Direct synthesis and characterization of mixed-valent Li-0.5-delta CoPO4, a Li-deficient derivative of the Cmcm polymorph of LiCoPO4.

While the majority of research activities on LiCoPO4 is focussed on the thermodynamically stable olivine-type Pnma polymorph, the metastable Pna2(1) 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 Li-0.5-delta CoPO4. 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 Li-0.5-delta CoPO4 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 Li-0.5-delta CoPO4 is metastable and exhibits a complex, multi-step thermal decomposition mechanism. In the first step at 394 degrees C, it decomposes to alpha-Co2P2O7 (P2(1)/c) and LiCoPO4 (Cmcm) upon O-2 release. The LiCoPO4 (Cmcm) intermediate is then irreversibly transformed to olivine-type LiCoPO4 (Pnma) at 686 degrees C. The material properties of Li-0.5-delta CoPO4 are further compared to the fully lithiated, isostructural LiCoPO4 (Cmcm) phase, for which an improved structure solution as well as Co L-2,L-3-edge X-ray absorption spectra are reported for the first time.