Synthesis and Structure of Pb[C₂O₅]: An Inorganic Pyrocarbonate Salt

We have synthesized Pb[C₂O₅], an inorganic pyrocarbonate salt, in a laser-heated diamond anvil cell (LH-DAC) at 30 GPa by heating a Pb[CO₃] + CO₂ mixture to ≈2000(200) K. Inorganic pyrocarbonates contain isolated [C₂O₅]^2- groups without functional groups attached. The [C₂O₅]^2- groups consist of two oxygen-sharing [CO₃]^3- groups. Pb[C₂O₅] was characterized by synchrotron-based single-crystal structure refinement, Raman spectroscopy, and density functional theory calculations. Pb[C₂O₅] is isostructural to Sr[C₂O₅] and crystallizes in the monoclinic space group P2₁/c with Z = 4. The synthesis of Pb[C₂O₅] demonstrates that, just like in other carbonates, cation substitution is possible and that therefore inorganic pyrocarbonates are a novel family of carbonates, in addition to the established sp² and sp³ carbonates.