Fragility, anharmonicity and anelasticity of silver borate glasses

The fragility and the anharmonicity of (Ag(2)O)(x)(B(2)O(3))(1-x) borate glasses have been quantified by measuring the change in the specific heat capacity at the glass transition temperature T(g) and the room-temperature thermodynamic Gruneisen parameter. Increasing the silver oxide content above X = 0.10 leads to an increase of both the parameters, showing that a growing fragility of a glass-forming liquid is predictive of an increasing overall anharmonicity of its glassy state. 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 (C) 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.