Quasiharmonic approximation for a double Morse-type local potential model: Application to aKH2PO4-type phase diagram

Abstract

In order to describe a structural phase transition at low temperature, a quantum particle within a local potential is considered. According to the general formalism presented by Salje et al. [Z. Phys. B 82, 399 (1991)], a quasiharmonic approximation is applied to the local potential and the interaction is replaced by the mean field one. The rigorous effective potential is reduced from a double Morse-type potential. The order parameter, the variance, and the effective soft frequency are given by analytic equations with potential parameters. As an application, a molecular motion of ${\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}$ in KDP is considered and the disappearance of the ordered phase at low temperature under high pressure is discussed. The difference between KDP and DKDP crystals are described successfully by the presented model.