High pressure polymorphs of LiCoPO4 and LiCoAsO4

Olivine-LiCoXO4 (X = P, As) compounds might transform to the denser spinel-type and Na2CrO4-type structures under pressure. In this work, the relative energetic stability of the three polymorphs and the pressure of the possible polymorphic transformations are investigated combining experiments and first principles calculations. Olivine-LiCoAsO4 is predicted to transform to the Na2CrO4-like structure at 0.4 GPa and to the spinel structure at 5.8 GPa (0 K). Quenching HP/HT experiments show that olivine-LiCoAsO4 treated at 6 GPa/1173 K transforms to the spinel-like structure. Computational results indicate that olivine-LiCoPO4 transforms to the Na2CrO4-like form at around 4 GPa (0 K), the latter being the stable form till very high pressures (21.6 GPa). In good agreement with this, olivine-LiCoPO4 when subjected to 6 GPa/1173 K and 15 GPa/1173 K is converted to the Na2CrO4-type polymorph. Crystallographic data of the new compound LiCoPO4 within the Na2CrO4 structural type are provided.