6Li magic-angle spinning (MAS) NMR study of electron correlations, magnetic ordering, and stability of lithium manganese(III) oxides

Three different lithium manganese(III) oxides (the orthorhombic LiMnO2 phase, monoclinic layered LiMnO2 and tetragonal Li2MN2O4) were studied with Li-6 magic-angle spinning (MAS) NMR spectroscopy. Much smaller shifts of 36-143 ppm are observed for the Mn(III) phases, in comparison to the other lithium manganese oxides with manganese oxidation states varying from +3.5 to +4. The NMR shift of this system is governed by the Fermi-contact interaction and consequently, is controlled by the lithium local environment. For orthorhombic LiMnO2, one resonance at 36 ppm is observed between -39 and 283 degreesC and a single resonance at -5 ppm is seen below -39 degreesC, indicating a magnetic phase transition involving a change from short-range electronic correlations to long-range antiferromagnetic ordering. The Li-6 NMR shift of the resonances of the monoclinic and tetragonal phases show very little change with temperature in the range studied (-136 to 283 degreesC), implying that short-range antiferromagnetic interactions between Mn cations also exist for these phases. No evidence for a magnetic phase transition to three-dimensional ordering, however, is observed. Two different lithium sites are identified in Li2Mn2O4, which are assigned to lithium on the 8c octahedral and 4a tetrahedral sites. Samples with lithium on the 8c site only are obtained using mild synthesis conditions, whereas occupation of both sites is obtained with more stringent conditions or at high temperatures.