Elastic and anelastic properties of densified vitreous B2O3: Relaxations and anharmonicity

The elastic and anelastic properties of densified B2O3 glasses, melt quenched under pressures of 2 and 4 GPa, were investigated by measuring the sound velocity and the acoustic attenuation of longitudinal and shear ultrasonic waves in the megahertz range over the temperature interval between 8 and 300 K. Densification from 1826 to 2373 kg/m3 leads to an extraordinarily large growth of both bulk and shear moduli but leaves the Poisson's ratio nearly constant. In the glass compacted at 4 GPa, the elastic moduli become larger by a factor of five than those characterizing normal vitreous B2O3 (v-B2O3) as a consequence of modifications of the chemical bonding in the network. The thermally activated relaxations of intrinsic structural defects, which dominate the acoustic behaviors of normal glass below 150 K, giving rise to an intense attenuation peak and a corresponding steep decrease in sound velocity, are increasingly depressed by growing densification. Above 150 K, the ultrasonic velocity is mainly regulated by the vibrational anharmonicity and shows a nearly linear decrease as the temperature is increased, with a substantially smaller slope with increasing densification. Modeling the relaxation losses and the related velocity variations by an asymmetric double-well potential model that has a distribution of both the barrier potential and the asymmetry, it has been possible to separate the relaxation and the anharmonic contributions to the sound velocity. The former has been ascribed to local motions of boroxol rings formed by connected BO3 planar triangles, the basic units building up the network of v-B2O3, while the latter has been interpreted in terms of the Akhiezer mechanism concerning the "thermal vibration viscosity."