Verwey-type transition and magnetic properties of the LiMn2O4 spinels

The transport and magnetic properties of stoichiometric and Li-rich LiMn2O4 spinels are investigated. Clear anomalies, which are associated with a structural phase transition from cubic to tetragonal, are observed in the temperature dependence of resistivity and susceptibility in the stoichiometric sample. The sharp increase in resistivity with decreasing temperature suggests a transition similar to the Verwey transition seen in the Fe3O4 spinel. Electrical conduction above the transition temperature is well described by a hopping model with an activation energy of about 0.16 eV. Effective paramagnetic moments obtained from Curie constants yield information on the average ionic valence of Mn. The average ionic valences of Mn increase in the Li-rich samples, suggesting that the excess Li ions substitute on the Mn site. A superexchange magnetic interaction between the Mn ions via oxygen atoms is also discussed. The magnetic interaction in a 90 degrees arrangement of Mn-O-Mn changes from antiferromagnetic to ferromagnetic with increasing Mn4+/Mn3+ ratio. (C) 1997 Academic Press.