High-pressure behavior of otavite (CdCO3)

The high-pressure, room temperature behavior of otavite (CdCO3) was investigated by angle-dispersive synchrotron radiation powder diffraction up to 40 GPa, Raman spectroscopy up to 23 GPa and quantum mechanical calculations based on density functional theory. The calcite-type structure of CdCO3 is stable up to at least ∼19 GPa as shown by Raman spectroscopy. The compression mechanism was obtained from structure refinements against the diffraction data. The quantum mechanical calculations propose a calcite–aragonite phase transition to occur at about 30 GPa. The existence of a pressure-induced phase transition is supported by the Raman and diffraction experiments. Evidence for the transformation is given by broadening of X-ray reflections and external Raman bands starting from about 19 GPa in both experiments.

Research highlights▶ The calcite-type structure of CdCO3 is stable up to at least ∼19 GPa. ▶ DFT calculations suggest a calcite–aragonite-type phase transition in CdCO3 at 30 GPa. ▶ Experimental data propose a phase transition to occur at ∼19 GPa in unknown structure. ▶ Higher pressure stability of otavite with respect to calcite can be attributed only to the significantly different electron configuration of Cd2+ and Ca2+. ▶ The pressure homologue rule does not necessarily hold for cations with different valence spheres.