Abstract
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, K(2)C(3)O(7), discovered in laser-heated diamond anvil cells at 55(3) and 45(2) GPa. The crystal structure of K(2)C(3)O(7) was determined in situ by using synchrotron single-crystal X-ray diffraction from a polycrystalline sample. It features nonplanar [C(3)O(7)](2-) anions, which consist of three corner-sharing planar CO(3) groups rotated relative to one another. This anion extends the homologous series of sp(2)-carbonates: [CO(3)](2-)─[C(2)O(5)](2-)─[C(3)O(7)](2-). Raman spectroscopy establishes the characteristic vibrational fingerprint of the [C(3)O(7)](2-) anion. Density functional theory (DFT) calculations corroborate the experimental results and suggest a thermodynamic stability of K(2)C(3)O(7) between 10 and 55 GPa. DFT calculations predict a phase transition between 80 and 90 GPa associated with polymerization of the [C(3)O(7)](2-) groups, accompanied by a change in the coordination polyhedra of two carbon atoms from triangles to tetrahedra. These results imply that other sp(2)-and mixed sp(2)/sp(3)-carbonates might be stabilized at a high pressure.