Phonon Spectra and Phase Transitions in CuInP 2 (Se x S 1 − x ) 6 Ferroelectrics

The investigations of dielectric permittivity, optic birefringence and Raman scattering spectra for CuInP 2 (Se x S 1 m x ) 6 mixed crystals give evidence about rapid lowering of the temperature and smearing of first order transition between monoclinic paraelectric and ferroelectric phases at sulfur by selenium replacement. In the concentration interval x = 0.3 m 0.75 the state similar to dipole glass obviously exists. The temperature dependence of the low energy part of Raman spectra for CuInP 2 S 6 crystals and concentration transformation of these spectra in CuInP 2 (Se x S 1 m x ) 6 solutions leads to suggestion that the morphotropic phase transitions between monoclinic and trigonal phases can be associated with the instability of a "rigid layer" shifting mode.