Soft-mode-driven lattice instabilities in Cs₂HgCl₄crystal: phenomenological treatment and far-infrared spectroscopy of the structurally modulated phases

Abstract

This paper reports a comprehensive phenomenological description and experimental infrared (IR) investigations of the soft-mode-driven lattice instabilities into various commensurately and incommensurately modulated phases of Cs(2)HgCl(4) crystals. Our theoretical analysis shows that the lattice instabilities along the a and c crystallographic directions are related to low-frequency transverse optical (TO) phonon branches of Σ(2) and Λ(3) symmetry, respectively, which merge together in the center of the Brillouin zone at the point of B(3g) symmetry. As the temperature decreases both branches fall down, leading first to the direct condensation of the soft TO Σ(2) mode in the symmetric Σ direction (k is parallel to a*). On the other hand, coupling of the TO and transverse acoustic (TA) modes of Λ(3) symmetry causes, at somewhat lower temperatures, a series of frozen modulated commensurate and incommensurate states developing along the symmetric Λ direction (k is parallel to c*). Polarized far-infrared (FIR) reflectivity spectra (15-600 cm(-1)) of Cs(2)HgCl(4) crystals were measured in a broad temperature region, 10-297 K. Despite a rich sequence of structurally modulated phases existing above 163 K we observed rather moderate temperature evolution of IR spectra where only a few new modes of different polarizations have been activated. However, the commensurately modulated phases occurring below 163 K made an essential impact on the spectra of all three polarizations. The process of activation of both the Raman- and the IR-active phonons in the structurally modulated phases is subjected to the phenomenological analysis.

Hydrostatic pressure induced structural phase transition and mechanical properties of fluoroperovskite

“…According to the difference of crystal structure between the new phase and the parent phase, structural phase transitions can be classified into two types, displacive and reconstructive. Based on the soft mode theory of the displacive phase transformation, Kityk et al [13] studied Pressure-induced ferroelastic instability and lattice dynamics of Cs 2 HgCl 4 crystals, besides, Shchur Ya et al [14] investigated the lattice instabilities into various modulated phases of Cs 2 HgCl 4 crystals. Furthermore, Kityk et al [15] confirmed the elastic instability under hydrostatic pressure is attributed to a critical lowering of a soft optical mode.…”