Lattice dynamics and baric behavior of phonons in the Hg$_{2}$Br$_{2}$ model ferroelastics

Raman spectra of Hg$_{2}$Br$_{2}$ model improper ferroelastic crystals have been investigated in a wide range of high hydrostatic pressures. Baric dependences of the phonon frequencies have been obtained; of greatest interest are the observed soft mode originating from the slowest $TA_1$ acoustic branch at the Brillouin zone boundary ($X$ point) of the tetragonal phase and the anomalous behavior of this mode. In the ferroelastic phase spectra, the ignition of the second acoustic $TA_2$ from the same point has also been detected and its baric behavior has been studied. Under sufficiently high pressures, splitting of doubly degenerate phonons with the $E_g$ symmetry has been observed and explained. The parameters of the Grüneisen constants have been determined from the baric dependences of the phonon frequencies and discussed.