Landau theory of the ferroelastic phase transition in ionic molecular solids: A microscopic approach

Starting from Bogolyubov's inequality, we derive a variational form for the Helmholtz free energy $F$ to study the ferroelastic phase transitions in simple ionic molecular solids. The coefficients of the Landau expansion of $F$ in terms of the five orientational order parameters (${\eta }_i,i=1,\dots ,5\mathrm{}$) are calculated from a rotation-translation Hamiltonian which has been used with reasonable success to explain the thermoelastic anomalies of the orientationally disordered phase. Detailed calculations have been performed for CsCN to understand the first-order phase transition from the pseudocubic ($\mathrm{Pm}3m$) to trigonal ($R3m$) phase. Various aspects of this transition resulting from the interplay of direct and lattice-mediated interactions and limitations of the currently used models to describe the physical properties of alkali cyanides are discussed.