The effect of oxygen deficiency on the structural phase transition and electronic and magnetic properties of the spinel LiMn2O4-delta

The effect of oxygen deficiency on a phase transition from cubic to tetragonal phase has been investigated for the LiMn2O4-delta spinels with delta = 0-0.1 by measurements of differential scanning calorimetry (DSC), magnetic susceptibility (chi), Li-7 nuclear magnetic resonance (Li-7 NMR) and electronic resistivity (rho). According to the DSC measurements, the transition temperature for a stoichiometric compound (T-C) is determined to be 245 +/- 1 K. As delta increases from zero to 0.066, the magnitude of T-C is found to decrease by about 4 K; then, the magnitude of T-C increases by about 41 K with further increase of delta up to 0.10; thus, the T-C-versus-delta curve exhibits a broad minimum at around delta = 0.05. This relationship between T-C and delta is also observed by the measurements of chi, Li-7 NMR and rho. The dependence of T-C on delta is considered to be a result of a competition between a decrease in the average valence of Mn ions and a dilution of the nearest-neighbour interaction between Mn ions. Both effects are caused by the oxygen deficiency; the former raises T-C, whereas the latter reduces it.