The Orthorhomic Structure of CaCo[subscript 3], SrCO[subscript 3], PbCO[subscript 3] and BaCO[subscript 3]: 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 < Sr < Pb < Ba). The CaCO{sub 3} structure is, therefore, the most distorted, whereas the BaCO{sub 3} 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 M{sup 2+} 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 CO{sub 3} group can be obtained reliably only by using neutron-diffraction data, especially in the presence of other heavy atoms.
- Research Organization:
- Argonne National Laboratory (ANL)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1006201
- Journal Information:
- Can. Mineral., Journal Name: Can. Mineral. Journal Issue: (5) ; 10, 2009 Vol. 47; ISSN CAMIA6; ISSN 0008-4476
- Country of Publication:
- United States
- Language:
- ENGLISH
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