Ab initio study of the dolomite to dolomite-II high-pressure phase transition

Ab initio calculations were performed on ordered and disordered dolomite and dolomite-II crystal structures. The mechanism for the dolomite to dolomite-II phase transition was investigated by the calculation of vibrational frequencies. In particular, a soft mode was observed at the F point of the rhombohedral Brillouin zone, whose frequency becomes imaginary at pressure (P) higher than 16.70 GPa. Static geometry optimizations of dolomite-II in the P range 0-32 GPa allowed to study both the dolomite structural evolution with P and its compressibility. As pressure decreases, the dolomite-II distortion decreases as well, becoming almost regular at similar to 17 GPa (phase transition) and increasing again after the phase transition, to values comparable to the low-pressure dolomite. The deformation style slightly changes in the dolomite-II like crystal structures before and after the phase transition. At low P (dolomite), K-0 = 95.4 (5) GPa and K'= 4.26(8), whereas dolomite-II after the phase transition has K-0 = 3.44(3), by fixing V-0 and K-0 at the value obtained for dolomite at the equilibrium. The influence of cation disorder on the baric behavior of dolomite was also investigated. No phase transition was predicted for disordered dolomite, at least up to 26 GPa. Thermodynamic calculations carried out on the two dolomite polymorphs allow the evaluation of temperature (T) effects on the phase transition in the range of validity of a fully ordered structural model. The dolomite to dolomite-II phase transition boundary is located at P = 16.75 GPa at T = 300 K, in agreement with considerations based on the static calculation and the analysis of the soft mode.