Raman spectroscopy and x-ray diffraction of sp₃ CaC O₃ at lower mantle pressures

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 nonambient 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>40GPa. We find that postaragonite CaCO₃ transforms to the previously predicted P21/cCaCO₃ with sp3-hybridized carbon at 105 GPa (∼30GPa higher than the theoretically predicted crossover pressure). The lowest-enthalpy transition path to P21/cCaCO₃ includes reoccurring sp2 and sp3CaCO₃ intermediate phases and transition states, as revealed by our variable-cell nudged-elastic-band simulation. Raman spectra of P21/cCaCO₃ show an intense band at 1025cm-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 sp2CaCO₃ 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.