Existence of an orientational glass state in(KCN)x(KBr)1−xmixed crystals

Recent neutron scattering experiments by Rowe et al. on mixed crystals of ${\left(\mathrm{KCN}\right)}_x{\left(\mathrm{KBr}\right)}_{1-x}$ cannot be explained by a previous dynamic theory by Michel et al. The appearance of a narrow central line in the scattering spectrum suggests the onset of an orientational glass state characterized by a freezing in of the CN orientations. Starting from a microscopic lattice dynamical description of the coupling of orientational impurities with acoustic lattice modes, we demonstrate that an orientational freezing in produces a freezing in of lattice strains. We introduce an orientational glass state parameter and a corresponding susceptibility. The neutron scattering law is formulated, taking into account interference between translations and orientations. The orientational glass state and its coupling to the lattice leads to an additional elastic line in the scattering law, the intensity of which reflects the temperature behavior of the order parameter. The bilinear coupling of translations and orientations leads to an explanation of characteristic asymmetries of equal intensity contours of the elastic line in $\overrightarrow{\mathrm{Q}}$ space. The temperature minima in the transverse acoustic-phonon frequencies are related to a cusp in the orientational glass state susceptibility.