Crystal and electronic structure of Co3O4 spinel under pressure probed by XANES and Raman spectroscopy

Crystal and electronic structure of Co3O4 spinel have been investigated by x-ray absorption near edge structure (XANES) at the Co K edge up to 58.5 GPa and Raman scattering up to 65 GPa. Several transitions have been observed upon pressurization, and the original structure was recovered on decompression. Experimental and theoretical XANES and Raman data are compatible with the occurrence of a monoclinic P21/c phase above ∼52.7 GPa. Vibrational modes analyzed in details by Raman scattering indicate that two other subtle transitions take place above ∼21.9 GPa (orthorhombic Fddd) and ∼43.0 GPa (monoclinic C2/m), in agreement with the previous x-ray diffraction experiments. Our combined experimental XANES and multiple scattering calculations indicate clear evidence of a tetrahedral to octahedral coordination crossover at the Co2+ sites, being completed upon transition to the monoclinic P21/c phase. The valence and spin states at two different Co sites remained unchanged at least up to the transition onset to the monoclinic P21/c phase ruling out the possibility of charge transfer and spin crossover in the intermediate phases as proposed previously.