Single crystal diffraction study of Ba[H₄C₄O₁₀][H₃C₄O₁₀][H₂CO₃][HCO₃], a hydrous mixed sp²/sp³-carbonate

We have synthesized the first hydrous sp³-carbonate by laser-heating Ba[CO₃], CO₂ and H₂O in a diamond anvil cell at 40(3) GPa. The crystal structure of Ba[H₄C₄O₁₀][H₃C₄O₁₀][H₂CO₃][HCO₃] was determined by synchrotron single crystal X-ray diffraction. The experiments were complemented by DFT-based calculations. This compound is the first example of a carbonate containing both trigonally-coordinated carbon in [CO₃]^2--groups and tetrahedrally coordinated carbon in [CO₄]^4--groups. The [CO₄]^4--groups polymerize by corner-sharing to form pyramidal [C₄O₁₀]^4--groups, which can bind three or four hydrogen atoms. As the pyramidal [C₄O₁₀]^4--groups are a constituent of several anhydrous sp³-carbonates, we now expect that further high-pressure hydrous carbonates can be obtained.