Quadrupolar relaxation in (KBr)1−x(KCN)x

Brillouin scattering was used to study the dynamic properties of the mixed crystals (KBr${)}_{1\mathrm{-}\mathrm{x}}$(KCN${)}_{\mathrm{x}}$. By analyzing the frequencies and linewidths of the $T_{2g}$-symmetry phonon, we were able to determine the cyanide quadrupolar relaxation rate as a function of temperature in the paraelastic regime. We found that this relaxation frequency is proportional to exp(-1/T), rather than proportional to the square root of temperature (√T ), as assumed in Michel’s random-field theory. Modifying this theory by incorporating the observed temperature dependence of the relaxation rate improves the agreement with experimental data but is still not adequate. The hierarchically constrained relaxation model of Palmer et al., on the other hand, does provide a phenomenological explanation for our data. In addition, we have determined the value of the random-field strength in these mixed systems.