High-pressure synthesis of acentric sodium pyrocarbonate, Na 2 [C 2 O 5 ]

The inorganic pyrocarbonate salt Na [C O ] crystallizes in the acentric, monoclinic space group 2 with = 2. It contains monovalent alkali metal cations together with isolated pyrocarbonate anions. The [C O ] -groups consist of two planar [CO ] -groups which are slightly tilted with respect to each other around the bridging oxygen atom. Na [C O ] was synthesized in a laser-heated diamond anvil cell at 20(2) GPa by heating a mixture of Na [CO ] + CO 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 [C O ] -complex anions.