Copper ordering in lamellar CuMP2S6 (M= Cr, In): Transition to an antiferroelectric or ferroelectric phase

Abstract The transition to an antiferroelectric or ferroelectric phase in the lamellar compound CuMP2S6 (M= Cr or In) is shown to be triggered by the appearance of an ordered copper sublattice. Cu(I) ions in CuCrP2S6, which are distributed over several sites at room temperature, freeze into an intermediate quasi-antipolar state between 190 and 175K, and then form an array of alternating up and down dipoles below 150K. Evidence for these transitions were obtained using calorimetry, diffraction and electric permittivity measurements. CuInP2S6 at 298K contains antiparallel polar sublattices of Cu(I) and In(III) which yield a Ps =2.6 μC-cm−2, as calculated from crystallographic results. The existence of copper disorder and the central octahedral location of In(III) in the paraelectric phase (T>315K) were also confirmed.