Twisted [C₂O₅]²⁻-groups in Ba[C₂O₅] pyrocarbonate

The inorganic pyrocarbonate salt Ba[C₂O₅] contains twisted pyrocarbonate anions ([C₂O₅]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[C₂O₅] was synthesized in a laser-heated diamond anvil cell at 30(2) GPa by heating a mixture of Ba[CO₃] + CO₂ 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 [CO₃]²⁻-groups of the [C₂O₅]²⁻-anion are strongly twisted around the bridging oxygen atom. Ba[C₂O₅] has been observed in the pressure range of 5–30 GPa, where its symmetry is P6/m with Z = 12.