LOW-TEMPERATURE PHASE-TRANSITIONS IN TLGAS2 LAYER CRYSTALS

Polarized Raman scattering spectra of TlGaS2 layer crystals have been studied for the first time as a function of temperature between 8.5 and 295 K. No evidence for a soft mode behaviour has been found. The anomalies observed in the temperature dependence of low- and high-frequency phonon modes at approximately 250 and approximately 180 K, respectively, are explained as due to the phase transitions. It is supposed that the phase transitions are caused by the deformation of structural complexes GaS4, rather than by slippage of Tl atom channels in [110] and [110BAR] directions, which is mainly responsible for the appearance of the low-temperature ferroelectric phase transitions in other representatives of TlBX2 layer compounds.