Acoustic phonon absorption and dispersion in hydroxyl-doped KCl crystals at low temperatures

The acoustic properties of hydroxyl-doped KCl crystals have been measured by Brillouin scattering in the temperature range 1 to 50 K. For samples with OH- content higher than 200 ppm, two features dominate the temperature dependence of the attenuation, namely an absorption peak around 20 K and a strong increase of the damping with decreasing temperature below 5 K. The results bear some similarities to the behaviour observed in glasses and have been accounted for by assuming relaxational and resonant phonon absorption processes.