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 an d 295 K. No evidence for a soft mode behaviour has been found. The ano malies observed in the temperature dependence of low- and high-frequen cy phonon modes at approximately 250 and approximately 180 K, respecti vely, are explained as due to the phase transitions. It is supposed th at the phase transitions are caused by the deformation of structural c omplexes GaS4, rather than by slippage of Tl atom channels in [110] an d [110BAR] directions, which is mainly responsible for the appearance of the low-temperature ferroelectric phase transitions in other repres entatives of TlBX2 layer compounds.