Effects of Pressure-Induced Phase Transition in Model Hg2Br2 Ferroelastics

Abstract

Raman spectra of model improper ferroelastics (Hg₂Br₂ crystals) have been analyzed in a wide range of hydrostatic pressures. The baric dependences of the phonon frequencies are obtained. The revealing and anomalous behavior of the soft mode, which is genetically related to the acoustic phonon (ТА₁) at the Brillouin zone boundary (point X) of the tetragonal phase, are most interesting. The buildup of the second acoustic phonon (ТА₂) from the same point has also been found in the ferroelastic-phase spectra, and its baric behavior has been investigated. The splitting of doubly degenerate phonons of the E_g symmetry has been revealed at fairly high pressures and explained.