The effect of hydrostatic pressure and temperature on the frequency dependencies of the a.c. conductivity of ionic (AgPO3) and (Ag2S)0.3(AgPO3)0.7 glasses

The dielectric properties of vitreous (Ag2S)(x)(AgPO3)(1-x) and AgPO3 have been measured in the radio frequency region under hydrostatic pressure up to about 0.2 GPa in the temperature range 293 to 373 K to determine the effects of pressure on ionic motion in a conducting glass, Measurements have also been made at atmospheric pressure down to 20 K to extend understanding of the conduction mechanisms. The d.c, conductivity increases with temperature and decreases with hydrostatic pressure. Analysis of the frequency dependencies of the conductivities of both glasses indicates the pressure and temperature dependencies of their conductivities to be caused by the effects of pressure and temperature on their respective ion hopping rates alone. The activation energy and volume for these two glasses have been determined as functions of pressure and temperature. At 293 K, the activation volumes are 4.15 and 2.52 cm(3)mol(-1) for AgPO, and (Ag2S)(0.3)(AEPO(3))(0.7) glasses, respectively, and become larger at higher temperatures. In an alternative interpretation, deformation potentials of 1.25 and 0.9 eV have been obtained for the two glasses. Results are consistent with the random-walk model but are an order of magnitude smaller than expected from the free volume theory. Introduction of Ag2S into the glass decreases the activation energy and volume Delta V, suggesting that the P-S mode vibration has a weaker pressure dependence than the P-O mode. The fact that the Delta V is less than the molar volume of the corresponding charge carrier evidences a higher polarisability of silver ions in this glass. (C) 1998 Elsevier Science B.V. All rights reserved.