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

The inorganic pyrocarbonate salt Na-2[C2O5] crystallizes in the acentric, monoclinic space group P2(1) 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. Na-2[C2O5] was synthesized in a laser-heated diamond anvil cell at 20(2) GPa by heating a mixture of Na-2[CO3] + CO2 to approximate 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.