Elasticity and thermal expansivity of (AgI)x(AgPO3)1-x glasses.

Abstract

We have measured the transit times of longitudinal and shear ultrasonic waves in (AgI${)}_{\mathit{x}}$(${\mathrm{AgPO}}_{3}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ glasses, with 0\ensuremath{\le}x\ensuremath{\le}0.46, at temperatures between 80 and 360 K and at pressures up to 4 kbar at 196 K and/or 296 K. We have also measured the thermal expansivity of (AgI${)}_{\mathit{x}}$(${\mathrm{AgPO}}_{3}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ glasses with x=0 and 0.31 between 90 and 295 K. From the transit-time data, we found that all the elastic stiffness moduli decrease with increases in AgI content or temperature. However, all moduli increase with increasing pressure. The shear-to-bulk modulus ratio, ${\mathit{C}}_{\mathrm{T}}$/B, is close to 0.25, the value calculated by Bergman and Kantor for a material having a one-dimensional backbone and an interstitial fluid near the percolation limit. The ${\mathit{C}}_{\mathrm{T}}$/B ratio decreases as AgI content or pressure increases, indicating a reduction in structural stability and that a structural phase transition involving macroscopic shear may occur at high pressure. The temperature dependence of each elastic modulus has a component due to elastic anharmonicity and, in some samples, an inflection due to Ag-ion relaxation which occurs at a lower temperature the higher the AgI content. The pressure dependences of the elastic moduli yielded positive ultrasonic mode Gr\uneisen parameters and an average high-temperature Gr\uneisen parameter greater than the mode Gr\uneisen parameters deduced from the temperature dependences of the moduli or from thermal expansion data. While the latter Gr\uneisen parameters agreed at room temperature the thermal Gr\uneisen parameters fell as temperature decreased and became negative at about 155 K for the x=0 sample, indicating the importance of excitations having a negative anharmonicity parameter (mode Gr\uneisen parameter).