Single crystal diffraction study of Ba[HCO][HCO][HCO][HCO], a hydrous mixed sp/sp-carbonate.

We have synthesized the first hydrous sp-carbonate by laser-heating Ba[CO], CO and HO in a diamond anvil cell at 40(3) GPa. The crystal structure of Ba[HCO][HCO][HCO][HCO] 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 [CO]-groups and tetrahedrally coordinated carbon in [CO]-groups. The [CO]-groups polymerize by corner-sharing to form pyramidal [CO]-groups, which can bind three or four hydrogen atoms. As the pyramidal [CO]-groups are a constituent of several anhydrous sp-carbonates, we now expect that further high-pressure hydrous carbonates can be obtained.