prepared by pouring the liquids onto a brass mold. Diffuse X-ray diffraction patterns confirmed the amorphous state of the samples. A 1:1:2 mixture of 99.9% pure-BaCO 3 , Ga 2 O 3 and GeO 2 to prepare a standard BaGa 2 Ge 2 O 8 crystal was melted at 15008C for 5 h in a platinum crucible and cooled to room temperature. In addition, a 1:1 mixture of

99.9% pure-PbO and Ga 2 O 3 to make the PbGa 2 O4 crystalline standard was melted at 13008C for 5 h in a platinum crucible. XRD patterns confirmed the formation of BaGa 2 Ge 2 O 8 and PbGa 2 O4 crystals. 2.2. EXAFS measurement Ga K-edge EXAFS spectra were recorded in the transmission mode at the VEPP-3 Ž2.0 GeV, 80–120 mA. storage ring ŽSiberian Synchrotron Radiation Center at Budker INP, Novosibirsk. on the EXAFS beamline with a SiŽ111. channel-cut monochromator at the fixed beam position. The monochromator was calibrated using L I , L II and L III edges of a Ta metal foil. Energy resolution was estimated to be 3 eV at the Ga K-edge. A fused silica mirror was used to reject higher harmonics. Samples with a Ga edge step of 0.5–1.0 were made by stacking about 10 layers of fine powders spread on the adhesive tape. Two ion chambers filled with Ar Ž5%. q He Ž95%. and Ar Ž100%. were used to detect the intensity of an incident and transmitted beam, respectively. In addition, measurements of Ga K-edge XANES spectra were done with a SiŽ511. channel-cut monochromator at the same beamline. Energy resolution under these conditions is estimated to be approximately 0.6 eV. All spectra were taken at room temperature. 2.3. Analyses of the extended X-ray absorption fine structure (EXAFS) spectra The experimental EXAFS spectrum, x Ž E . in energy space, of each sample was obtained from the measured absorption spectrum, m Ž E ., using the relation

x Ž E. s

m Ž E . y m0 Ž E . D m 0 Ž E0 .

,

Ž 1.

where E0 is the energy at the absorption edge, m 0 Ž E . is the atomic-like absorption after the edge and D m 0 Ž E0 . is the height at the edge step. E0 was determined from the first inflection point in the derivative of each Ga K-edge absorption spectrum recorded for each sample. The normalization constant, D m 0 Ž E0 ., for each spectrum was determined from the difference in the amplitude between m 0 Ž E . and the pre-edge total absorption, m Ž E ., at the ab-

Y.G. Choi et al.r Journal of Non-Crystalline Solids 221 (1997) 199–207

sorption edge energy E0 . m 0 Ž E . was obtained from the fourth order splines, which were selected to optimize the R-components of the Fourier transform of the EXAFS spectrum below R bkg . R bkg was kept at 0.08 nm w14x in order not to distort the first peak of the spectra. After each raw absorption spectrum was converted into an energy space EXAFS function, more than four x Ž E . were converted into x Ž k ., where k is the momentum of the photo-electron, and averaged at each composition to increase the signal to noise ratio. Fourier transforms of x Ž k . with a k 3-weighing factor and a Hanning window function, were performed w15x. The small-k window parameter Ž Dk1. and large-k window parameter Ž Dk 2. was main-

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tained at 1.0 and 0.2, respectively, for all spectra in order not to affect the left-hand side of the first coordination shell. k min and k max was kept to 3.0 " 0.1 and 12.0 " 0.3 Ay1 , respectively, for all Fourier transforms. Data-fitting procedures began with the theoretical calculation of the scattering amplitudes and phaseshift functions of the Ga–O pair using the FEFF6 program w16x. The calculated spectrum was compared with the experimental results from the standard BaGa 2 Ge 2 O 8 crystal to find the optimum over-all amplitude scaling factor, S0 , resulting in the best-fitting between two spectra. Fitting was made in Rspace within a range of the first peak between 0.09 and 0.22 nm in RDF of Ga K-edge EXAFS spec-

Fig. 1. k 3 -weighted Ga K-edge x Ž k . spectra from three glasses with composition Ža. 70PbO–30Ga 2 O 3 , Žb. 75PbO–25Ga 2 O 3 , Žc. 80PbO–20Ga 2 O 3 and crystalline standards Žd. BaGa 2 Ge 2 O 8 , Že. PbGa 2 O4 and Žf. b-Ga 2 O 3 .

Y.G. Choi et al.r Journal of Non-Crystalline Solids 221 (1997) 199–207

202

trum. Then, the value of S0 was used for further analyses of spectra recorded from the glasses. All parameters used in the data reduction processes were kept constant for both glasses and crystals. Extraction of the EXAFS interference functions, Fourier transforms into K or R spaces as well as non-linear least-squares fitting in R-space were carried out using the UWXAFS 3.0 software package w17x. The validity of the fitting was evaluated by the R-factors calculated as follows w18x: Rs

N Ý is1 Re Ž f i . N Ý is1

½ ½ Re Ž x˜ . i

2 2

q Im Ž f i . q Im Ž x˜ i .

2

5 , 5

2

Ž 2.

where f i is the discrepancy between the experimen-

tal results and the theoretical model. N is the number of the function evaluations.

3. Results Experimental Ga K-edge spectra were converted into K-space as in Fig. 1a–c for glass samples and Fig. 1d–f for standard crystal samples . Fig. 2 is the radial distribution function ŽRDF. curves obtained from each spectrum in Fig. 1. The over-all amplitude scaling factor, S0 , for the optimum fitting was 0.93 which is in the range that Mustre de Leon et al. w19x suggested. Results of the fitting between the experimental and calculated spectra for the standard

Fig. 2. Magnitude of Fourier transform of k 3-weighted x Ž k . spectra for Ža. 70PbO–30Ga 2 O 3 , Žb. 75PbO–25Ga 2 O 3 , Žc. 80PbO–20Ga 2 O 3 glasses and the standard crystals Žd. BaGa 2 Ge 2 O 8 , Že. PbGa 2 O4 and Žf. b-Ga 2 O 3 . Single-shell fitting for Ža. – Žd. and two-shell fitting for Že. and Žf. were presented. Solid lines and dots are the experimental and the fitted results, respectively. Note that the phase shifts were not corrected.

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Table 1 ˚ Results of the fitting by assuming the presence of GaO4 tetrahedra with a uniform bond distance of 1.83 A Sample Žmol%. BaGa 2 Ge 2 O 8 70PbO–30Ga 2 O 3 75PbO–25Ga 2 O 3 80PbO–20Ga 2 O 3

E0 ŽeV.

NGa – O

˚. R Ga – O ŽA

˚2. s 2 ŽA

R-factor

10369.4 10371.1 10370.3 10369.6

4.00 Žfixed. 4.01 Ž0.28. 4.03 Ž0.26. 4.05 Ž0.21.

1.833 Ž0.003. 1.856 Ž0.005. 1.858 Ž0.004. 1.861 Ž0.003.

0.0030 Ž0.0005. 0.0038 Ž0.0006. 0.0043 Ž0.0006. 0.0041 Ž0.0004.

0.001 0.002 0.002 0.001

Values in parentheses are estimated uncertainties.

BaGa 2 Ge 2 O 8 crystal were good as indicated by the small uncertainties and R-factors in Table 1. Peaks from the first-nearest neighbors were well resolved compared to those from the atoms located further away, which is typical in the spectra for glasses. Therefore, higher-order peaks in Fig. 2a–c were excluded from further analysis even though there seems to be a systematic amplitude change with composition. Peaks originating from the secondnearest neighbors in BaGa 2 Ge 2 O 8 crystal were not clearly identified in Fig. 2d probably because of the complexity of the structure and the imperfect crystallinity of this home-made crystal. Nevertheless, the peak from the first Ga–O shell was clear enough to be used as a standard since the main analysis of the present work is concentrated on the first Ga–O shell environment. In fact, S0 s are comparable to those obtained by Newville w20x, which supports the adequacy of our calculation. Therefore, the BaGa 2 Ge 2 O 8 crystal was used as a standard since a simple singleshell fitting incorporating the first-nearest neighbors only was possible with a small Debye–Waller factor. First, curve fittings were attempted by considering the first-nearest neighbors incorporating four oxygens around the central Ga atom at the distance of 0.183 nm. Results of the fitting obtained from the average of at least four experimental EXAFS spectra from each composition are in Table 1. Fitted Ga–O distances of 0.1833 nm for a BaGa 2 Ge 2 O 8 crystal well coincides with the reported value of 0.183 nm w21x. Ga–O bond distances for glasses were within 0.1856–0.1861 nm with the coordination numbers of ; 4.03. Agreement between the fitted curves and the experimental results within the first-nearest neighbors was excellent as shown in Fig. 2a–c. The peak at the left hand-side of the main peak, originating from the background of EXAFS spectrum and Žor. the truncation ripple and therefore, does not seem to carry any valuable structural information.

Thermal and positional disorder can affect the determination of accurate structural parameters in the glasses. Effects of the disorder on the interatomic distances were evaluated from the model of Eisenberger and Brown w22x. If both s 2rR and s 2rl are smaller than 0.001 nm, where s 2 is the Debye– Waller-type factor, R is the bond length and l is the electron mean free path, the disorder is not large enough to affect the coordination number and bond lengths derived from EXAFS spectra. Results in Table 1 satisfied the above criteria and therefore, thermal and positional disorders do not seem to affect the fitting procedure significantly.

4. Discussion Ga–O bond distances Ž R Ga – O . of 0.1857 " 0.0003 nm for PbO–Ga 2 O 3 glasses are longer than these in BaGa 2 Ge 2 O 8 crystal ŽTable 1. which is approximately 0.183 nm. Considering the typical errors Ž"10%. involved in the calculation of the coordination number, the possibility of the GaO6 octahedra formation needs to be addressed. In fact, Miyaji et al. w7x suggested that approximately 10% of all Ga are surrounded by six oxygens in 50PbO–50GaO1.5 glass based on the calculated average coordination number of 4.2 in their neutron diffraction spectrum. Furthermore, the average bond distance tends to increase with the formation of GaO6 as reported for several gallate crystals w21,23–25x. The longer Ga–O bond lengths in PbO–Ga 2 O 3 glasses compared to those in crystals with four-coordinated Ga Ži.e., BaGa 2 Ge 2 O 8 . seem to indicate that some of the gallium atoms may form GaO6 octahedra. Fukumi and Sakka w26x used Ga–O bond lengths to estimate the portion of GaO6 octahedra in alkali

Y.G. Choi et al.r Journal of Non-Crystalline Solids 221 (1997) 199–207

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Table 2 ˚ and 2.00 A, ˚ Results of fitting by assuming the presence of both GaO4 tetrahedra and GaO6 octahedra where bond length of Ga–O is 1.83 A respectively Sample Žmol%.

GaO4 tetrahedra NGa – O

b-Ga 2 O 3 70PbO–30Ga 2 O 3 75PbO–25Ga 2 O 3 80PbO–20Ga 2 O 3

4.0 Žfixed. 3.94 3.85 4.20

GaO6 octahedra

˚. R Ga – O ŽA 1.836 1.871 1.875 1.855

s

2

˚ ŽA

2.

0.0034 0.0026 0.0031 0.0029

and alkaline–earth metal gallate glasses. The fraction Ž X . of GaO6 octahedra was calculated by R Ga – O s

 1.83 = 4 Ž 1 y X . q 2.00 = 6 X 4 .  4Ž 1 y X . q 6 X 4

Ž 3.

Results of the calculation by substituting the values in Table 1 into Eq. Ž3. suggest that approximately

R-factor

NGa – O

˚. R Ga – O ŽA

s

6.0 Žfixed. 0.3 0.0001 0.015

1.988 1.823 1.739 1.812

0.0075 y0.014 y0.021 y0.013

2

˚ ŽA

2.

0.007 0.004 0.005 0.006

13% of the total Ga are surrounded by six oxygens in 80PbO–20Ga 2 O 3 glass. To identify the presence of the Ga coordinated with six oxygens, another fitting was carried out assuming the presence of both tetrahedra and octahedra where the Ga–O bond distance is 0.183 and 0.200 nm, respectively as in b-Ga 2 O 3 crystal w23x. Results of the non-linear

Fig. 3. Fourier-filtered k 3 x Ž k . of the first gallium coordination shell for Ža. 70PbO–30Ga 2 O 3 , Žb. 75PbO–25Ga 2 O 3 , Žc. 80PbO–20Ga 2 O 3 , Žd. BaGa 2 Ge 2 O 8 , Že. PbGa 2 O4 and Žf. b-Ga 2 O 3 . Solid and dotted lines represent the real and imaginary part of the EXAFS function, respectively.

Y.G. Choi et al.r Journal of Non-Crystalline Solids 221 (1997) 199–207

least-squares fitting in Table 2, however, showed larger R-factors compared to those in Table 1 obtained for the case of GaO4 tetrahedra. Further, Debye–Waller factors calculated from the glasses with the assumption of GaO6 formation became negative which is physically meaningless. All these fitting parameters suggest the inadequacy of the proposed model of GaO6 octahedra formation. Therefore, it seems more reasonable to interpret the structure of these glasses based on the results in Table 1 with a single uniform Ga–O distance. The Fourier-filtered EXAFS functions in the range 0.09–0.22 nm for the samples and crystal samples are presented in Fig. 3. Smooth Gaussian-like amplitude envelops for all glasses indicate the adequacy of the single-shell fitting. On the other hand, non-Gaussian amplitude envelops with a broad shoulder Žindicated with an arrow in Fig. 3e,f. for PbGa 2 O4 and b-Ga 2 O 3 crystal samples indicate the presence of two different Ga–O bond distances Ži.e., two different coordination schemes. in these crystals. We, therefore, conclude that there is a negligible amount of GaO6 octahedra our samples, and most Ga have four oxygens at a distance of ; 0.1858 nm in the first coordination shell. Ga K-edge XANES spectra of the glasses and b-Ga 2 O 3 , normalized with the edge continuum jumps after a removal of the background, are shown in Fig. 4. One can clearly find that XANES spectra for all glass samples are similar. However, the spectrum recorded from the b-Ga 2 O 3 crystal sample deviates considerably. It shows a broad peak at the edge and the second peak at an energy approximately 3 eV larger than that of the first peak. Presence of these two peaks in the XANES spectrum recorded from the crystalline b-Ga 2 O 3 is due to the atomic absorption effect influenced by the coordination scheme of Ga w27x. It is well known that Al K-edge XANES spectra in alluminosilicate glasses have two peaks near the edge separated by approximately 3 eV when both four and six coordinated Al exist w28x. A similar situation was also found when Ge forms two different coordination polyhedra in Ge-containing oxides w29x. In this case, the peak due to the Ge surrounded by six oxygens is shifted to higher energy side relative to that from the four-coordinated Ge. In addition, the amplitude of the former is larger than that of the latter. Therefore, the second peak located

205

Fig. 4. Ga K-edge XANES spectra of Ža. b-Ga 2 O 3 crystal and Žb. 70PbO–30Ga 2 O 3 , Žc. 75PbO–25Ga 2 O 3 and Žd. 80PbO–20Ga 2 O 3 glasses.

at the higher energy side in b-Ga 2 O 3 XANES spectrum is most probably associated with Ga surrounded by six oxygens. We constructed the approximate edge spectra of b-Ga 2 O 3 crystal and glasses. The normalized absorption AŽ E . of the edge region could be described with an arctangent function and two Lorentzian functions by the following equation; AŽ E . s

½

1

1 q

2

p

arctan

ž

E y E0

G

/5

q Lorentz Ž E y E1 . q Lorentz Ž E y E2 . , Ž 4. where E0 is the inflection point of edge and G is related with the width of the edge step. E1 and E2 are the positions of energy where a resonance of four- and six-coordinated Ga takes place, respectively. The first term of Eq. Ž4. is a well-known lineshape function of the continuum step w30x. The result of the fitting for the b-Ga 2 O 3 crystal, as

206

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changes in the coordination of Ga. However, the longer Ga–O bond distance found for PbO–Ga 2 O 3 glasses compared to those in crystalline BaGa 2 Ge 2 O 8 seems to suggest the presence of the three-coordinated oxygens. In case of PbGa 2 O4 crystal, a wide range of Ga–O bond lengths between 0.181 and 0.187 nm, has been identified w31x. This difference in bond lengths is closely related to the coordination scheme of oxygens. In this crystal, oxygens bonded to three cations have two Ga at a distance of ; 0.186 nm with one lead at 0.235 nm. On the other hand, those bonded to two Ga only have a Ga–O bond distance of 0.182 nm. Valence of the bond as a function of bond length can be calculated using the bond–valence method w32–36x. The most common empirical expression being used to calculate the valence, n i j , of the bond with a length, R i j , is w32x

n i j s exp

Fig. 5. Simulated absorption edge spectra of b-Ga 2 O 3 crystal and 70PbO–30Ga 2 O 3 glass, composed of Ža. a arctangent function and two Lorentzian functions which correspond to resonance of Žb. four-coordinated Ga and Žc. six-coordinated Ga, respectively. Dots and solid line is experimental spectrum and calculated spectrum, respectively.

shown in Fig. 5, indicated that E1 , E2 and G are 10371.5, 10375.0 and 0.46 eV, respectively. For glass samples, values of E1 and E2 obtained from the fitting of b-Ga 2 O 3 crystal were used. Resulting G from three glasses was in the range 0.5–0.7 eV. From the relative proportion between GaO4 tetrahedra and GaO6 octahedra in b-Ga 2 O 3 crystal, which is 50:50, we can calculate the fraction of each polyhedron in samples from the areas of each Lorentzian peak. Calculated fraction of six-coordinated Ga in this glass sample was approximately 5%. In addition, overall coordination of oxygens around Ga in Table 1 also suggested that the fraction of six-coordinated Ga is less than 3%. Therefore, we conclude that most of the Ga atoms Ž; 95%. in glasses have a coordination number which is ; 4. Changes in the properties of this glass suggested by Shelby w2x do not appear to be related to the structural changes in glasses since there are no abrupt

½

Ž Bi j y R i j . b

5

,

Ž 5.

where Bi j is the bond valence parameter and b is commonly taken as an universal constant of 0.037 nm w34x. The bond valence parameter Ž Bi j . for Ga and Pb is reported to be 1.730 and 2.112, respectively w33x. Pauling’s second rule w37x states that the overall valence of the bond which any specific oxygen makes should have 2 Ž"0.1. valence units Žvu.. Oxygens bonded to two Ga atoms and one Pb at the distance of 0.186 nm and 0.235 nm, respectively, have the calculated valence of a single Ga–O and Pb–O bond of 0.704 vu and 0.526 vu. Then, total valence of the three coordinated oxygens becomes 1.934 vu which satisfies the Pauling’s rule. A similar bonding scheme can be expected in PbO–Ga 2 O 3 glasses. A neutron diffraction study w7x on the glass in this system showed an Pb–O distance in PbO4 tetrahedra of 0.23 nm. For three-coordinated oxygens with a Ga–O bond distance of 0.186 nm, total valence becomes 2.01 vu which is again within Pauling’s criterion. Formation of the three-coordinated oxygens is also suggested from the Raman spectra of alkali and alkaline–earth gallate glasses w25x. However, existence of oxygens bonded to only two Ga can not be totally ignored. First, it would be difficult to form a random network structure if all of the oxygens bonded to Ga are three coordinated even

Y.G. Choi et al.r Journal of Non-Crystalline Solids 221 (1997) 199–207

though the amount of Ga 2 O 3 in glasses is small. Further, Raman spectra of these glasses suggested the presence of the vibration between two GaO4 tetrahedra connected through ‘two-coordinated oxygen’ w5,6,10x. In this case, one needs charge compensating cations to maintain the charge neutrality against the net negative charge of the GaO4 tetrahedra. The structure of crystalline PbGa 2 O4 indicated that there are Pb 2q ions located in the vicinity of two-coordinated oxygens to compensate the negative charge of GaO4 tetrahedra. A similar charge-compensating role of Pb 2q in addition to its network-forming role in the form of PbO 3 and Žor. PbO4 was also suggested from an XPS study w38x. At present, a relative amount of two-coordinated oxygens among those bonded to Ga seems to be small compared to three-coordinated ones. This investigation does not give any direct information on the coordination scheme of oxygens bonded to Pb. A more detailed analysis on the Ga–K edge spectra recorded at the liquid nitrogen temperature is in progress.

5. Conclusion Analyses of the Ga K-edge EXAFS spectra suggested that most of the gallium ions are surrounded by four oxygens forming GaO4 tetrahedra. The amount of GaO6 octahedra is less than 5% of total Ga-polyhedra. Ga–O bond lengths in glasses were in the 0.1856–0.1861 nm range which is longer than those normally found in the crystalline compounds. This variation in bond length suggests the presence of three-coordinated oxygens in the network structure. The bond-valence analysis suggested that two of the bonds forming three-coordinated oxygens are connected to Ga while the other is to one Pb.

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