Low-Temperature Acoustic Properties of(KBr)1−x(KCN)xin the Orientationally Disordered State

The attenuation and the variation of the phase velocity of high-frequency ultrasonic waves have been measured in ${\left(\mathrm{KBr}\right)}_{1-x}{\left(\mathrm{KCN}\right)}_x$ crystals for $x=0.25\mathrm{} \mathrm{and} 0.50$ in the temperature range 0.1 to 8 K. For both crystals we observe the low-temperature acoustic anomalies characteristic of structural glasses. The results are interpreted in terms of a tunneling model with a broad spectrum of energy splittings and relaxation times. Spectral density and phonon-coupling constants are determined and discussed with respect to their dependence on $x$.