SUCCESSION OF THE LOW-TEMPERATURE PHASE-TRANSITIONS IN TLINS2 CRYSTALS

Two additional incommensurate phase transitions (PT's) at Ti = 206 K and T-tr = 79 K were observed in TlInS2 crystals. Peculiarities of the temperature behaviour of the dielectric susceptibility and the dielectric loops allowed to consider the PT's at Tc1 = 204 K and Tc2 = 201 K as incomplete lock-in transitions and the PT at T approximate to 195 K as a temperature range of the final lock-in transition. The PT at T-tr = 79 K was considered as an isomorphous transition. The experimental results of the second harmonic generation signal at low temperature are also in favour of such a conclusion. Peculiarities of the temperature behaviour of the dielectric properties at low temperature are qualitatively explained by means of the theory of two nonequivalent sublattices.