Dielectric measurement study of lamellar CuInP2Se6: successive transitions towards a ferroelectric state via an incommensurate phase?

The dielectric properties of lamellar CuInP2Se6 are studied as a function of temperature. Two phase transitions are detected: a second-order one at Ti=248 K and a first-order transition at Tc=236 K. These observations confirm the assumed paraelectric and ferroelectric character of the high- and low-temperature phases and give the first indication of an intermediate dipole state existing in this material. The hypothesis of an incommensurate phase occurring between Ti and Tc and the possible appearance of a Lifshitz point in CuInP2(SexS1−x)6 are discussed.