Pressure-induced phase transformations of PbCO3 by X-ray diffraction and Raman spectroscopy

By employing synchrotron radiation, X-ray diffraction and Raman spectroscopy, the high pressure structural transformations of lead carbonate PbCO3 was investigated in diamond anvil cells up to similar to 50 GPa at room temperature. Three pressure-induced transitions have been observed at similar to 8.5, similar to 15 and similar to 26 GPa, respectively. The transition from PbCO3-I to PbCO3-II is a displacive transformation featured with anti-rotation of [CO3](2-) triangles. PbCO3-II is a metastable phase because the [CO3](2-) groups are in a unfixed state until they reach the equilibrium positions in PbCO3-III. PbCO3-III adopts a monoclinic symmetry with primitive lattice, and tendentiously exhibits a more compressible b-axis relative to c-axis. Isothermal pressure-volume relationship of PbCO3-III is well described by the Birch-Murnaghan equation of state with K-0 = 131(4) GPa, [GRAPHICS] (fixed) and V-0 = 246(1) angstrom(3). However, little information on the crystal structure of PbCO3-IV can be extracted from the present experiment. The transformation process of PbCO3 exhibits similarity to that of calcite and dolomite.