Synthesis and Characterization of Lithium Pyrocarbonate (Li ₂ [C ₂ O ₅ ]) and Lithium Hydrogen Pyrocarbonate (Li[HC ₂ O ₅ ])

The anhydrous pyrocarbonate and the first hydrogen pyrocarbonate Li[HC₂O₅] have been synthesized in a laser-heated diamond anvil cell at moderate pressures ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>≈</mml:mo> <mml:mn>25</mml:mn></mml:mrow> <mml:annotation>${ \approx 25}$</mml:annotation> </mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>⋯</mml:mo> <mml:annotation>${ \cdots }$</mml:annotation> </mml:semantics> </mml:math> O hydrogen bond upon pressure release.