FERRIELECTRIC ORDERING IN LAMELLAR CUINP2S6

The lamellar compound CuInP2S6 is presented as an unusual example of a collinear, two-sublattice ferri-electric system. Single-crystal x-ray diffraction is used to study the thermal evolution of the polar chara cter of its structure. The results are consistent with the occurrence of a first-order, order-disorder transition at T-c = 315 K and provide indirect evidence for copper hopping motions. Estimates for the spont aneous polarization are calculated at various temperatures from the at omic coordinates and found to agree well with dielectric measurements. The example of CuInP2S6 suggests that cooperative dipole behavior may generally arise when d(10) cations (e.g., Cu-I and In-III) susceptibl e to second-order Jahn-Teller distortions are embedded in a matrix of restricted dimensionality. [S0163-1829(97)01238-1].