Fragility, Anharmonicity and Anelasticity of Silver Borate Glasses

The fragility and the anharmonicity of silver borate glasses have been quantified by measuring the change in the specific heat capacity at the glass transition temperature Tg and the room-temperature thermodynamic Gruneisen parameter. Increasing the silver oxide content above X=0.1 leads to an increase of both the parameters, showing that a growing fragility of a glassforming liquid is predictive of an increasing overall anharmonicity of its glassystate. The attenuation and velocity of ultrasonic waves of frequencies in the range of 10-70 MHz have also been measured in silver borate glasses as a function of temperature between 1.5 and 300 K. The experimental data reveal anelastic behaviours which are governed by (i) quantum-mechanical tunnelling below 20 K, (ii) thermally activated relaxations between 20 and 200 K and (iii) vibrational anharmonicity at even higher temperatures. Evaluation of tunnelling (I) and relaxation (C) strengths shows that C is independent of the structural changes affecting the borate network with increasing metal oxide content and is at least one order of magnitude smaller than C. The latter observation implies that only a small fraction of the locally mobile defects are subjected to tunnelling motions.