Tetrahedral frameworks and chains in oP32 CaC₂O₅ and mP80 CaCO₃ calcium carbonates above 100 GPa

Recent advances in the exploration of carbonates have established their high-pressure crystal chemistry as mainly based on carbon in the sp³ configuration. Such carbonates, built upon isolated or vertex-sharing ${\mathrm{CO}}_4^{4-}$ tetrahedra, exhibit striking structural diversity. Despite extensive research, synthesis of layered or framework carbonates remained a long-standing challenge. Herein we report on the synthesis and full structural characterization of a novel carbonate, oP32 CaC₂O₅ (Pna2₁), obtained at 122 GPa and 2800 K in a laser-heated diamond anvil cell, with a structure based on a vertex-sharing tetrahedral framework. In addition, mP80 CaCO₃ (P2₁/c) was obtained at the same conditions, featuring pyroxene-like chains of vertex-sharing tetrahedra. In contrast to previously reported CaCO₃ phases, we propose a novel racemic model based on both clockwise and counter-clockwise helical chirality of the chains. Ab initio calculations support experimental findings and indicate thermodynamic stability of oP32 CaC₂O₅ and mP80 CaCO₃ in the megabar pressure range.