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

The structures of the lamellar selenodiphosphates CuMP2Se6 (M=Cr, In) are r eexamined using powder diffraction and auxiliary techniques. Antiferromagne tic behavior observed in CuCrP2Se6 below 40 K is shown to be consistent wit h the existence of separate, triangular Cu and Cr sublattices rather than a random cation distribution. Analyses of neutron powder diffraction pattern s recorded at various temperatures for CuCrP2Se6 confirm this as well as re veal partial occupation by copper of equivalent sites shifted away from the layer midplane. Diffraction evidence is also given for site disorder invol ving off-center positions in the copper sublattice of CuInP2Se6 at room tem perature. 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 con trast 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-paraele ctric transition in CuInP2S6 (T-c=315 K). The possible nature of the low T- phrtse of CuInP2Se6 is discussed based on preliminary diffraction data and the known thermal behavior of CuInP2S6. (C) 1999 Elsevier Science S.A. All rights reserved.