Phonon-driven phase transitions in calcite, dolomite, and magnesite

Inelastic x-ray scattering and thermal diffuse scattering experiments on calcite, dolomite, and magnesite show an anomalous dispersion of the transverse acoustic Γ − F branch in calcite and dolomite. We show that the high-pressure transition from calcite to calcite-II is associated with a freezing of the transverse acoustic phonon at the F point of the Brillouin zone. We also observe a softening of the transverse acoustic, F -point phonon in dolomite, and extrapolation of our experimental data is consistent with the presence of a phase transition to dolomite-II at 17 GPa. No anomaly of the Γ − F branch was observed in magnesite. We have performed density functional theory based calculations to obtain the displacement pattern of the soft phonon, which allows us to conclude that both phase transitions from calcite to calcite-II and from dolomite to dolomite-II are driven by the soft phonon at the F point of the Brillouin zone.