On the Lamellar Compounds CuBiP₂Se₆, AgBiP₂Se₆ and AgBiP₂S₆. Antiferroelectric Phase Transitions Due to Cooperative Cu⁺ and Bi³⁺ Ion Motion

CuBiP₂Se₆, AgBiP₂Se₆, and AgBiP₂S₆ were prepared from the corresponding elements. CuBiP₂Se₆ and AgBiP₂Se₆ crystallize in the space group R3̄ with a = 6.5532(16) Å and c = 39.762(13) Å for CuBiP₂Se₆ and a = 6.6524(13) Å and c = 39.615(15) Å for AgBiP₂Se₆. AgBiP₂S₆ crystallizes in the triclinic space group P1̄ with a = 6.3833(13) Å, b = 7.1439(14) Å, c = 9.5366(19) Å, α = 91.89(3)°, β = 91.45(3)°, γ = 94.05(3)°. CuBiP₂Se₆ was found to exhibit a temperature-dependent antiferroelectric ordering of the Cu⁺ and Bi³⁺ ions in the lattice. An intermediate and a fully ordered structure were refined at 173 and 97 K, respectively. Electronic band and total energy calculations at the DFT level clearly suggest that the antiferroelectric model is energetically favored over the paraelectric and hypothetical ferrielectric models. This phase transition can be classified as a second-order Jahn−Teller distortion. The antiferroelectric state of CuBiP₂Se₆ is an indirect gap semiconductor. The compounds were characterized with differential thermal analysis and solid-state UV/vis diffuse reflectance spectroscopy. Generalized implications regarding the expected ferroelectric behavior of compounds in the CuMP₂Se₆ system (M = trivalent metal) are discussed.