A New High Pressure Polymorph of Na2[C2O5] Synthesized at 41 GPa

The inorganic pyrocarbonate Na2[C2O5] was synthesized at 41(3) GPa and 1000(300) K using a laser-heated diamond anvil cell. The compound was obtained by heating a mixture of Na2[CO3] and CO2 under high-pressure conditions. Its crystal structure was determined by single crystal synchrotron X-ray diffraction. This high pressure polymorph of Na2[C2O5] crystallizes in the space group Fdd2 with Z = 48 and an extremely anisotropic unit cell shape. The lattice parameters at 41(3) GPa are a = 50.466(7) Å, b = 23.681(6) Å, and c = 2.7212(4) Å, corresponding to a unit cell volume of 3252.1(10) Å3. The two [CO3]2−-groups which form the [C2O5]2−-groups are slightly tilted with respect to each other. Density functional theory based calculations confirmed the structure and supported the interpretation of the Raman spectra. Na2[C2O5]−Fdd2 is a high pressure polymorph of the previously reported Na2[C2O5]−P21.