PRESSURE-INDUCED FERROELASTIC INSTABILITY AND LATTICE-DYNAMICS OF CS2HGCL4 CRYSTALS WITHIN THE SEMIEMPIRICAL RIGID-ION MODEL

An analysis of the lattice dynamics of Cs2HgCl4 crystals and their rel ation to the pressure-induced proper ferroelastic instability is repor ted using a semiempirical rigid-ion model in the quasiharmonic approxi mation. The model parameters (effective radii and charges of ions) hav e been adjusted to satisfy the equilibrium configuration of the observ ed structure in the normal phase. The influence of unharmonicity has b een 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 t he lattice instability leading to the appearance of the proper ferroel astic phase. It has been shown that the lattice loses its stability wi th respect to the transverse-acoustical mode TA(Z)(k parallel to/b) wh ereas the soft optical mode (B-3g symmetry at k = 0) remains stable. T his is in agreement with the phenomenological theory. The rigid-ion mo del also reproduces the line of the proper ferroelastic phase transiti ons in the P,T-phase diagram which is close to the experimentally obse rved phase diagram. [S0163-1829(98)03729-1].