Pressure behavior of phonons and phase transition effects in Hg2I2 model virtual ferroelastics

The Raman spectra of virtual ferroelastic Hg2I2 crystals have been studied in a broad range of hydrostatic pressures. Linear pressure dependences of phonon frequencies are obtained at P < Pc, which exhibit jumps and breaks at the phase transition point (Pc = 9 kbar). Grüneisen parameters of the Hg2I2 crystals are determined. The spectra of a ferroelastic phase (P > Pc) in various polarizations exhibit the excitation of new lines from the X point of the Brillouin zone, including transverse (TA) and longitudinal (LA) acoustic phonons. Based on analysis of the experimental and theoretical results, the Raman spectra of ferroelastic Hg2I2 crystals are interpreted and a model of the phase transition in these crystals is proposed.