Cr3+-Containing Carbonates and Cr2O3-Pbcn at Extreme Conditions

We synthesized three carbonates, Cr2[CO3]3, Cr2[C2O5][CO3]2, and Cr2[CO3][O]2, containing Cr3+ by laser heating of chromium oxides in CO2 at pressures between 45 and 55 GPa. All phases were characterized by single-crystal X-ray diffraction and Raman spectroscopy, while density functional theory calculations complemented the experimental results. The structure of Cr2[CO3]3-P63/m contains coplanar groups, and a 4f Wyckoff site is half occupied by Cr3+. Cr2[C2O5][CO3]2-C2/c is characterized by coplanar [CO3]2− and pyro-[C2O5]2− anions and is isostructural to Al2[C2O5][CO3]2. It was obtained only after prolonged laser heating. The oxycarbonate Cr2[CO3][O]2-Pbcn was synthesized at the highest pressure (55 GPa) studied here. In this structure, half of the oxygen atoms coordinate one carbon and two Cr atoms, while the other half coordinate three chromium atoms. The oxidation state of chromium in the carbonates is +3, independent of the oxidation state (+3 or +4) of chromium in the oxide starting material. We observed the transformation of Cr2O3-R3̅c to Cr2O3-Pbcn above 45(2) GPa after laser heating.