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EXAFS study of the gallium and iron environments in cation conducting pyrophosphate glasses
ELSEVIER
Physica B 208&209 (1995) 595-596
EXAFS study of the gallium and iron environments in cation conducting pyrophosphate glasses S. Poisson a'*, P. Berthet a, A. Belkebir b, A. Rulmont b aLaboratoire de Chimie des Solides, URA 446 CNRS, Bat. 414, Universitb Paris Sud, 91405 Orsay Cedex, France bLaboratoire de Chimie Inorganique Structurale, Institut de Chimie, Bdt. B6, Universitb de Liege, 4000 Liege 1, Belgium
Abstract EXAFS study of ion conducting pyrophosphate glasses prepared in the L i : O - G a 2 0 3 - P : O 5 and Na20-(Ga~-xFex)203-P205 systems may indicate that gallium atoms are distributed on tetrahedral and octahedral sites whereas iron atoms are always found in sixfold coordination. The analysis of our results suggests that some complex units such as G a P 2 0 ~ and F e P 2 0 ~ must be taken into account to describe the glassy network.
1. Introduction Phosphate glasses containing Li or Na ions exhibit cationic conductivity and have potential use as solid electrolytes. Among them, ternary glasses containing iron or gallium have been investigated by EXAFS [1,2]. A previous study of glasses prepared in the N a 2 0 - G a 2 O a - P 2 0 5 ternary system has shown that gallium environment depends on the condensation rate of the phosphate groups [2]. Furthermore, for pyrophosphate compositions the mean coordination of gallium atoms changes with the P / G a ratio. A more detailed study of pyrophosphate glasses was undertaken by substituting iron for gallium or replacing sodium by lithium.
2. Experimental Glasses melted were prepared by quenching mixtures with the required compositions between 800 ° C and 1200°C in air or between copper plates. *Corresponding author.
EXAFS spectra were recorded in transmission at the Ga and Fe K-edges on the EXAFS 1 beam line at LURE-DCI (Orsay, France) using a Si (3 3 1) channel-cut monochromator with a 2eV energy step. The samples used for the experiments were made of finely ground glasses (q~ < 25 ~tm) mixed with starch (c6nloOs) n powder and deposited between two parallel adhesive tapes. For all the samples, the absorption step was close to 1 and the absorption in the EXAFS region lower than 2. The data were analyzed using conventional procedure. The origin of the photoelectron energy was fixed at 10366eV for the Ga K-edge and 7121.5eV for the Fe K-edge. These values correspond to the inflexion points of the absorption steps of crystalline GaAsO,~ and FePO4 used as model compounds. Radial distribution functions (RDF) were obtained by Fourier transforming ka~((k) multiplied by a Hanning window. The main peak of each RDF was Fourier filtered in order to fit its structural parameters between 3.5 and 12 + 1~, -~. Back scattering amplitudes and phase shifts were extracted from the spectra of GaAsO4-quartz (4 x G a - O at 1.825/~ [2]) and FePO4-quartz (4x F e - O at 1.85 A [3]), fixing the Debye-Waller factor (a) at 0.05 A. The electron mean free path was written as 2 = k/y and
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 6 6 - 7
596
S. Poisson et al./Physica B 208&209 (1995) 595 596
Table 1 Structural parameters obtained at the Ga K-edge N
Table 2 Structural parameters obtained at the Fe K-edge
a
d~a--o 8Eo
r
(/~)
(rio
(eV)
(%)
NaaGa2(P2OT)9/4 Na2Ga(P207)5/4 NavGa3(P2OT)4 Na6Ga2(P2OT)3 NasGa(P207)2 Na9Ga(P2OT)3 Na13Ga(P2Ovh
5.5 5.5 5.9 6.1 6.1 6.2 6.4
0.091 0.092 0,096 0.095 0.094 0.097 0,097
1.88 1.89 1.89 1.90 1.90 1.90 1.91
- 1,5 - 2.9 - 1,0 0,8 - 0,6 - 0,9 - 0,4
0.3 0.3 0.5 0.6 0.8 1.1 0.3
NasGao.75Feo.25(P2OT)2 Na5Gao.5oFeo.so(P2Ov)z NasGao.EsFeo.75(P207)2
5.8 5.8 5.6
0.096 0.092 0.089
1.91 1.90 1.89
- 2,4 - 1,9 - 2.4
0.3 0.4 0.6
LiGaP207 Li7Ga3(P2OTh Li3Ga2(P2OT)3 LisGa(P207)2
5.0 5.0 5.3 5.7
0.087 0.089 0.093 0.094
1.86 1.86 1.88 1.89
-
0.4 0.5 0.6 0.6
1.1 2.1 0.9 0.1
a constant value ~, = 1 A - 2 was assumed for the analysis of all the data.
3. Results and discussion
3.1. Iron-free glasses
For all the glasses, the high value of the Debye Waller factor leads to consider the structural parameters (N and d) reported in Table 1 with some caution. Nevertheless, it is worth noting that, in the sodium glasses series as in the lithium series, the coordination numbers and the Ga O distance increase with the P / G a ratio. On the other hand, for a constant P / G a ratio, the structural parameters are lower for lithium glasses than for sodium glasses. The analysis of vibrational spectroscopy data reported previously I-2] led to discard a fivefold coordination for gallium in the glasses under study. Therefore, gallium atoms are distributed on both tetrahedral and octahedral sites. The evolution of this distribution with the P / G a ratio suggests that this distribution could result
8Eo (eV)
r (%)
N
a (/~)
dwo (A,)
NasFe(P207)2 NasGao.75Feo.25(P2OTh NasGao.5oFeo.5o(P2OT)2
7.5 7.0 7.4
0.095 0.079 0.090
1.97 1.97 1.97
- 2.0 - 2.4 - 2.5
0.4 0.9 0.5
NaFeP207 crystalline
6.9
0.081
1.97
- 0.9
1.5
from an equilibrium in the melt: GaOl
+ P20~ ~---GaP20191.
(1)
The above-mentioned differences between lithium and sodium glasses indicate that this equilibrium depends on the nature of the alkaline ion. 3.2. Iron-containing glasses
F r o m the structural parameters determined at the Fe K-edge (Table 2), which are similar to those obtained for crystalline N a F e P 2 0 7 [4], it is clear that all the iron atoms are found in sixfold coordination in the glasses. Thus, a significant difference is found between the glassy networks of NasFe(P2OT) 2 and NasGa(P2Ov)2: the GaO4 tetrahedra existing in the latter have no equivalent in the former. For mixed iron gallium glasses, the evolution of the gallium environment suggests that the fraction of chelated gallium atoms decreases when the iron content increases. It is likely that sixfold coordinated iron atoms are chelated ( F e P 2 O l l units) and therefore that equilibrium (1) is shifted by a lower number of available P207 groups.
References
[1] [2] [3] [4]
Lin Yunfei et al., J. Non-Cryst. Solids 112 (1989) 139. P. Berthet et al., Solid State Ionics 70-71 (1994) 476. A. Goiffon et al., Rev. Chim. Min. 23 (1986) 99. M. Gabelica-Robert et al., J. Solid State Chem. 45 (1982) 389.
Physica B 208&209 (1995) 595-596
EXAFS study of the gallium and iron environments in cation conducting pyrophosphate glasses S. Poisson a'*, P. Berthet a, A. Belkebir b, A. Rulmont b aLaboratoire de Chimie des Solides, URA 446 CNRS, Bat. 414, Universitb Paris Sud, 91405 Orsay Cedex, France bLaboratoire de Chimie Inorganique Structurale, Institut de Chimie, Bdt. B6, Universitb de Liege, 4000 Liege 1, Belgium
Abstract EXAFS study of ion conducting pyrophosphate glasses prepared in the L i : O - G a 2 0 3 - P : O 5 and Na20-(Ga~-xFex)203-P205 systems may indicate that gallium atoms are distributed on tetrahedral and octahedral sites whereas iron atoms are always found in sixfold coordination. The analysis of our results suggests that some complex units such as G a P 2 0 ~ and F e P 2 0 ~ must be taken into account to describe the glassy network.
1. Introduction Phosphate glasses containing Li or Na ions exhibit cationic conductivity and have potential use as solid electrolytes. Among them, ternary glasses containing iron or gallium have been investigated by EXAFS [1,2]. A previous study of glasses prepared in the N a 2 0 - G a 2 O a - P 2 0 5 ternary system has shown that gallium environment depends on the condensation rate of the phosphate groups [2]. Furthermore, for pyrophosphate compositions the mean coordination of gallium atoms changes with the P / G a ratio. A more detailed study of pyrophosphate glasses was undertaken by substituting iron for gallium or replacing sodium by lithium.
2. Experimental Glasses melted were prepared by quenching mixtures with the required compositions between 800 ° C and 1200°C in air or between copper plates. *Corresponding author.
EXAFS spectra were recorded in transmission at the Ga and Fe K-edges on the EXAFS 1 beam line at LURE-DCI (Orsay, France) using a Si (3 3 1) channel-cut monochromator with a 2eV energy step. The samples used for the experiments were made of finely ground glasses (q~ < 25 ~tm) mixed with starch (c6nloOs) n powder and deposited between two parallel adhesive tapes. For all the samples, the absorption step was close to 1 and the absorption in the EXAFS region lower than 2. The data were analyzed using conventional procedure. The origin of the photoelectron energy was fixed at 10366eV for the Ga K-edge and 7121.5eV for the Fe K-edge. These values correspond to the inflexion points of the absorption steps of crystalline GaAsO,~ and FePO4 used as model compounds. Radial distribution functions (RDF) were obtained by Fourier transforming ka~((k) multiplied by a Hanning window. The main peak of each RDF was Fourier filtered in order to fit its structural parameters between 3.5 and 12 + 1~, -~. Back scattering amplitudes and phase shifts were extracted from the spectra of GaAsO4-quartz (4 x G a - O at 1.825/~ [2]) and FePO4-quartz (4x F e - O at 1.85 A [3]), fixing the Debye-Waller factor (a) at 0.05 A. The electron mean free path was written as 2 = k/y and
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 6 6 - 7
596
S. Poisson et al./Physica B 208&209 (1995) 595 596
Table 1 Structural parameters obtained at the Ga K-edge N
Table 2 Structural parameters obtained at the Fe K-edge
a
d~a--o 8Eo
r
(/~)
(rio
(eV)
(%)
NaaGa2(P2OT)9/4 Na2Ga(P207)5/4 NavGa3(P2OT)4 Na6Ga2(P2OT)3 NasGa(P207)2 Na9Ga(P2OT)3 Na13Ga(P2Ovh
5.5 5.5 5.9 6.1 6.1 6.2 6.4
0.091 0.092 0,096 0.095 0.094 0.097 0,097
1.88 1.89 1.89 1.90 1.90 1.90 1.91
- 1,5 - 2.9 - 1,0 0,8 - 0,6 - 0,9 - 0,4
0.3 0.3 0.5 0.6 0.8 1.1 0.3
NasGao.75Feo.25(P2OT)2 Na5Gao.5oFeo.so(P2Ov)z NasGao.EsFeo.75(P207)2
5.8 5.8 5.6
0.096 0.092 0.089
1.91 1.90 1.89
- 2,4 - 1,9 - 2.4
0.3 0.4 0.6
LiGaP207 Li7Ga3(P2OTh Li3Ga2(P2OT)3 LisGa(P207)2
5.0 5.0 5.3 5.7
0.087 0.089 0.093 0.094
1.86 1.86 1.88 1.89
-
0.4 0.5 0.6 0.6
1.1 2.1 0.9 0.1
a constant value ~, = 1 A - 2 was assumed for the analysis of all the data.
3. Results and discussion
3.1. Iron-free glasses
For all the glasses, the high value of the Debye Waller factor leads to consider the structural parameters (N and d) reported in Table 1 with some caution. Nevertheless, it is worth noting that, in the sodium glasses series as in the lithium series, the coordination numbers and the Ga O distance increase with the P / G a ratio. On the other hand, for a constant P / G a ratio, the structural parameters are lower for lithium glasses than for sodium glasses. The analysis of vibrational spectroscopy data reported previously I-2] led to discard a fivefold coordination for gallium in the glasses under study. Therefore, gallium atoms are distributed on both tetrahedral and octahedral sites. The evolution of this distribution with the P / G a ratio suggests that this distribution could result
8Eo (eV)
r (%)
N
a (/~)
dwo (A,)
NasFe(P207)2 NasGao.75Feo.25(P2OTh NasGao.5oFeo.5o(P2OT)2
7.5 7.0 7.4
0.095 0.079 0.090
1.97 1.97 1.97
- 2.0 - 2.4 - 2.5
0.4 0.9 0.5
NaFeP207 crystalline
6.9
0.081
1.97
- 0.9
1.5
from an equilibrium in the melt: GaOl
+ P20~ ~---GaP20191.
(1)
The above-mentioned differences between lithium and sodium glasses indicate that this equilibrium depends on the nature of the alkaline ion. 3.2. Iron-containing glasses
F r o m the structural parameters determined at the Fe K-edge (Table 2), which are similar to those obtained for crystalline N a F e P 2 0 7 [4], it is clear that all the iron atoms are found in sixfold coordination in the glasses. Thus, a significant difference is found between the glassy networks of NasFe(P2OT) 2 and NasGa(P2Ov)2: the GaO4 tetrahedra existing in the latter have no equivalent in the former. For mixed iron gallium glasses, the evolution of the gallium environment suggests that the fraction of chelated gallium atoms decreases when the iron content increases. It is likely that sixfold coordinated iron atoms are chelated ( F e P 2 O l l units) and therefore that equilibrium (1) is shifted by a lower number of available P207 groups.
References
[1] [2] [3] [4]
Lin Yunfei et al., J. Non-Cryst. Solids 112 (1989) 139. P. Berthet et al., Solid State Ionics 70-71 (1994) 476. A. Goiffon et al., Rev. Chim. Min. 23 (1986) 99. M. Gabelica-Robert et al., J. Solid State Chem. 45 (1982) 389.