Evidence for the existence of a PbCO3-II phase from high pressure X-ray measurements

The high-pressure room temperature behavior of PbCO3 was investigated by angle-dispersive synchrotron powder diffraction up to 16.16(5) GPa. A phase transition to a high-pressure polymorph II of lead carbonate was observed at a pressure of approximately pc = 8.7 GPa. Thereby, the symmetry is reduced from orthorhombic Pmcn to monoclinic P121/c1. The cell parameters at p = 8.90(5) GPa are a = 5.101(1) (angstrom), b = 8.303(3) (angstrom), c = 5.602(2) (angstrom), β = 89.51(4)o, Z = 4. The transformation is supposed to be of 2nd order.