Twisted [C 2 O 5 ] 2- -groups in Ba[C 2 O 5 ] pyrocarbonate.

The inorganic pyrocarbonate salt Ba[C 2 O 5 ] contains twisted pyrocarbonate anions ([C 2 O 5 ] 2- ), an atomic arrangement previously not observed in other pyrocarbonates. This unexpected additional structural degree of freedom points towards an enlarged chemical variability in this novel group of compounds. Ba[C 2 O 5 ] was synthesized in a laser-heated diamond anvil cell at 30(2) GPa by heating a mixture of Ba[CO 3 ] + CO 2 to ≈ 1500(200) K. Its crystal structure was solved from single crystal synchrotron X-ray diffraction data and confirmed by density functional theory-based calculations. The two planar [CO 3 ] 2- -groups of the [C 2 O 5 ] 2- -anion are strongly twisted around the bridging oxygen atom. Ba[C 2 O 5 ] has been observed in the pressure range of 5-30 GPa, where its symmetry is P 6/ m with Z = 12.