Peculiarities of phase transitions in polytypes of monoclinic TlInS2

The existence of two polytypes at room temperatures, C-TlInS2 and 2C-TlInS2, with different monoclinic cell parameters, c and 2c, has been revealed. Significant differences in crystal lattice dynamics of these polytypes have been found. In particular, two phase transitions (PTs) have been detected for the polytype C-TlInS2 as the temperature varies: a second-order PT from paraphase to incommensurate phase at T i = 215 K and a first-order ferroelectric PT accompanied by a quadrupling of the parameter c at T c = 197 K. No PT accompanied by an increase in unit cell parameter c has been found in the polytype 2C-TlInS2, but a global temperature hysteresis characteristic of crystals with an incommensurately modulated structure has been detected at T = 180–230 K.

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