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 {{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. CO4 tetrahedra are expected to polymerize in layers and frameworks at high pressure, but the synthesis of layered or framework carbonates remains challenging. Here, the authors prepare and characterize oP32 CaC2O5 (Pna21) — with a structure based on a vertex-sharing tetrahedral framework — and mP80 CaCO3 (P21/c) — featuring pyroxene-like chains of vertex-sharing tetrahedra — and propose a racemic model based on clockwise and counter-clockwise helical chain chirality.