Spectroscopy of the pressure-induced virtual phase transition in Hg2I2 crystals

Raman spectra of incipient ferroelastics, namely, Hg2I2 crystals, have been investigated over a wide range of hydrostatic pressures. Linear pressure dependences of phonon frequencies have been obtained at P < P (c) ). It has been found that jumps and breaks are observed in these dependences at the phase transition point P (c) = 9 kbar. The Gruneisen parameters of the Hg2I2 crystals have been determined and discussed. The Raman spectra of the ferroelastic phase (P > P (c) ) in different polarizations exhibit excitation of acoustic vibrations from the X point of the Brillouin zone boundary, including transverse acoustic (TA(1) and TA(2)) and longitudinal acoustic (LA) phonons. The Raman spectra of the ferroelastic phase have been interpreted based on the analysis of the experimental results, and a model of the phase transition in these crystals has been proposed.