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

The crystal structures of four isostructural orthorhombic carbonates, CaCO{sub 3} (aragonite), SrCO{sub 3} (strontianite), PbCO{sub 3} (cerussite), and BaCO{sub 3} (witherite), were obtained by Rietveld refinements using data acquired by synchrotron high-resolution powder X-ray diffraction (HRPXRD). For BaCO{sub 3}, 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 CO{sub 3} group is slightly non-planar, and the two independent C-O distances are slightly different. The CO{sub 3} group becomes more symmetrical and less aplanar from CaCO{sub 3} to BaCO{sub 3} (M{sub radii}{sup 2+}: Ca and distances, average angle, and aplanarity. These linear trends are the result of the effective size of the divalent ionic radius of the M cations thatmore » are coordinated to nine oxygen atoms. The geometrical features of the CO{sub 3} group can be obtained reliably only by using neutron-diffraction data, especially in the presence of other heavy atoms.« less