Magnetic Properties and Magnetic Structures of Cu3(OD)4XO4, X = Se or S: Cycloidal versus Collinear Antiferromagnetic Structure

Abstract

We report a comparative study of the magnetic properties of synthetic Cu3(OH)4(SO4)x(SeO4)1-x and the magnetic structures of the parent compounds. All compounds are isostructural and belong to the orthorhombic class of parent compounds. They consist of 3-legged ribbons of edge-sharing copper octahedra connected by micro3-OH and XO4 (X=S or Se). XO4 acts both as one-atom and three-atom bridges to connect seven Cu atoms (six Cu(2) and one Cu(1)) belonging to three neighboring ribbons. The two end members behave as low-dimensional AF with a long-range antiferromagnetic state below 5 (X=S) and 8 K (X=Se); the former shows evidence of a canting. Analyses of the neutron powder diffraction data for X=S were shown to display an ordered magnetic state (k=0 0 0) where the moments of Cu(2) within the two outer legs are collinear and parallel within each leg but antiparallel from each other; the orientation of the moments of Cu(2) is the c axis. In contrast, for X=Se k=approximately 1/7 0 0 and the magnetic structure is cycloidal and transforms progressively from being incommensurate (T>3 K) to commensurate (T<or=3 K). The moments of Cu(2) of each leg are oriented antiparallel as for X=S, but they rotate about the b axis while propagating along the a axis. In both cases the moments of Cu(1) of the inner leg remain random. The magnetic entropy of 15.2+/-1 J/kmol for the end members, estimated from integrating the heat capacity/temperature, is close to that expected (3R ln 2) for three Cu2+ (S=1/2).