Acoustic behaviour of normal and densified vitreous GeO2

The acoustic properties of GeO2 glass were investigated by measuring acoustic attenuation and sound velocity between 1.5 and 300 K over the frequency range 10-50 MHz. A low-temperature plateau of attenuation followed by a large loss peak, paralleled by corresponding dispersive behaviour in sound velocity, were the main experimental observations. The temperature behaviour of ultrasonic attenuation is explained in terms of thermally activated relaxations of intrinsic structural defects at high temperatures (T 10 K) and of tunnelling motions at low temperatures (T 10 K). A direct comparison between the values of tunnelling strength, obtained over the MHz- and kHz-intervals, indicates that the internal-friction plateau arising from the phonon-assisted tunnelling of two-level systems (TLS) is slightly frequency-dependent. Analysis of the temperature dependence of sound velocity showed that anomalous hardening, typical of glasses having tetrahedrally coordinated structures, starts above ~ 100 K. Finally, a comparison of sound velocity behaviour in normal and permanently densified GeO2 glasses demonstrates that densification leads to a significant reduction in the number of relaxing particles.