THE ORTHORHOMBIC STRUCTURE OF CaCO3, SrCO3, PbCO3 AND BaCO3: LINEAR STRUCTURAL TRENDS

The crystal structures of four isostructural orthorhombic carbonates, CaCO3 (aragonite), SrCO3 (strontianite), PbCO3 (cerussite), and BaCO3 (witherite), were obtained by Rietveld refinements using data acquired by synchrotron high-resolution powder X-ray diffraction (HRPXRD). For BaCO3, powder neutron-diffraction data were obtained and refined by the Rietveld method. For aragonite, we also carried out a refinement of the structure by single-crystal X-ray diffraction. These carbonates belong to the space group Pmcn, with Z = 4. The CO3 group is slightly non-planar, and the two independent C-O distances are slightly different. The CO3 group becomes more symmetrical and less aplanar from CaCO3 to BaCO3 (M-radii(2+): Ca < Sr < Pb < Ba). The CaCO3 structure is, therefore, the most distorted, whereas the BaCO3 structure is the least distorted. Several linear structural trends are observed in plots of selected parameters as a function of the unit-cell volume, V. These parameters are radii of the nine-coordinated M2+ cations, the unit-cell axes, the average M-O and C-O distances, average O-C-O angle, and aplanarity. These linear trends are the result of the effective size of the divalent ionic radius of the M cations that are coordinated to nine oxygen atoms. The geometrical features of the CO3 group can be obtained reliably only by using neutron-diffraction data, especially in the presence of other heavy atoms.