doi:10.1039/c7ra04043aengLudwig, J.Alarcón-Suesca, C.Geprägs, S.Nordlund, D.Doeff, M.M.Puente Orench, I.Nilges, T.Direct synthesis and characterization of mixed-valent Li0.5-dCoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4ART-2017-104788While the majority of research activities on LiCoPO4 is focussed on the thermodynamically stable olivine-type 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 = 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.2017http://zaguan.unizar.es/record/6975610.1039/c7ra04043ahttp://zaguan.unizar.es/record/69756oai:zaguan.unizar.es:69756RSC Advances 7, 45 (2017), 28069-28081by-nchttp://creativecommons.org/licenses/by-nc/3.0/es/info:eu-repo/semantics/openAccess