Effects of pressure-induced phase transition in model Hg$_{2}$Br$_{2}$ ferroelastics

Raman spectra of model improper ferroelastics (Hg$_{2}$Br$_{2}$ 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 (ТА$_1$) at the Brillouin zone boundary (point $X$) of the tetragonal phase, are most interesting. The buildup of the second acoustic phonon (ТА$_2$) 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.