The synthesis of a novel type of carbonate, namely of the inorganic pyrocarbonate salt Sr­[C2O5], which contains isolated [C2O5]2–-groups, significantly extends the crystal chemistry of inorganic carbonates beyond the established sp 2- and sp 3-carbonates. We synthesized Sr­[C2O5] in a laser-heated diamond anvil cell by reacting Sr­[CO3] with CO2. By single crystal synchrotron diffraction, Raman spectroscopy, and density functional theory (DFT) calculations, we show that it is a pyrocarbonate salt. Sr­[C2O5] is the first member of a novel family of inorganic carbonates. We predict, based on DFT calculations, that further inorganic pyrocarbonates can be obtained and that these will be relevant to geoscience and may provide a better understanding of reactions converting CO2 into useful inorganic compounds.

We have synthesized the orthocarbonate Sr2CO4, in which carbon is tetrahedrally coordinated by four oxygen atoms, at moderately high pressures [20(1) GPa] and high temperatures (≈3500 K) in a diamond anvil cell by reacting a SrCO3 single crystal with SrO powder. We show by synchrotron powder X-ray diffraction, Raman spectroscopy, and density functional thoery calculations that this phase, and hence sp3-hybridized carbon in a CO4 4– group, can be recovered at ambient conditions. The C–O bond distances are all of similar lengths [≈1.41(1) Å], and the O–C–O angles deviate from the ideal tetrahedral angle by a few degrees only.

We have synthesized the orthocarbonate Sr 3 [CO 4 ]O in a laserheated diamond anvil cell at 20 and 30 GPa by heating to ≈3000 (300) K. Afterward, we recovered the orthocarbonate with [CO 4 ] 4− groups at ambient conditions. Single-crystal diffraction shows the presence of [CO 4 ] 4− groups, i.e., sp 3hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO 4 ] 4− tetrahedra are located in a cage formed by corner-sharing OSr 6 octahedra, i.e., octahedra with oxygen as a central ion, forming an antiperovskite-type structure. At high pressures, the octahedra are nearly ideal and slightly rotated. The highpressure phase is tetragonal (I4/mcm). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase (Pnma), where the octahedra tilt and deform slightly.

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.

CaC2O5-I4̅2d was obtained by reacting CO2 and CaCO3 at lower Earth mantle pressures and temperatures ranging between 34 and 45 GPa and between 2000 and 3000 K, respectively. The crystal structure was solved by single-crystal X-ray diffraction and contains carbon atoms tetrahedrally coordinated by oxygen. The tetrahedral CO4 4– groups form pyramidal [C4O10]4– complex anions by corner sharing. Raman spectroscopy allows an unambiguous identification of this compound, and the experimentally determined spectra are in excellent agreement with Raman spectra obtained from density functional theory calculations. CaC2O5-I4̅2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO2 under ambient conditions. As polymorphs of CaCO3 and CO2 are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC2O5-I4̅2d, which therefore needs to be considered instead of end-member CaCO3 in models of the mantle mineralogy.