Magnetic Structure and Magnetic Properties of Synthetic Lindgrenite, Cu₃(OH)₂(MoO₄)₂

Abstract

Synthetic Cu3(OH)2(MoO4)2 consists of Cu3(OH)2 brucite ribbons of edge-sharing copper octahedra connected by MoO4 into a 3D network as in the mineral, lindgrenite, for all temperatures between 1.5 and 300 K. Each ribbon consists of a triangular connection between two different types of copper atom (Cu(1) and 2 Cu(2)) via mu3-OH. The MoO4 acts both as one- and three-atom bridges to connect six Cu atoms belonging to three adjacent ribbons. The magnetic properties are consistent with those of ferrimagnetic chains, and the resulting moment of each chain is parallel below the long-range magnetic ordering at 13 K. The Curie constant is 0.468(1) emu K mol-1 of Cu; the Weiss temperature is -14.2(2) K, and the saturation magnetization at 2 K in 50 kOe is 0.41 N muB mol-1 of Cu. Analyses of the neutron powder diffraction reveal an ordered magnetic state where the moment of Cu(1) is antiparallel to those of the two Cu(2); all of them point along the a axis without any sign of geometrical frustration. Any degeneracy that may be present because of the triangular topology of the Cu atoms (s = 1/2) appears to be lifted by the distortion from an ideal equilateral geometry of the triangle. The entropy, estimated from the heat capacity measurements, attains 50% of the total of 17.7 J K-1 mol-1, close to that expected for three Cu atoms (3R ln 2), up to the long range ordering temperature, and the remaining is associated with the low dimensionality of the material.