Weak ferromagnetism of LiMnPO4

Structural and magnetic properties of the novel materials for lithium batteries LiFePO4 and LiMnPO4 were studied by X-ray diffraction, SQUID magnetometry and EPR spectroscopy. LiMnPO4 has an olivine-type structure with a Mn-ion square lattice in the b-c plane. The occupation factors for Li and those oxygen atoms, which bridge Mn ions in the b-c plane showed noticeable deviation from the stoichiometry. In addition, the oxygen atoms, which are in the same layer as Li ions, exhibit a remarkable mean-square displacement in LiMnPO4 but not in LiFePO4. The olivine structure suggests quasi-two-dimensional (quasi-2D) antiferromagnetic structure of Mn(II) ions (S = 5/2) with sizable interlayer exchange interactions. Magnetization measurements clearly revealed a transition to a weak ferromagnetic state below T-N = 45 K. On the other hand we find that LiFePO4 orders antiferromagnetically below 50 K. The difference in the magnetic properties of LiMnPO4 and LiFePO4 reflect the differences in the electronic states between these two compounds and may be very important for the electrochemical inactivity of LiMnPO4. EPR measurements also suggest that at temperatures above TN the low-energy magnetic excitations in LiMnPO4 are characteristic for the quasi-2D magnetic structure with the soliton excitation energy E-S = 139 K. (C) 2004 Elsevier Ltd. All rights reserved.