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−${[{\mathrm{C}\mathrm{O}}_3]}^{2-}$${[{\mathrm{C}\mathrm{O}}_3]}^{2-}$ and pyro-[C2O5]2−${[{\mathrm{C}}_2{\mathrm{O}}_5]}^{2-}$${[{\mathrm{C}}_2{\mathrm{O}}_5]}^{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.