Sr[C₂O₅] is an Inorganic Pyrocarbonate Salt with [C₂O₅]^2– Complex Anions

Abstract

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.

High‐Pressure Crystal Structures of Pb2CO4 and PbC2O5 with Tetrahedral [CO4] and Pyrocarbonate [C2O5] atomic groups

“…PbC 2 O 5 ‐ P 2 1 /c is the third known pyrocarbonate structure. Two other structures are SrC 2 O 5 ‐ P 2 1 /c , [6] to which PbC 2 O 5 ‐ P 2 1 /c is isostructural, and CaC 2 O 5 ‐ Cc [7] . The structure appearing on compression of SrC 2 O 5 ‐ P 2 1 /c is not yet known.…”

“…The following earlier known structures have been used for the enthalpy calculations: PbCO 3 ‐ Pnma , [22] Pb 3 CO 5 ‐ Pnma , [23] Pb 2 CO 4 ‐ P 2 1 2 1 2 1 , [24] and PbCO 3 ‐ R 3 m [25] . In addition, we used structural models of Ca‐ and Sr‐carbonates with Ca and Sr replaced by Pb: Ca 3 CO 5 ‐ Cmcm , [1] Ca 3 CO 5 ‐ I 4/ mcm , [1] CaC 2 O 5 ‐ Pc , [7] CaC 2 O 5 ‐F dd 2, [1] CaC 2 O 5 ‐, [1] CaC 2 O 5 ‐ , [5] CaC 2 O 5 ‐ Cc , [7] and SrC 2 O 5 ‐ P 2 1 /c [6] . These structures were loaded as seeds for further crystal structure prediction using USPEX.…”

“…These compositions were chosen based on the analogy with other MOÀ CO 2 systems (M=Mg, Ca, Sr, Ba, and Mn), where crystal structures of only 3 : 1, 2 : 1, 1 : 1 and 1 : 2 are realized at high pressure both in theory [1][2][3][4] and experiment. [5,6,8,9,12,18] The search was performed for 1-4 formula units per unit cell. The size of the first generation in the USPEX calculations was equal to 65 structures.…”

“…[5] Recently, the crystal structure of pyrocarbonate SrC 2 O 5 with doubled (connected through the common oxygen atom) [CO 3 ] triangles was synthesized at 30 GPa. [6] After this finding, pyrocarbonate structure was also found for CaC 2 O 5 at pressure above 10 GPa. [7] Despite the sp 3 hybridization of carbon atoms, which is untypical for carbonates at ambient pressure, some of the found structures turned out to be dynamically stable at 0 GPa.…”

“…The sp 3 ‐hybridized structure of this compound was experimentally synthesized at 34–45 GPa [5] . Recently, the crystal structure of pyrocarbonate SrC 2 O 5 with doubled (connected through the common oxygen atom) [CO 3 ] triangles was synthesized at 30 GPa [6] . After this finding, pyrocarbonate structure was also found for CaC 2 O 5 at pressure above 10 GPa [7]…”