Pressure-induced ferroelastic instability and lattice dynamics ofCs2HgCl4crystals within the semiempirical rigid-ion model

Abstract

An analysis of the lattice dynamics of ${\mathrm{Cs}}_{2}{\mathrm{HgCl}}_{4}$ crystals and their relation to the pressure-induced proper ferroelastic instability is reported using a semiempirical rigid-ion model in the quasiharmonic approximation. The model parameters (effective radii and charges of ions) have been adjusted to satisfy the equilibrium configuration of the observed structure in the normal phase. The influence of unharmonicity has been introduced into the lattice dynamics simulation indirectly through the changes of the lattice parameters that occur under the action of temperature and hydrostatic pressure. Such a simple model reproduces the lattice instability leading to the appearance of the proper ferroelastic phase. It has been shown that the lattice loses its stability with respect to the transverse-acoustical mode ${\mathrm{TA}}_{Z}$$(\mathbf{k}\mathbf{\ensuremath{\Vert}}\mathbf{b})$ whereas the soft optical mode (${B}_{3g}$ symmetry at $\mathbf{k}=0$) remains stable. This is in agreement with the phenomenological theory. The rigid-ion model also reproduces the line of the proper ferroelastic phase transitions in the $P,T$-phase diagram which is close to the experimentally observed phase diagram.