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

The inorganic pyrocarbonate salt Na 2 [C 2 O 5 ] crystallizes in the acentric, monoclinic space group P 2 1 with Z = 2. It contains monovalent alkali metal cations together with isolated pyrocarbonate anions. The [C 2 O 5 ] 2- -groups consist of two planar [CO 3 ] 2- -groups which are slightly tilted with respect to each other around the bridging oxygen atom. Na 2 [C 2 O 5 ] was synthesized in a laser-heated diamond anvil cell at 20(2) GPa by heating a mixture of Na 2 [CO 3 ] + CO 2 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 2 O 5 ] 2- -complex anions.