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

Abstract

ABSTRACTBy 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 ∼50 GPa at room temperature. Three pressure-induced transitions have been observed at ∼8.5, ∼15 and ∼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 K0 = 131(4) GPa, (fixed) and V0 = 246(1) Å3. 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.