Stability of carbonate structures with sp3-hybridized carbon atoms on decompression to ambient pressure

One of the discoveries of high-pressure crystal chemistry of the last 10–20 years is the finding of transition of carbonates from structures with triangular [CO3]-groups to structures with [CO4]-tetrahedra and the finding of intermediate compounds with tetrahedrally coordinated carbon atoms in the MO–CO2 (M = Mg, Ca, Sr, Ba) and M’2O–CO2 (M’ = Li, Na, K) systems. The possibility of dynamic stability of the structures of these intermediate compounds at atmospheric pressure was unexpected. In this work, based on the calculation of phonon dispersion curves, we carry out a systematic study of the dynamic stability of sp3-hybridized carbonate structures at atmospheric pressure and compare the obtained results with the available experimental data. As a result, for the first time, we demonstrate that sp3-structures of simple carbonates can also be dynamically stable. Namely, the phonon dispersion curve of MgCO3-P with three-membered rings of [CO4]-tetrahedra is characterized by the absence of imaginary frequencies at 0 GPa. The extensive comparison with experimental data shows that, based on the calculation of phonon dispersion curves, the possibility of the full decompression and extraction from high pressure environment of sp3-structures can be reliably predicted.