Direct synthesis and characterization of mixed-valent Li _0.5−δCoPO ₄, a Li-deficient derivative of the Cmcm polymorph of LiCoPO ₄

While the majority of research activities on LiCoPO4 is focussed on the thermodynamically stable olivinetype Pnma polymorph, the metastable Pna21 and Cmcm modifications have recently attracted considerable attention due to their interesting material properties. In this study, we present the first Li-deficient structural derivative of the Cmcm modification with the nominal composition Li0.5
dCoPO4. As opposed to the substoichiometric olivine (Pnma) phases LixCoPO4 (x ¼ 0; 2/3), which are exclusively accessible by electrochemical or chemical Li extraction techniques, this is also the first time that a direct soft-chemical synthesis route towards a LixCoPO4-type material is accomplished. X-ray and neutron diffraction studies indicate that Cmcm-type Li0.5
dCoPO4 shows vacancies on both the Li and Co sites, whereas X-ray absorption spectra demonstrate that the structure features heterovalent Co ions (+2/+3) to compensate for the Li deficit. Magnetic measurements reveal a long-range antiferromagnetic order below 10.5 K. A thorough investigation of the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and temperature-dependent in situ X-ray powder diffraction demonstrates that Li0.5
dCoPO4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 C, it decomposes to a-Co2P2O7 (P21/c) and LiCoPO4 (Cmcm) upon O2 release. The LiCoPO4 (Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO4 (Pnma) at 686 C. The material properties of Li0.5
dCoPO4 are further compared to the fully lithiated, isostructural LiCoPO4 (Cmcm) phase, for which an improved structure solution as well as Co L2,3-edge X-ray absorption spectra are reported for the first time.