Brillouin scattering study of hypersonic relaxation in a Ca(NO3)2 –KNO3 mixture

Abstract

Brillouin spectroscopy has been used to provide information about the hypersonic velocity and absorption of thermal waves in a glass-forming 60% KNO3–40% Ca(NO3)2 mixture (mol %) in a temperature range of 90–380°C for gigahertzian frequencies. A relaxation process was revealed in both the longitudinal velocity and attenuation. The data are consistent with ultrasonic results for the limiting low-frequency velocity. The limiting high-frequency velocity is considerably larger than that reported from ultrasonic studies and the position of the maximum of the amplitude absorption coefficient is shifted to higher temperatures. The variation of the relaxation time with temperature was found to be Arrhenius with a temperature-independent activation energy in excellent accordance with the model proposed by Weiler, Bose, and Macedo. At low temperatures, a transverse mode due to shear waves was observed close to the Rayleigh line.