Phase transitions and low-temperature structure of lithium manganese oxide spinel

Low-temperature structures of the lithium manganese spinels were determined using TOF neutron Rietveld analysis. The spinels LiMn2O4-delta with different delta values, (0.016, 0.040, and 0.132) showed the cubic-orthorhombic phase transitions, and the lattice distortion decreased with decreasing delta. Although no anomaly corresponding to the cubic-orthorhombic phase transition was observed in the DSC curve for the spinet with delta similar to 0.016, the transition was observed by the structure analysis, which is consistent with the broad C-P anomaly at 250 K. The cubic-orthorhombic phase transition is closely correlated to the existence of the vacancy. The charge disproportionation into trivalent and tetravalent state proceeds gradually with decreasing temperature, and the extent of the disproportionation is dependent on the vacancy. Based on the structure analysis, the phase transitions in the spinet are discussed.