Copper sublattice ordering in layered CuMP2Se6 (M=In, Cr)

The structures of the lamellar selenodiphosphates CuMP2Se6 (M=Cr, In) are reexamined using powder diffraction and auxiliary techniques. Antiferromagnetic behavior observed in CuCrP2Se6 below 40 K is shown to be consistent with the existence of separate, triangular Cu and Cr sublattices rather than a random cation distribution. Analyses of neutron powder diffraction patterns recorded at various temperatures for CuCrP2Se6 confirm this as well as reveal partial occupation by copper of equivalent sites shifted away from the layer midplane. Diffraction evidence is also given for site disorder involving off-center positions in the copper sublattice of CuInP2Se6 at room temperature. The deduced off-center shift for copper in CuCrP2Se6 is less than in CuInP2Se6; moreover, these displacements are significantly smaller than in the thiophosphates CuMP2S6 (M=Cr, In). No indication of dipole ordering within the copper sublattice of CuCrP2Se6 was found down to T=10 K, in contrast with the appearance of antipolar copper order in CuCrP2S6 below 150 K. On the other hand, calorimetry detected a transition in CuInP2Se6 between T=200 and 240 K, a range much lower and broader than for the ferro-paraelectric transition in CuInP2S6 (Tc=315 K). The possible nature of the low T-phase of CuInP2Se6 is discussed based on preliminary diffraction data and the known thermal behavior of CuInP2S6.