Phase transitions and incommensurability in the layered semiconductor TlInS ₂an NMR study

We report on the first NMR study of powder and single crystal samples of thallium indium sulfide, TlInS ₂. The crystal under study is a pure single-layer TlInS ₂ 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=192K to T _i=205K. 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 TlInS ₂ is discussed and ascribed to polytypism of the TlInS ₂ crystals.