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° Mn–O–Mn exchange for determining the magnetic order.