ELASTIC AND ANELASTIC PROPERTIES, VIBRATIONAL ANHARMONICITY, AND FRACTAL BOND CONNECTIVITY OF SUPERIONIC GLASSES

To quantify the thermally activated relaxations of the mobile silver i ons in superionic silver phosphosulphate (Ag2SO4)(x)(AgPO3)((1-x)) and phosphosulphide (Ag2S)(x)(AgPO3)((1-x)) glasses, the broad attenuatio n peaks reported previously have been analyzed in terms of a Gaussian- type energy distribution. The parameters obtained are used to determin e the influence of thermally activated relaxation processes on the tem perature dependence of the ultrasonic wave velocity measured between 1 .5 and 300 K. After subtraction of the relaxation effects together wit h those due to anharmonic interactions, another contribution to the te mperature dependence of the ultrasonic velocity remains below 100 K, w hich follows a Linear temperature dependence - as predicted by the sof t-potential model (SPM) for relaxation of soft harmonic oscillators. T he soft HO relaxation contribution to the ultrasonic velocity temperat ure dependences of silver phosphate-based glasses has a similar magnit ude to those determined previously for lanthanide metaphosphate glasse s. While the silver phosphate-based glasses have skeletons that are co mprised of long chains of phosphate ions, the lanthanide metaphosphate glasses are close to having a three-dimensional structure. The agreem ent of the excess contribution to the temperature dependence of the ul trasonic velocity with the predictions of the SPM for both types of gl ass, in spite of their complete differences in structure, is further e vidence for the universal applicability of the soft-potential model. T o determine the vibrational anharmonicity of the long wavelength acous tic modes in superionic glasses, the hydrostatic pressure derivatives of the second-order elastic stiffness tensor components have been meas ured for these (Ag2SO4),(AgPO3)((1-x)), (Ag2S)(x)(AgPO3)((1-x)), vitre ous AgPO3 and also for silver iodide molybdate (AgI)(0.75)(Ag2MoO4)(0. 25). TO examine further the effects of vibrational anharmonicity, the thermal expansions of the vitreous phosphates have also been measured. The linear thermal expansion coefficient becomes anomalously negative at lower temperatures for the (Ag2SO4)(x)(AgPO3)((1-x)) and (Ag2S)(x) (AgPO3)((1-x)) glasses. The wide variations found between the elastic and nonlinear acoustic properties of superionic silver phosphate, moly bdate, and berate glasses stem from differences in the bonding and con nectivities of the glass skeletons.