Thermally activated relaxations and vibrational anarmonicity in alkali borate glasses: a study of Brillouin scattering

Measurements of Brillouin light scattering have been performed in (M(2)O)(0.14)(B(2)O(3))(0.86) alkali-borate glasses, where M=Li and K, as a function of temperature between 15 and 300 K. The temperature behaviors of hypersonic attenuation and velocity have been explained in terms of thermally activated relaxations of intrinsic structural defects and of anharmonic interactions between hypersonic waves and thermal vibrational modes. In the temperature region above 150 K, where the mean free path of thermal modes is shorter than the acoustic wavelength, it has been shown that the sound propagation is mainly regulated by the Akhiezer mechanism of "phonon viscosity." It causes a linear increase in the hypersonic attenuation and a linear decrease in the sound velocity with increasing temperature.