Spinel-rock salt transformation in LiCoMnO_4-δ

The transformation on heating LiCoMnO₄, with a spinel structure, to LiCoMnO₃, 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 Li₂MnO₃, 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 Co²⁺ and Mn³⁺ in LiCoMnO₃. Subsolidus phase diagram determination of the Li₂O-CoO_x-MnO_ysystem has established the compositional extent of spinel solid solutions at approximately 500 C.