The importance for the global carbon cycle, the P–T phase diagram of CaCO3 has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO3. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO3-VII. CaCO3-VII is a structural analogue of CaCO3-P21/c-l, predicted theoretically earlier. The P–T phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO3-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO3-VI was found to be metastable in the whole pressure–temperature range.

The exceptional ability of carbon to form sp 2 and sp 3 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 CaCO 3 at P > 40GPa. We find that post-aragonite CaCO 3 transforms to the previously predicted P2 1 /c-CaCO 3 with sp 3 -hybridized carbon at 105 GPa (~30 GPa higher than the theoretically predicted crossover pressure). The lowest enthalpy transition path to P2 1 /c-CaCO 3 includes reoccurring sp 2 -and sp 3 -CaCO 3 intermediate phases and transition states, as reveled by our variable-cell nudged elastic band simulation. Raman spectra of P2 1 /c-CaCO 3 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 sp 2 -CaCO 3 , due to the C-O bond length increase across the sp 2 -sp 3 transition, and can be used as a fingerprint of tetrahedrally-coordinated carbon in other carbonates.