On the phase transition in alkali cyanides

The phase transition in alkali cyanides is studied starting from a Hamiltonian containing a bilinear coupling term between translational and rotational degrees of freedom, thus leading to an effective rotational interaction. It is shown that the problem can be reformulated in terms of elasticity theory and the effective rotational interaction then becomes an interaction between elastic dipoles. It is demonstrated that the phase transition is of first order. The long range nature of the elastic dipole forces which would lead to a shape dependence of the system for monodomain samples is responsible for the breakup of the ordered phase into domains.