PARAELECTRIC FERROELECTRIC TRANSITION IN THE LAMELLAR THIOPHOSPHATE CUINP2S6

The occurrence of a first-order paraelectric-ferroelectric phase trans ition at T(C) = 315(5) K in CuInP2S6 is revealed by calorimetry, X-ray powder diffraction, and dielectric measurements. This is the first ob servation of such a phenomenon in a thiophosphate of the lamellar MPS3 family. The data are consistent with the polar character of the room- temperature structure attributable to the acentric positions of the Cu (I) and In(III) ions and may be related to the probable T dependence o f the site-occupation ratios for the former cation and displacement of the latter. Results of the dielectric measurements are suggestive of a two-component phase transition.