Synthesis and Characterization of Lithium Pyrocarbonate (Li2[C2O5]) and Lithium Hydrogen Pyrocarbonate (Li[HC2O5])

The anhydrous pyrocarbonate and the first hydrogen pyrocarbonate Li[HC₂O₅] have been synthesized in a laser-heated diamond anvil cell at moderate pressures ($$  GPa). The structures of the two compounds have been obtained from single crystal X-ray diffraction data. Raman spectroscopy and DFT calculations have been employed to further characterize their structure-property relations. The present results significantly enlarge the group of inorganic pyrocarbonates by the discovery of the hydrogenated pyrocarbonate anion Li[HC₂O₅]⁻. In the structure of Li[HC₂O₅] there is a symmetric O−H−O arrangement at high pressures, which converts to a conventional O−H$$ O hydrogen bond upon pressure release.