The $c_{44}$ shear elastic constant has been measured at 10 MHz in the cubic high-temperature phase of ${\mathrm{K}}_{\mathrm{x}}$${\mathrm{Rb}}_{1\mathrm{-}\mathrm{x}}$CN single crystals. As the consequence of a bilinear coupling term in the free energy, $c_{44}$ is the soft mode associated with orientational ordering of the ${\mathrm{CN}}^{\mathrm{-}}$ ions at low temperatures. The behavior of $c_{44}$ as a function of composition and temperature has been analyzed in terms of an elastic Curie-Weiss expression and an extended mean-field model that includes quenched random strain fields. The critical temperature $T_c$(x) for long-range ordering exhibits a roughly parabolic dependence on x, which can be modeled empirically with regular solution theory.

• Received 7 March 1988

DOI:https://doi.org/10.1103/PhysRevB.38.5640