Raman spectroscopic study of PbCO₃ at high pressures and temperatures

Cerussite (PbCO₃) has been investigated by high-pressure and high-temperature Raman spectroscopy up to pressures of 17.2 GPa and temperatures of 723 K. Two pressure induced phase transitions were observed at about 8.0(2) and 16.0(2) GPa, respectively. The post-aragonite transition (PbCO₃-II) at 8.0(2) GPa is accompanied by softening of the v₂-out-of-plane mode of the CO₃^2- group and disappearance of the B_1g (v₄-in-plane band of the CO₃^2- group) mode. Stronger shifts of the carbonate group modes after the phase transition suggest that the new structure is more compressible. The formation of a second high-pressure polymorph begins at about 10 GPa. It is accompanied by the occurrence of three new bands at different pressures and splitting of the v₁-symmetric C-O stretching mode of the CO₃^2- group. The transitions are reversible on pressure release. A semi-quantitative phase diagram for PbCO₃ as a function of pressure and temperature is proposed.