High-pressure synthesis of acentric sodium pyrocarbonate, Na₂[C₂O₅]

The inorganic pyrocarbonate salt Na₂[C₂O₅] crystallizes in the acentric, monoclinic space group P2₁ with Z = 2. It contains monovalent alkali metal cations together with isolated pyrocarbonate anions. The [C₂O₅]^2--groups consist of two planar [CO₃]^2--groups which are slightly tilted with respect to each other around the bridging oxygen atom. Na₂[C₂O₅] was synthesized in a laser-heated diamond anvil cell at 20(2) GPa by heating a mixture of Na₂[CO₃] + CO₂ to ≈800(200) K. Its crystal structure was obtained by single crystal synchrotron X-ray diffraction and confirmed by density functional theory-based calculations in combination with Raman spectroscopy. Second harmonic generation measurements verified the acentric space group symmetry. The crystal structure is characterized by alternating layers of Na⁺-cations and [C₂O₅]^2--complex anions.