Jahn-Teller-like model for the 208-K phase transition in the solid electrolyte RbAg4I5

Because the optical birefringence in Rb${\mathrm{Ag}}_4$ ${\mathrm{I}}_5$ has been found to depend linearly on the order parameter near the 208-K phase transition, it is necessary to consider a model in which the order parameter couples linearly to the static strain. In this paper, such a model is developed in which coupling to lattice strains lifts the degeneracy of nearest-neighbor pairs of Ag ions and leads to a Hamiltonian which is very similar to that for cooperative Jahn-Teller systems. The large number of such pairs (12) requires the use of group-theoretical methods in choosing the appropriate combinations. This Hamiltonian is treated in the Landau theory, and is shown to predict the occupancy of Ag sites in the rhombohedral lower-temperature phase, to explain the softening of the $C_{44}$ elastic constant, and to lead to the observed linear dependence of the birefringence on the order parameter.