Ferroelasticity and glass-like behavior in alkali halide-alkali cyanide mixed crystals

Abstract

Single crystal neutron diffraction studies in (KBr)_1-x(KCN)_x are summarized. Mixed crystals with CN⁻ concentrations x > 0.6 exhibit ferroelastic phase transitions from a high-temperature plastic phase into a low-temperature elastically ordered phase in which the CN⁻ orientations show long range orientational order and the center of mass lattice exhibits shear distortions. For concentrations x ≤ 0.6 orientational disorder is frozen-in and transitions into an orientational glass state occur. Close to the critical concentration x_c ∼ 0.6 the diffraction profiles at the transition temperatures are dominated by diffuse-scattering contributions. These results are compared to model calculations from Mayer and Cowley predicting a two-dimensional melting transition in a crystal with a planar instability. In the spirit of this model we suggest that the glass state close to x_c can be described as a frozen-in two dimensional liquid. A schematic phase diagram is constructed which bears similarities with a phase diagram as proposed by Galam within the framework of a compressible ferroelastic model.