Structure and magnetic properties of LiMVO4 (M = Mn, Cu)

LiM2+VO4 compounds with spinel-related structures show interesting electrochemical, magnetic and multiferroic properties. Neutron powder diffraction data for LiMVO4 (M = Cu, Mn) were collected at room temperature and 1.5 K. In contrast to single crystal NPD results, no evidence of magnetic ordering was detected for LiCuVO4 at 1.5 K due to the incommensurate order and low Cu2+ magnetic moment. For LiMnVO4 (space group Cmcm) a clear ordered magnetic state was determined for the first time: the chains of edge-shared MnO6 octahedra have antiferromagnetic (AFM) order within each chain, and also between chains to give C' centring. The magnetic behaviour of the MnO6 chain can be well described by the classical one-dimensional Heisenberg model with an intra-chain exchange energy of J/k = -4.32 K. It is believed that direct d(xy) - d(xy) AFM interactions are dominant over 90 degrees Mn-O-Mn exchange for determining the magnetic order.