The exceptional ability of carbon to form sp2 and sp3 bonding states leads to a great structural and chemical diversity of carbon-bearing phases at non-ambient conditions. Here we use laser-heated diamond anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in CaCO3 at P > 40 GPa. We find that post-aragonite CaCO3 transforms to the previously predicted P21/c-CaCO3 with sp3-hybridized carbon at 105 GPa (~30 GPa higher than the theoretically predicted crossover pressure). The lowest enthalpy transition path to P21/c-CaCO3 includes reoccurring sp2- and sp3-CaCO3 intermediate phases and transition states, as reveled by our variable-cell nudged elastic band simulation. Raman spectra of P21/c-CaCO3 show an intense band at 1025 cm-1, which we assign to the symmetric C-O stretching vibration based on empirical and first principles calculations. This Raman band has a frequency that is ~20 % lower than the symmetric C-O stretching in sp2-CaCO3, due to the C-O bond length increase across the sp2-sp3 transition, and can be used as a fingerprint of tetrahedrally-coordinated carbon in other carbonates.
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