Inorganic Tricarbonate: High-Pressure Synthesis and Structure of K ₂ C ₃ O ₇

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.