Ferrielectric ordering in lamellarCuInP2S6

The lamellar compound ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ is presented as an unusual example of a collinear, two-sublattice ferrielectric system. Single-crystal x-ray diffraction is used to study the thermal evolution of the polar character of its structure. The results are consistent with the occurrence of a first-order, order-disorder transition at ${T}_{c}=315\mathrm{K}$ and provide indirect evidence for copper hopping motions. Estimates for the spontaneous polarization are calculated at various temperatures from the atomic coordinates and found to agree well with dielectric measurements. The example of ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ suggests that cooperative dipole behavior may generally arise when ${d}^{10}$ cations (e.g., ${\mathrm{Cu}}^{\mathrm{I}}$ and ${\mathrm{In}}^{\mathrm{III}}$) susceptible to second-order Jahn-Teller distortions are embedded in a matrix of restricted dimensionality.