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Mixed former effect in sodium phospho tellurate glasses
SOUD STATE Solid State Ionics 86-88
ELSEYIER
(1996) 447-45
IONlr?c
1
Mixed former effect in sodium phospho tellurate glasses G.D.L.K. Jayasinghe”‘“,
F?W.S.K. Bandaranayake”,
J.L. Souquetb
“Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka hLaboratoire d’lonique et d’Electrochimie des Solides de Grenoble (URA CNRS D 1213), INPG, ENSEEG, BP 75, 38402, Saint Martin d ‘H&es, France
Abstract Glasses in the binary system xNa,P,O,-( 1 - x)Na,Te,O, have been synthesised for 0 5 n 5 1. For all compositions, the Nevertheless for compositions near x = 0.4 glasses seem to be glasses are transparent and appear homogeneous. heterogeneous on SEM observations. Glass transition temperatures for all the compositions are between 220 and 310°C. Conductivity data have been obtained using impedance techniques in the IOO-250°C temperature range. The aT product obeys an Arrhenius relationship with a composition-independent pre-exponential term A. The value of log A = 4.8 is in agreement with an indirect interstitial mechanism mode1 for sodium cation migration. Isothermal conductivity curves with x show a maximum in the ionic conductivity. This mixed network former effect may be justified by assuming an endothermic mixture between sodium phosphate and sodium tellurate.
Keywords: Glasses; Conductivity;
Glass transition
temperature;
Network
1. Introduction Ionic conductivity in inorganic oxide glasses is mainly due to the migration of monovalent cations such as alkali or silver cations. These cations are ionically bonded to a covalent macromolecular skeleton made of network former oxides. It has often been observed that, at constant network modifier content, ionic conductivity increases when the network former is progressively substituted by another. Several systematic studies, most of the investigated systems containing a mixture of B,O, and P205 as network formers, have been reported [l-3]. From a
*Corresponding
author.
0167-2738/96/$15.00 Copyright PII SOl67-2738(96)00173-7
01996
Elsevier
Science
B.V. All rights
general point of view such an ionic conductivity enhancement appears when the macromolecular network is made of a mixture of two network formers with different coordination polyhedra. For example, in the case of B,O, and P205 the coordination polyhedra in pure oxides are triangles and tetrahedra respectively. In the present work we have associated P205 with TeO, in which tellurium forms trigonal bi-pyramids with surrounding oxygens atoms. Sodium oxide has been chosen as a modifier to continue the previous study on the Na,O-TeO, binary glassy system [4]. In order to compare glasses with the same number of network former cations we have chosen to write TeO, as Te,O, and then prepare glasses in the 0.5Na,O-0.5[xP,O,-( 1 - x)Te,O,] system. The same system may also be described as a solution of reserved
G.D.L.K.
448
Jayusinghe
et ul. I Solid
State lonics
86-88
(1996)
447-4.51
sodium metaphosphate NaPO, with sodium tellurate Na,Te,O, according to the formula xNa,PZO,-( 1 x)Na,Te,O,.
2. Experimental
2.1. Glass preparution Glasses were obtained by melting appropriate quantities of Na,CO, (99% Janssen Chimica), TeO, (99% Janssen Chimica) and NH,H,PO, (99% Janssen Chimica) in a platinum crucible. Mixtures were melted at 800°C for 40 min at a heating rate of 3 K min ‘. The melts were quenched in an aluminium mould in air at room temperature. Glass pellets of 12 mm diameter and 1.5 mm thickness were prepared for ten compositions in between 0 5 x 9 1. The amorphous state was confirmed by X-ray diffraction. Chemical analysis by plasma emission spectroscopy showed less than 1% difference between nominal compositions and synthesised samples. All glasses are transparent and the colour varies from light yellow to colourless as TeO, is substituted by P20s. Glass transition temperatures T, were determined by DTA or DSC with a heating rate of 10°C min _I. The variation of T, as a function of x is shown in Fig. 1. T, values range from 220-310°C with a maximum near x = 0.7. The two limiting composi-
225
0
0.2
06
0.4
08
IO
x Fig. 1. Variation in glass transition temperatures T, with x in the xNazP,O,-(
1-x)NazTe20,
glass system.
glass
tions for x = 0 and x = 1 correspond to defined crystalline compounds Na,Te,O, and NaPO,, for which melting temperatures T, of the crystalline state are 435°C and 588°C respectively. T, values for these compositions verify the empirical relationship T,(K)IT,(K) I- 2/3. The homogeneity of glass samples were checked by SEM on fresh surfaces. Most of the glasses appear to be homogeneous except for compositions near x = 0.4 for which a heterogeneous aspect, probably due to a phase separation, could be observed as shown in Fig. 2. 2.2. Conductivity
325
200
Fig. 2. Heterogeneous aspect of 0.4NazP20,-0.6NaZTe,03 observed by SEM.
measurements
Both flat surfaces of glass pellets were painted with a solution of colloidal graphite in isopropanol (DAG 154 Achenson). The a.c. conductivity measurements were carried out by an impedance spectroscopy technique in the frequency range 5 Hz-13 MHz using a Hewlett Packard impedance analyzer 4192A LF. Impedance plots were obtained in the temperature range lOO-250°C. The bulk impedance defines an arc of a circle whose center lies below the real axis. The angle of depression is related to the width of the distribution of electrical relaxation time. Interestingly, this angle of depression has a maximum value near x =0.5 where a heterogeneous aspect was observed by SEM (Fig. 3).
G.D.L.K. Jayasinghr et al. I Solid State Ionics 86-88
449
(1996) 447-451
-55
-6.0
-z -6.5 " t", 0 x -70
-75
0
0.2
04
06
06
v
1.--// cl
-80 0
1.0
0.2
04
U6
08
X
X Fig. 3. Depression angle of the bulk impedance as a function of
Fig. 4. Isothermal conductivity variations with the network former
glass composition.
composition at 190°C.
3. Results Below their respective glass transition temperatures, conductivity data of all glass samples obey an Arrhenius equation,
Activation energies E, and values of pre-exponential factors A for each composition are listed in Table 1. In the whole composition range we have observed that the pre-exponential term is composition-independent with a mean value of log A=4.8?0.40. Consequently, isothermal variations of conductivity (Fig. 4) are correlated with variations of activation energy (Fig. 5) only.
065 -
0
Table
04
0.6
06
1.0
X
Fig. 5. Variation network
4. Discussion For cationic charge carriers
0.2
former
of the conductivity
activation
energy
vs. the
composition.
pairs or, in other words, double occupied sodium sites [5,6]. The concentration of these interstitial pairs, C,, may be very small compared with the concentration of total alkali cations C and is a
conductive glasses, generally the are positively charged interstitial
I
Conductivity
parameters as a function of x in the xNa,P,O,-(
I -x)Na,Te,O,
glassy
system
x
0
0.10
0.20
0.37
0.40
0.48
0.60
0.70
0.80
0.90
I .oo
E,, W’)
0.80
0.79
0.78
0.75
0.71
0.70
0.65
0.65
0.67
0.69
0.73
4.58
4.72
4.95
4.85
4.75
4.82
4.48
4.67
4.85
5.22
5.15
LogA(Scm
’
K)
450
G.D.L.K. fayasinghe
et al. I Solid State lonics 86-88
temperature dependent exponential function of the free energy necessary to form an interstitial pair: C, = Cexp(s) where AC,= AH, - TAS, is the free energy associated with the formation of an interstitial pair. When an electric field is applied the electrochemical mobility U, of the interstitial pair:
(1996) 447-451
for log A are then 4.8 for NaPO, and 4.9 for Na,Te,O,. These estimated values are very close to the experimental mean value of log A =4.8 indicating that the interstitial pair formation and migration processes are purely enthalpic for these glassy electrolytes and the entropic terms AS, and AS, are negligible. 4.2. Comment on conductivity composition
variations
with
(3) where 1 is the mean jump distance between cationic sites, “0 is the attempt frequency and AC, the migration free energy. An expression for the conductivity u+ = FC, U, can be obtained by substituting Eqs. (2) and (3) and this can be rearranged as r+T=
cg3!
exp(g!i)
exp(+l) (4)
This expression can be compared with the experimental Arrhenius equation (1) and the constants can be identified as FWV A= o 6R
exp
(
iAS,
+ AS,
)
According to Eq. (6) the activation energy, E,, is a function of the defect formation enthalpy, AH,, and migration enthalpy, AH,,,, and it is difficult to state which is responsible for variation in E, with x. In the weak electrolyte model the mobility and hence AH,,, are assumed to be independent of the macromolecular network and variations in E, are related to variations in AH, only [7]. This approach depicts the interstitial pair formation as a dissociation of the network modifier, Na,O, in the network former mixture (P,O,-Te,O,) according to the dissociation equilibrium, Na,OsNa’
+ NaO
In this description the Na+ species may be identified as the positively charged interstitial pair and
,
E, = +AH, + AH,. Comparison of these theoretical values for A and EC, with experimental data in Table 1 leads to the following remarks.
Ionized species in very low concentration are expected to have an ideal behaviour. Consequently, the variation in AH, depends only on the variation of the partial enthalpy of the network modifier, aN,+.
4.1. Comments
In the pseudo-binary system xNa,P,O,--( lx)Na,Te,O,, Na,O is a common constituent. In case of an endothermic mixture between sodium phosphate and sodium tellurate, the partial enthalpy would increase leading to a decrease of AH, and activation energy E,. The microscopic origin of this endothermic mixing enthalpy could be related to the structural incompatibility between PO, tetrahedra and TeO, bipyramids. In such a case we may also expect that when the entropic term is not large enough to compensate for the positive enthalpic term the mixture would tend to phase separate as observed by SEM for composition near x=0.5.
on the pre-exponential
term A
Assuming a homogeneous distribution of the alkali cations in the glass, 1 and C can be estimated from density data. Densities for the two limiting compositions NazTe,O, and NaPO, measured by picnometry are 4.4 g cm -’ and 2.5 g cmm3 respectively. The sodium cation concentration C and the jump distance 1=( 1lC)“3 calculated from the density data 0 are 1.4X 102’ atom cm-’ and 4.16 A for vitreous Na,Te,O, and 1.5X IO’* atom cmm3 and 4.05 A for NaPO,. Taking the approximate value of 10” Hz for V, and neglecting the entropic term, estimated values
G.D.L.K. Juyasinghe
et (11. I Solid State Ionics 86-88
Acknowledgments GDLKJ and PWSKB are grateful to the European Economic Community for providing fellowships to work in France under research grant no. C 1I-CT910948.
References [I] T. Tsuchiya (1987) 857.
and T. Moriya,
J. Non-Cryst.
Solids
95/96
(1996) 447-451
4.51
[2] A. Magistris, G. Chiodelli and M.J. Duclot, Solid State Ionics 9/lO (1983) 611. [3] M. Tatsumisago, K. Yoneda, N. Machida and T. Minami, .I. Non-Cryst. Solids 95/96 (1987) 197. [4] G.D.L.K. Jayasinghe, D. Coppo, P.W.S.K. Bandaranayake and J.L. Souquet, Solid State Ionics 76 (1995) 297. [S] M.D. Ingram, J. Amer. Ceram. Sot. 63 (1980) 248. [6] S.R. Elliot, Solid State Ionics 27 (1988) 131. [7] D. Ravaine and J.L. Souquet, Phys. Chem. Glasses I8 (1977) 21.
ELSEYIER
(1996) 447-45
IONlr?c
1
Mixed former effect in sodium phospho tellurate glasses G.D.L.K. Jayasinghe”‘“,
F?W.S.K. Bandaranayake”,
J.L. Souquetb
“Department of Physics, University of Peradeniya, Peradeniya, Sri Lanka hLaboratoire d’lonique et d’Electrochimie des Solides de Grenoble (URA CNRS D 1213), INPG, ENSEEG, BP 75, 38402, Saint Martin d ‘H&es, France
Abstract Glasses in the binary system xNa,P,O,-( 1 - x)Na,Te,O, have been synthesised for 0 5 n 5 1. For all compositions, the Nevertheless for compositions near x = 0.4 glasses seem to be glasses are transparent and appear homogeneous. heterogeneous on SEM observations. Glass transition temperatures for all the compositions are between 220 and 310°C. Conductivity data have been obtained using impedance techniques in the IOO-250°C temperature range. The aT product obeys an Arrhenius relationship with a composition-independent pre-exponential term A. The value of log A = 4.8 is in agreement with an indirect interstitial mechanism mode1 for sodium cation migration. Isothermal conductivity curves with x show a maximum in the ionic conductivity. This mixed network former effect may be justified by assuming an endothermic mixture between sodium phosphate and sodium tellurate.
Keywords: Glasses; Conductivity;
Glass transition
temperature;
Network
1. Introduction Ionic conductivity in inorganic oxide glasses is mainly due to the migration of monovalent cations such as alkali or silver cations. These cations are ionically bonded to a covalent macromolecular skeleton made of network former oxides. It has often been observed that, at constant network modifier content, ionic conductivity increases when the network former is progressively substituted by another. Several systematic studies, most of the investigated systems containing a mixture of B,O, and P205 as network formers, have been reported [l-3]. From a
*Corresponding
author.
0167-2738/96/$15.00 Copyright PII SOl67-2738(96)00173-7
01996
Elsevier
Science
B.V. All rights
general point of view such an ionic conductivity enhancement appears when the macromolecular network is made of a mixture of two network formers with different coordination polyhedra. For example, in the case of B,O, and P205 the coordination polyhedra in pure oxides are triangles and tetrahedra respectively. In the present work we have associated P205 with TeO, in which tellurium forms trigonal bi-pyramids with surrounding oxygens atoms. Sodium oxide has been chosen as a modifier to continue the previous study on the Na,O-TeO, binary glassy system [4]. In order to compare glasses with the same number of network former cations we have chosen to write TeO, as Te,O, and then prepare glasses in the 0.5Na,O-0.5[xP,O,-( 1 - x)Te,O,] system. The same system may also be described as a solution of reserved
G.D.L.K.
448
Jayusinghe
et ul. I Solid
State lonics
86-88
(1996)
447-4.51
sodium metaphosphate NaPO, with sodium tellurate Na,Te,O, according to the formula xNa,PZO,-( 1 x)Na,Te,O,.
2. Experimental
2.1. Glass preparution Glasses were obtained by melting appropriate quantities of Na,CO, (99% Janssen Chimica), TeO, (99% Janssen Chimica) and NH,H,PO, (99% Janssen Chimica) in a platinum crucible. Mixtures were melted at 800°C for 40 min at a heating rate of 3 K min ‘. The melts were quenched in an aluminium mould in air at room temperature. Glass pellets of 12 mm diameter and 1.5 mm thickness were prepared for ten compositions in between 0 5 x 9 1. The amorphous state was confirmed by X-ray diffraction. Chemical analysis by plasma emission spectroscopy showed less than 1% difference between nominal compositions and synthesised samples. All glasses are transparent and the colour varies from light yellow to colourless as TeO, is substituted by P20s. Glass transition temperatures T, were determined by DTA or DSC with a heating rate of 10°C min _I. The variation of T, as a function of x is shown in Fig. 1. T, values range from 220-310°C with a maximum near x = 0.7. The two limiting composi-
225
0
0.2
06
0.4
08
IO
x Fig. 1. Variation in glass transition temperatures T, with x in the xNazP,O,-(
1-x)NazTe20,
glass system.
glass
tions for x = 0 and x = 1 correspond to defined crystalline compounds Na,Te,O, and NaPO,, for which melting temperatures T, of the crystalline state are 435°C and 588°C respectively. T, values for these compositions verify the empirical relationship T,(K)IT,(K) I- 2/3. The homogeneity of glass samples were checked by SEM on fresh surfaces. Most of the glasses appear to be homogeneous except for compositions near x = 0.4 for which a heterogeneous aspect, probably due to a phase separation, could be observed as shown in Fig. 2. 2.2. Conductivity
325
200
Fig. 2. Heterogeneous aspect of 0.4NazP20,-0.6NaZTe,03 observed by SEM.
measurements
Both flat surfaces of glass pellets were painted with a solution of colloidal graphite in isopropanol (DAG 154 Achenson). The a.c. conductivity measurements were carried out by an impedance spectroscopy technique in the frequency range 5 Hz-13 MHz using a Hewlett Packard impedance analyzer 4192A LF. Impedance plots were obtained in the temperature range lOO-250°C. The bulk impedance defines an arc of a circle whose center lies below the real axis. The angle of depression is related to the width of the distribution of electrical relaxation time. Interestingly, this angle of depression has a maximum value near x =0.5 where a heterogeneous aspect was observed by SEM (Fig. 3).
G.D.L.K. Jayasinghr et al. I Solid State Ionics 86-88
449
(1996) 447-451
-55
-6.0
-z -6.5 " t", 0 x -70
-75
0
0.2
04
06
06
v
1.--// cl
-80 0
1.0
0.2
04
U6
08
X
X Fig. 3. Depression angle of the bulk impedance as a function of
Fig. 4. Isothermal conductivity variations with the network former
glass composition.
composition at 190°C.
3. Results Below their respective glass transition temperatures, conductivity data of all glass samples obey an Arrhenius equation,
Activation energies E, and values of pre-exponential factors A for each composition are listed in Table 1. In the whole composition range we have observed that the pre-exponential term is composition-independent with a mean value of log A=4.8?0.40. Consequently, isothermal variations of conductivity (Fig. 4) are correlated with variations of activation energy (Fig. 5) only.
065 -
0
Table
04
0.6
06
1.0
X
Fig. 5. Variation network
4. Discussion For cationic charge carriers
0.2
former
of the conductivity
activation
energy
vs. the
composition.
pairs or, in other words, double occupied sodium sites [5,6]. The concentration of these interstitial pairs, C,, may be very small compared with the concentration of total alkali cations C and is a
conductive glasses, generally the are positively charged interstitial
I
Conductivity
parameters as a function of x in the xNa,P,O,-(
I -x)Na,Te,O,
glassy
system
x
0
0.10
0.20
0.37
0.40
0.48
0.60
0.70
0.80
0.90
I .oo
E,, W’)
0.80
0.79
0.78
0.75
0.71
0.70
0.65
0.65
0.67
0.69
0.73
4.58
4.72
4.95
4.85
4.75
4.82
4.48
4.67
4.85
5.22
5.15
LogA(Scm
’
K)
450
G.D.L.K. fayasinghe
et al. I Solid State lonics 86-88
temperature dependent exponential function of the free energy necessary to form an interstitial pair: C, = Cexp(s) where AC,= AH, - TAS, is the free energy associated with the formation of an interstitial pair. When an electric field is applied the electrochemical mobility U, of the interstitial pair:
(1996) 447-451
for log A are then 4.8 for NaPO, and 4.9 for Na,Te,O,. These estimated values are very close to the experimental mean value of log A =4.8 indicating that the interstitial pair formation and migration processes are purely enthalpic for these glassy electrolytes and the entropic terms AS, and AS, are negligible. 4.2. Comment on conductivity composition
variations
with
(3) where 1 is the mean jump distance between cationic sites, “0 is the attempt frequency and AC, the migration free energy. An expression for the conductivity u+ = FC, U, can be obtained by substituting Eqs. (2) and (3) and this can be rearranged as r+T=
cg3!
exp(g!i)
exp(+l) (4)
This expression can be compared with the experimental Arrhenius equation (1) and the constants can be identified as FWV A= o 6R
exp
(
iAS,
+ AS,
)
According to Eq. (6) the activation energy, E,, is a function of the defect formation enthalpy, AH,, and migration enthalpy, AH,,,, and it is difficult to state which is responsible for variation in E, with x. In the weak electrolyte model the mobility and hence AH,,, are assumed to be independent of the macromolecular network and variations in E, are related to variations in AH, only [7]. This approach depicts the interstitial pair formation as a dissociation of the network modifier, Na,O, in the network former mixture (P,O,-Te,O,) according to the dissociation equilibrium, Na,OsNa’
+ NaO
In this description the Na+ species may be identified as the positively charged interstitial pair and
,
E, = +AH, + AH,. Comparison of these theoretical values for A and EC, with experimental data in Table 1 leads to the following remarks.
Ionized species in very low concentration are expected to have an ideal behaviour. Consequently, the variation in AH, depends only on the variation of the partial enthalpy of the network modifier, aN,+.
4.1. Comments
In the pseudo-binary system xNa,P,O,--( lx)Na,Te,O,, Na,O is a common constituent. In case of an endothermic mixture between sodium phosphate and sodium tellurate, the partial enthalpy would increase leading to a decrease of AH, and activation energy E,. The microscopic origin of this endothermic mixing enthalpy could be related to the structural incompatibility between PO, tetrahedra and TeO, bipyramids. In such a case we may also expect that when the entropic term is not large enough to compensate for the positive enthalpic term the mixture would tend to phase separate as observed by SEM for composition near x=0.5.
on the pre-exponential
term A
Assuming a homogeneous distribution of the alkali cations in the glass, 1 and C can be estimated from density data. Densities for the two limiting compositions NazTe,O, and NaPO, measured by picnometry are 4.4 g cm -’ and 2.5 g cmm3 respectively. The sodium cation concentration C and the jump distance 1=( 1lC)“3 calculated from the density data 0 are 1.4X 102’ atom cm-’ and 4.16 A for vitreous Na,Te,O, and 1.5X IO’* atom cmm3 and 4.05 A for NaPO,. Taking the approximate value of 10” Hz for V, and neglecting the entropic term, estimated values
G.D.L.K. Juyasinghe
et (11. I Solid State Ionics 86-88
Acknowledgments GDLKJ and PWSKB are grateful to the European Economic Community for providing fellowships to work in France under research grant no. C 1I-CT910948.
References [I] T. Tsuchiya (1987) 857.
and T. Moriya,
J. Non-Cryst.
Solids
95/96
(1996) 447-451
4.51
[2] A. Magistris, G. Chiodelli and M.J. Duclot, Solid State Ionics 9/lO (1983) 611. [3] M. Tatsumisago, K. Yoneda, N. Machida and T. Minami, .I. Non-Cryst. Solids 95/96 (1987) 197. [4] G.D.L.K. Jayasinghe, D. Coppo, P.W.S.K. Bandaranayake and J.L. Souquet, Solid State Ionics 76 (1995) 297. [S] M.D. Ingram, J. Amer. Ceram. Sot. 63 (1980) 248. [6] S.R. Elliot, Solid State Ionics 27 (1988) 131. [7] D. Ravaine and J.L. Souquet, Phys. Chem. Glasses I8 (1977) 21.