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

Abstract The structures of the lamellar selenodiphosphates CuMP 2 Se 6 (M=Cr, In) are reexamined using powder diffraction and auxiliary techniques. Antiferromagnetic behavior observed in CuCrP 2 Se 6 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 CuCrP 2 Se 6 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 CuInP 2 Se 6 at room temperature. The deduced off-center shift for copper in CuCrP 2 Se 6 is less than in CuInP 2 Se 6 ; moreover, these displacements are significantly smaller than in the thiophosphates CuMP 2 S 6 (M=Cr, In). No indication of dipole ordering within the copper sublattice of CuCrP 2 Se 6 was found down to T =10 K, in contrast with the appearance of antipolar copper order in CuCrP 2 S 6 below 150 K. On the other hand, calorimetry detected a transition in CuInP 2 Se 6 between T =200 and 240 K, a range much lower and broader than for the ferro-paraelectric transition in CuInP 2 S 6 ( T c =315 K). The possible nature of the low T -phase of CuInP 2 Se 6 is discussed based on preliminary diffraction data and the known thermal behavior of CuInP 2 S 6 .