Inorganic Tricarbonate: High-Pressure Synthesis and Structure of K₂C₃O₇

The synthesis of carbonates with novel types of anions is important for geoscience, chemistry, and materials science. Herein, we present the first inorganic tricarbonate salt, K₂C₃O₇, discovered in laser-heated diamond anvil cells at 55(3) and 45(2) GPa. The crystal structure of K₂C₃O₇ was determined in situ by using synchrotron single-crystal X-ray diffraction from a polycrystalline sample. It features nonplanar [C₃O₇]^2- anions, which consist of three corner-sharing planar CO₃ groups rotated relative to one another. This anion extends the homologous series of sp²-carbonates: [CO₃]^2-─[C₂O₅]^2-─[C₃O₇]^2-. Raman spectroscopy establishes the characteristic vibrational fingerprint of the [C₃O₇]^2- anion. Density functional theory (DFT) calculations corroborate the experimental results and suggest a thermodynamic stability of K₂C₃O₇ between 10 and 55 GPa. DFT calculations predict a phase transition between 80 and 90 GPa associated with polymerization of the [C₃O₇]^2- groups, accompanied by a change in the coordination polyhedra of two carbon atoms from triangles to tetrahedra. These results imply that other sp²-and mixed sp²/sp³-carbonates might be stabilized at a high pressure.