ULTRASONIC-ATTENUATION AND INTERNAL-FRICTION IN (AGI)X(AGPO3)1-X SUPERIONIC GLASSES

We have measured the attenuation of longitudinal and shear ultrasonic waves in (AgI)x(AgPO3)1-x glasses, with 0 less-than-or-equal-to x less-than-or-equal-to 0.39, at temperatures between 80 and 360 K. We observe a large attenuation contribution, which increases with increasing frequency or AgI content. In samples with x greater-than-or-equal-to 0.31, this attenuation goes through a single broad peak as a function of temperature. The peak occurs at a higher temperature the greater the measuring frequency and its height increases with AgI content. These effects indicate that Ag-ion relaxation processes are involved. We fit our attenuation peaks and the internal friction peaks of Liu and Angell by using a modification of the "universal" relaxation function that Jonscher developed to account for non-Debye dielectric relaxation. From the fits we deduce relaxation activation enthalpies, exponents of the frequency-relaxation-time product, and relaxation amplitudes. We associate the activation enthalpies with potential barriers between alternative Ag sites, which may be affected by ion-ion correlation, discuss ways of accounting for the exponents, and account for the amplitudes by using different deformation potentials for Ag ions surrounded by oxide ions and for Ag ions surrounded by iodide ions, with both deformation potentials decreasing as AgI content increases.