Spinel–rock salt transformation in LiCoMnO 4− δ

The transformation on heating LiCoMnO 4 , with a spinel structure, to LiCoMnO 3 , with a cation-disordered rock salt structure, accompanied by loss of 25% of the oxygen, has been followed using a combination of diffraction, microscopy and spectroscopy techniques. The transformation does not proceed by a topotactic mechanism, even though the spinel and rock salt phases have a similar, cubic close-packed oxygen sublattice. Instead, the transformation passes through two stages involving, first, precipitation of Li 2 MnO 3 , leaving behind a Li-deficient, Co-rich non-stoichiometric spinel and, second, rehomogenization of the two-phase assemblage, accompanied by additional oxygen loss, to give the homogeneous rock salt final product; a combination of electron energy loss spectroscopy and X-ray absorption near edge structure analyses showed oxidation states of Co 2+ and Mn 3+ in LiCoMnO 3 . Subsolidus phase diagram determination of the Li 2 O-CoO x -MnO y system has established the compositional extent of spinel solid solutions at approximately 500°C.