Glass and phase transitions in (KBr)1−x(KCN)x

Results of a single-crystal neutron-diffraction study of the mixed molecular system (KBr${)}_{1\mathrm{-}\mathrm{x}}$(KCN${)}_{\mathrm{x}}$ are presented. The crystal with a ${\mathrm{CN}}^{\mathrm{-}}$ concentration x=0.65 exhibits a ferro- elastic transition from a plastic high-temperature phase of cubic symmetry to a low-temperature rhombohedral phase. The crystals with concentrations x=0.53 and 0.57 undergo transitions into a quadrupolar glass state. In all the crystals investigated, the diffraction profiles near the transition temperatures are dominated by diffuse-scattering contributions. These diffuse intensities are due to random strains which are generated by the substitutional ${\mathrm{Br}}^{\mathrm{-}}$ atoms acting as static impurity centers. The results demonstrate the importance of random strains for the order of the low-temperature state: A delicate balance between the rotation-translation coupling and the random strain fields determines the low-temperature phase. If the latter dominate, the elastic-order transition is suppressed and local order is frozen in. The temperature dependences of the diffuse scattered intensities are analyzed: Unusual line shapes and an unusual wave-vector dependence of the linewidths are found. In addition, the temperature dependence of Debye-Waller factors and of the orientational distribution of the ${\mathrm{CN}}^{\mathrm{-}}$ molecules is studied in detail.