Phase transitions and incommensurability in the layered semiconductor TlInS2-an NMR study

We report on the first NMR study of powder and single crystal samples of thallium indium sulfide, TlInS2. The crystal under study is a pure single-layer TlInS2 polytype. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from T-c = 192 K to T-i = 205 K. On approaching the phase transition at T-i from above, the crystal exhibits a soft mode behavior. A discrepancy in the literature data on the phase transitions in TlInS2 is discussed and ascribed to polytypism of the TlInS2 crystals.