Investigation of the vibrational spectra of a SbSI (Sb2S3)0.15 crystals in harmonic and anharmonic approximations

Abstract

The vibrational spectra of SbSI (Sb2S3)0.15 crystals in harmonic and anharmonic approximations are calculated by Density Functional Theory (DFT) method. Investigation of IR, R and A low frequency normal modes at points k = 0 and k = 0.5 of the Brillouin zone in SbSI (Sb2S3)0.15 crystals reveal that soft behavior of these modes mainly depends on the temperature dependent force constant Cij between Sb and I atoms and symmetry coordinates. Investigation of the mean value potential energy ¯Vp(z) per one atom dependence on the amplitude of the soft mode B1u symmetry coordinates in c(z) axis direction on the phase transition region have explained why ferroelectric phase transition temperature TC depends on compression in x-y plane of the SbSI chains in SbSI (Sb2S3)x crystals.