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Effects of pressure on the ionic conductivity of AgI–AgPO3 and KI–AgPO3 glasses
Solid State Ionics 105 (1998) 103–107
Effects of pressure on the ionic conductivity of AgI–AgPO 3 and KI–AgPO 3 glasses a, a a a b M.D. Ingram *, B. Macmillan , A.J. Pappin , B. Roling , J.M. Hutchinson a
b
Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, AB24 3 UE, UK Department of Engineering, University of Aberdeen, Meston Walk, Aberdeen, AB24 3 UE, UK
Abstract Activation volumes (DVA ) and activation energies (EA ) are obtained for Ag 1 -ion motion in glasses in AgPO 3 –KI and AgPO 3 –AgI systems, from studies of the pressure and temperature dependent DC conductivities. In AgPO 3 –AgI glasses, DVA and EA both decrease monotonically with increasing AgI content whereas, in AgPO 3 –KI glasses, both DVA and EA reach maximum values before decreasing at high KI contents. This apparently strong coupling between activation volumes and energies is a new result, which suggests that free volume plays a dominant role in conductivity enhancement in these strongly doped glassy electrolytes. Keywords: Pressure effect; Ion conductivity; Activation energy; Activation volume; Silver phosphate glasses; Doping; Glassy electrolytes
1. Introduction In the study of ionic mobility in glasses, the use of pressure as a variable has been rather neglected. This is probably because of the many additional experimental difficulties that it entails. Nevertheless, there may be useful insight into the mechanisms of ion transport in glasses to be gained from such studies [1–4]. For example, a comparison of the activation energies and activation volumes of glasses as a function of their composition could provide interesting information about the ionic sites and their effect on the conduction process. In the present paper, we examine the effect of hydrostatic pressure
*Corresponding author. Tel.: 144 1224 272905; fax: 144 1224 272921; e-mail: [email protected]
on the ionic conductivity of some potassium iodide– silver metaphosphate (KI–AgPO 3 ) glasses, which are compared with some silver iodide–silver metaphosphate (AgI–AgPO 3 ) glasses. We have selected the KI–AgPO 3 system for particular study in the light of some unusual behaviour reported by Doreau et al. [5]. These authors report a decrease in the conductivity at low KI contents before a rapid increase in the conductivity in the range of 10% to 20% KI. This is mirrored by the activation energy in this system, which first increases slightly and then decreases dramatically over the same range of KI content. We wished here to investigate whether or not the activation volume in the KI–AgO 3 system followed the same kind of behaviour. This could have important implications for the role of free volume in the mechanism of conductivity enhancement, as has been suggested by
0167-2738 / 98 / $19.00 1998 Elsevier Science B.V. All rights reserved. PII S0167-2738( 97 )00455-4
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M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
¨ Swenson and Borjesson [6], and could also provide evidence for a mixed cation (Ag 1 / K 1 ) effect [7].
2. Experimental
2.1. Materials Glasses in the KI–AgPO 3 and AgI–AgPO 3 systems were prepared with molar dopant concentrations of 0, 5, 10, 15 and 20% within each system, and additionally with 23% KI and 25% AgI, according to the following procedure. The KI and AgI were dried in an oven at 4008C for 1 h, then allowed to cool in a desiccator before use. Appropriate amounts of AgNO 3 and (NH 4 )H 2 PO 4 were weighed out, mixed and ground together. The required amounts of iodide were then added and ground into the mixture. The mixtures were heated in porcelain crucibles until bubbling had ceased (15–20 min) and then transferred to a furnace at 7008C for an hour. Glass specimens were made by pouring the melts into a brass mould to give cylinders of approximately 5 mm length and 5 mm diameter. The dimpled ends of these samples were rubbed down with glass paper to improve the contact in the conductivity jig and the samples were then sputter coated with blocking gold electrodes. They were stored in a vacuum desiccator until required.
2.2. Pressure studies Pressures of up to approximately 7 kbar (700 MPa) could be achieved using a pressure cell mounted in a 200 ton hydraulic press [8,9]. The pressure cell consists of a piston-in-cylinder arrangement, with the sample held at the tip of the piston (see Fig. 1). Electrical leads, for temperature control and measurement and for the measurement of conductivity, pass from the sample holder through the centre of the piston to the exterior. The pressurising medium is castor oil, and the temperature was maintained close to ambient (218C).
2.3. Conductivity measurements The conductivity of samples within the pressure cell was measured in situ using a Hewlett Packard
Fig. 1. Schematic illustration of the hydrostatic pressure cell.
LF Impedance Analyser (Model 4192A, 5 Hz–13 MHz) over a frequency range from approximately 100 Hz to 300 kHz. Comparison was made with conductivities measured on the same samples at ambient temperature and pressure outside the pressure apparatus, using the Schlumberger 1260 Impedance Analyser, and good agreement was found for the DC conductivities.
3. Results
3.1. Pressure dependence The dependence of the (log) DC conductivity on pressure up to approximately 6.5 kbar is shown in Fig. 2 for the KI–AgPO 3 glasses. For each pressure and each glass, the DC conductivity was evaluated at the plateau in the frequency dependent conductivity. All conductivities were corrected to a temperature of 295 K to allow for the small changes in temperature that occurred during the application of these high pressures. Corrections were made using an activation
M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
105
Fig. 3. The dependence of the activation volume on % KI content for the KI–AgPO 3 glasses. Open circles represent data obtained here; filled circles represent data obtained by Grincourt et al. [10]. The dashed line is drawn to guide to the eye.
Fig. 2. The dependence of the (log) DC conductivity on pressure up to approximately 6.5 kbar for the KI–AgPO 3 glasses, for the compositions indicated. The lines represent least squares fits.
energy evaluated for each glass at ambient pressure (see later). From the pressure dependence of the conductivity in Fig. 2, the activation volumes DVA for the KI– AgPO 3 glasses have been found, according to: d ln s DV S]]] D 5 2] , dP RT dc
A
T
(1)
and the results are shown in Fig. 3; also included (full circles) are values obtained by Grincourt et al. [10] for silver metaphosphate glass. It is clear that the data for KI–AgPO 3 glasses display a non-linear relationship between activation volume and potassium iodide content. These results for the KI doped glasses are compared with those for the AgI–AgPO 3 system in Fig. 4. Here, a reasonably linear dependence of activation volume on silver iodide content can be seen.
Fig. 4. The dependence of the activation volume on % AgI content for the AgI–AgPO 3 glasses. The line is drawn as a guide to the eye.
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M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
Fig. 5. The dependence of the (log) DC conductivity on reciprocal temperature for 5% KI–95% AgPO 3 glass. The line is a least squares fit.
Fig. 7. The dependence of the activation energy on % AgI content for the AgI–AgPO 3 glasses. The line is drawn as a guide to the eye.
3.2. Temperature dependence The temperature dependence of these glasses was measured at ambient pressure using the Schlumberger Impedance Analyser. A typical example of the dependence of the (log) conductivity on reciprocal temperature is shown in Fig. 5 for the KI–AgPO 3 glass. From these Arrhenius plots, the activation energies for the KI–AgPO 3 glasses have been found, and are shown in Fig. 6. Also included (full circles) are the values obtained by Doreau et al. [5] for the same range of glass compositions.
Fig. 6. The dependence of the activation energy on % KI content for the KI–AgPO 3 glasses. Open circles represent data obtained here; filled circles represent data obtained by Doreau et al. [5]. The dashed line is drawn to guide the eye.
These results may again be compared with those for the AgI–AgPO 3 system, shown in Fig. 7. Once again these latter glasses show a linear variation in behaviour with AgI content.
4. Discussion There is a remarkable distinction between the behaviours of silver phosphate glasses, doped with silver and potassium iodide respectively. For the silver iodide doped glasses, the activation energy decreases monotonically with increasing silver iodide content, and this is paralleled by the decrease in the activation volume. For the potassium iodide glasses, in contrast, the activation energy increases at small KI contents and then falls rapidly at higher contents. Again, this is paralleled in the activation volume data (and also by the values of the conductivity itself – see Fig. 2). That there is a close correlation between the behaviours of the activation energy and the activation volume even in a system where there is an unusual composition dependence is highly significant. This is the first time that such a clear correlation has been observed. It suggests that free volume and ionic conductivity are closely linked [6], and that the enhancement of ionic conductivity by salt doping occurs by a mechanism directly related to the expansion of the host glass.
M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
This suggestion fits easily with ideas recently ¨ advanced by Swenson and Borjesson [6]. They have argued that the role of added salts as dopants in enhancing the conductivity depends on their ability to expand the network structure (or anionic framework) of the host material. In some way, this global expansion is accompanied by an increase in free volume, which in turn opens up additional (or improved) pathways for ion transport. Our results indicate that the extra volume required for ion migration, i.e. DVA , increases in poorly conducting glasses and decreases when the conductivity goes up – fully in accord with this simple concept. We intend to extend these investigations to include a wider range of highly doped ‘superionic’ glasses. These studies will focus both on the correlations between network dilatation [6] and DVA , and on a more detailed analysis of frequency-dependent conductivities (compare Ref. [11]), to determine how the conductivity spectra fit into the universal pattern of ‘scaling behaviour’ [12] which is now well established for glassy electrolytes under ambient pressure conditions.
5. Conclusions These preliminary results for the effects of temperature and pressure on the ionic conductivities of iodide-doped silver phosphate glasses show a remarkable parallel between variations in activation energy and activation volume. This holds for both KI and AgI doped glasses, for which the dependencies of conductivity on dopant content are quite different. The implication is that free volume effects are
107
dominant in determining the ionic mobility in these systems.
Acknowledgements We acknowledge the former EPSRC high pressure facility (Harlow) for provision of the press, Aberdeen University for financial support for B Macmillan, and Prof J. Penman for the loan of the Hewlett Packard Impedance Analyser. We also thank Craig Watson for performing preliminary experiments in the AgPO 3 –KI system.
References [1] M.J. Ryan, S.I. Smedley, J. Non-Cryst. Solids 65 (1984) 29. [2] H. Senapati, G. Parthasarathy, S.T. Lakshmikumar, K.J. Rao, Phil. Mag. B 47 (1983) 291. [3] Y. Oyama, J. Kawamura, Solid State Ionics 53–56 (1992) 1221. [4] C.A. Angell, J. Zhou, Solid State Ionics 34 (1989) 243. [5] M. Doreau, J.P. Malugani, G. Robert, Electrochimica Acta 26 (1981) 711. ¨ [6] J. Swenson, L. Borjesson, Phys. Rev. Lett. 77 (1996) 3569. [7] M.D. Ingram, Phys. Chem. Glasses 28 (1987) 215. [8] J.M. Hutchinson, M.D. Ingram, A.H.J. Robertson, Phil. Mag. B 66 (1992) 449. [9] A.J. Pappin, M.D. Ingram, J.M. Hutchinson, G.D. Chryssikos, E.I. Kamitos, Phys. Chem. Glasses 36 (1995) 164. [10] Y. Grincourt, M. Henault, M. Duclot, J.L. Souquet, Phys. Chem. Glasses 37 (1996) 236. [11] C. Fanggao, G.A. Saunders, Z. Wei, M. Cutroni, A. Mandanici, A. Piccolo, Solid State Ionics 86–88 (1996) 425. [12] B. Roling, A. Happe, K. Funke, M.D. Ingram, Phys. Rev. Lett. 78 (1997) 2160.
Effects of pressure on the ionic conductivity of AgI–AgPO 3 and KI–AgPO 3 glasses a, a a a b M.D. Ingram *, B. Macmillan , A.J. Pappin , B. Roling , J.M. Hutchinson a
b
Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, AB24 3 UE, UK Department of Engineering, University of Aberdeen, Meston Walk, Aberdeen, AB24 3 UE, UK
Abstract Activation volumes (DVA ) and activation energies (EA ) are obtained for Ag 1 -ion motion in glasses in AgPO 3 –KI and AgPO 3 –AgI systems, from studies of the pressure and temperature dependent DC conductivities. In AgPO 3 –AgI glasses, DVA and EA both decrease monotonically with increasing AgI content whereas, in AgPO 3 –KI glasses, both DVA and EA reach maximum values before decreasing at high KI contents. This apparently strong coupling between activation volumes and energies is a new result, which suggests that free volume plays a dominant role in conductivity enhancement in these strongly doped glassy electrolytes. Keywords: Pressure effect; Ion conductivity; Activation energy; Activation volume; Silver phosphate glasses; Doping; Glassy electrolytes
1. Introduction In the study of ionic mobility in glasses, the use of pressure as a variable has been rather neglected. This is probably because of the many additional experimental difficulties that it entails. Nevertheless, there may be useful insight into the mechanisms of ion transport in glasses to be gained from such studies [1–4]. For example, a comparison of the activation energies and activation volumes of glasses as a function of their composition could provide interesting information about the ionic sites and their effect on the conduction process. In the present paper, we examine the effect of hydrostatic pressure
*Corresponding author. Tel.: 144 1224 272905; fax: 144 1224 272921; e-mail: [email protected]
on the ionic conductivity of some potassium iodide– silver metaphosphate (KI–AgPO 3 ) glasses, which are compared with some silver iodide–silver metaphosphate (AgI–AgPO 3 ) glasses. We have selected the KI–AgPO 3 system for particular study in the light of some unusual behaviour reported by Doreau et al. [5]. These authors report a decrease in the conductivity at low KI contents before a rapid increase in the conductivity in the range of 10% to 20% KI. This is mirrored by the activation energy in this system, which first increases slightly and then decreases dramatically over the same range of KI content. We wished here to investigate whether or not the activation volume in the KI–AgO 3 system followed the same kind of behaviour. This could have important implications for the role of free volume in the mechanism of conductivity enhancement, as has been suggested by
0167-2738 / 98 / $19.00 1998 Elsevier Science B.V. All rights reserved. PII S0167-2738( 97 )00455-4
104
M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
¨ Swenson and Borjesson [6], and could also provide evidence for a mixed cation (Ag 1 / K 1 ) effect [7].
2. Experimental
2.1. Materials Glasses in the KI–AgPO 3 and AgI–AgPO 3 systems were prepared with molar dopant concentrations of 0, 5, 10, 15 and 20% within each system, and additionally with 23% KI and 25% AgI, according to the following procedure. The KI and AgI were dried in an oven at 4008C for 1 h, then allowed to cool in a desiccator before use. Appropriate amounts of AgNO 3 and (NH 4 )H 2 PO 4 were weighed out, mixed and ground together. The required amounts of iodide were then added and ground into the mixture. The mixtures were heated in porcelain crucibles until bubbling had ceased (15–20 min) and then transferred to a furnace at 7008C for an hour. Glass specimens were made by pouring the melts into a brass mould to give cylinders of approximately 5 mm length and 5 mm diameter. The dimpled ends of these samples were rubbed down with glass paper to improve the contact in the conductivity jig and the samples were then sputter coated with blocking gold electrodes. They were stored in a vacuum desiccator until required.
2.2. Pressure studies Pressures of up to approximately 7 kbar (700 MPa) could be achieved using a pressure cell mounted in a 200 ton hydraulic press [8,9]. The pressure cell consists of a piston-in-cylinder arrangement, with the sample held at the tip of the piston (see Fig. 1). Electrical leads, for temperature control and measurement and for the measurement of conductivity, pass from the sample holder through the centre of the piston to the exterior. The pressurising medium is castor oil, and the temperature was maintained close to ambient (218C).
2.3. Conductivity measurements The conductivity of samples within the pressure cell was measured in situ using a Hewlett Packard
Fig. 1. Schematic illustration of the hydrostatic pressure cell.
LF Impedance Analyser (Model 4192A, 5 Hz–13 MHz) over a frequency range from approximately 100 Hz to 300 kHz. Comparison was made with conductivities measured on the same samples at ambient temperature and pressure outside the pressure apparatus, using the Schlumberger 1260 Impedance Analyser, and good agreement was found for the DC conductivities.
3. Results
3.1. Pressure dependence The dependence of the (log) DC conductivity on pressure up to approximately 6.5 kbar is shown in Fig. 2 for the KI–AgPO 3 glasses. For each pressure and each glass, the DC conductivity was evaluated at the plateau in the frequency dependent conductivity. All conductivities were corrected to a temperature of 295 K to allow for the small changes in temperature that occurred during the application of these high pressures. Corrections were made using an activation
M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
105
Fig. 3. The dependence of the activation volume on % KI content for the KI–AgPO 3 glasses. Open circles represent data obtained here; filled circles represent data obtained by Grincourt et al. [10]. The dashed line is drawn to guide to the eye.
Fig. 2. The dependence of the (log) DC conductivity on pressure up to approximately 6.5 kbar for the KI–AgPO 3 glasses, for the compositions indicated. The lines represent least squares fits.
energy evaluated for each glass at ambient pressure (see later). From the pressure dependence of the conductivity in Fig. 2, the activation volumes DVA for the KI– AgPO 3 glasses have been found, according to: d ln s DV S]]] D 5 2] , dP RT dc
A
T
(1)
and the results are shown in Fig. 3; also included (full circles) are values obtained by Grincourt et al. [10] for silver metaphosphate glass. It is clear that the data for KI–AgPO 3 glasses display a non-linear relationship between activation volume and potassium iodide content. These results for the KI doped glasses are compared with those for the AgI–AgPO 3 system in Fig. 4. Here, a reasonably linear dependence of activation volume on silver iodide content can be seen.
Fig. 4. The dependence of the activation volume on % AgI content for the AgI–AgPO 3 glasses. The line is drawn as a guide to the eye.
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M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
Fig. 5. The dependence of the (log) DC conductivity on reciprocal temperature for 5% KI–95% AgPO 3 glass. The line is a least squares fit.
Fig. 7. The dependence of the activation energy on % AgI content for the AgI–AgPO 3 glasses. The line is drawn as a guide to the eye.
3.2. Temperature dependence The temperature dependence of these glasses was measured at ambient pressure using the Schlumberger Impedance Analyser. A typical example of the dependence of the (log) conductivity on reciprocal temperature is shown in Fig. 5 for the KI–AgPO 3 glass. From these Arrhenius plots, the activation energies for the KI–AgPO 3 glasses have been found, and are shown in Fig. 6. Also included (full circles) are the values obtained by Doreau et al. [5] for the same range of glass compositions.
Fig. 6. The dependence of the activation energy on % KI content for the KI–AgPO 3 glasses. Open circles represent data obtained here; filled circles represent data obtained by Doreau et al. [5]. The dashed line is drawn to guide the eye.
These results may again be compared with those for the AgI–AgPO 3 system, shown in Fig. 7. Once again these latter glasses show a linear variation in behaviour with AgI content.
4. Discussion There is a remarkable distinction between the behaviours of silver phosphate glasses, doped with silver and potassium iodide respectively. For the silver iodide doped glasses, the activation energy decreases monotonically with increasing silver iodide content, and this is paralleled by the decrease in the activation volume. For the potassium iodide glasses, in contrast, the activation energy increases at small KI contents and then falls rapidly at higher contents. Again, this is paralleled in the activation volume data (and also by the values of the conductivity itself – see Fig. 2). That there is a close correlation between the behaviours of the activation energy and the activation volume even in a system where there is an unusual composition dependence is highly significant. This is the first time that such a clear correlation has been observed. It suggests that free volume and ionic conductivity are closely linked [6], and that the enhancement of ionic conductivity by salt doping occurs by a mechanism directly related to the expansion of the host glass.
M.D. Ingram et al. / Solid State Ionics 105 (1998) 103 – 107
This suggestion fits easily with ideas recently ¨ advanced by Swenson and Borjesson [6]. They have argued that the role of added salts as dopants in enhancing the conductivity depends on their ability to expand the network structure (or anionic framework) of the host material. In some way, this global expansion is accompanied by an increase in free volume, which in turn opens up additional (or improved) pathways for ion transport. Our results indicate that the extra volume required for ion migration, i.e. DVA , increases in poorly conducting glasses and decreases when the conductivity goes up – fully in accord with this simple concept. We intend to extend these investigations to include a wider range of highly doped ‘superionic’ glasses. These studies will focus both on the correlations between network dilatation [6] and DVA , and on a more detailed analysis of frequency-dependent conductivities (compare Ref. [11]), to determine how the conductivity spectra fit into the universal pattern of ‘scaling behaviour’ [12] which is now well established for glassy electrolytes under ambient pressure conditions.
5. Conclusions These preliminary results for the effects of temperature and pressure on the ionic conductivities of iodide-doped silver phosphate glasses show a remarkable parallel between variations in activation energy and activation volume. This holds for both KI and AgI doped glasses, for which the dependencies of conductivity on dopant content are quite different. The implication is that free volume effects are
107
dominant in determining the ionic mobility in these systems.
Acknowledgements We acknowledge the former EPSRC high pressure facility (Harlow) for provision of the press, Aberdeen University for financial support for B Macmillan, and Prof J. Penman for the loan of the Hewlett Packard Impedance Analyser. We also thank Craig Watson for performing preliminary experiments in the AgPO 3 –KI system.
References [1] M.J. Ryan, S.I. Smedley, J. Non-Cryst. Solids 65 (1984) 29. [2] H. Senapati, G. Parthasarathy, S.T. Lakshmikumar, K.J. Rao, Phil. Mag. B 47 (1983) 291. [3] Y. Oyama, J. Kawamura, Solid State Ionics 53–56 (1992) 1221. [4] C.A. Angell, J. Zhou, Solid State Ionics 34 (1989) 243. [5] M. Doreau, J.P. Malugani, G. Robert, Electrochimica Acta 26 (1981) 711. ¨ [6] J. Swenson, L. Borjesson, Phys. Rev. Lett. 77 (1996) 3569. [7] M.D. Ingram, Phys. Chem. Glasses 28 (1987) 215. [8] J.M. Hutchinson, M.D. Ingram, A.H.J. Robertson, Phil. Mag. B 66 (1992) 449. [9] A.J. Pappin, M.D. Ingram, J.M. Hutchinson, G.D. Chryssikos, E.I. Kamitos, Phys. Chem. Glasses 36 (1995) 164. [10] Y. Grincourt, M. Henault, M. Duclot, J.L. Souquet, Phys. Chem. Glasses 37 (1996) 236. [11] C. Fanggao, G.A. Saunders, Z. Wei, M. Cutroni, A. Mandanici, A. Piccolo, Solid State Ionics 86–88 (1996) 425. [12] B. Roling, A. Happe, K. Funke, M.D. Ingram, Phys. Rev. Lett. 78 (1997) 2160.