Brillouin scattering and rotation translation coupling of (KCN)xX1−x mixed crystals

Brillouin scattering is used to study the concentration and temperature dependence of longitudinal and transverse phonon frequencies of KCN_χBr_1?χ, KCN_χC1_1?χ, and KCN_0.85I_0.15 mixed crystals. Unlike pure KCN, mixed crystals with χ < 0.7 show no phase transition upon cooling, and the transverse acoustic frequency ω_TA(110) displays a minimum at some characteristic temperature T_i.T_i is below the phase transition of pure KCN and depends on the C1^? concentration. The Brillouin data are discussed in terms of coupling of the acoustic phonons with orientational motion of CN^? ions and mean field theory.