Time-Resolved Structural Analysis of K- and Ba-Exchange Reactions in Synthetic Na-birnessite using Synchrotron X-ray Diffraction
Time-resolved Rietveld refinements using synchrotron X-ray diffraction (XRD) have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. Potassium- and Ba-birnessite, like Na-birnessite, were found to have triclinic symmetry. Rietveld analyses of the XRD patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and {beta} unit-cell parameters, with a decrease of 1.7 and 0.5%, respectively, in unit-cell volume relative to Na-birnessite. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Split electron density maxima with centers at (0 0 0.5) were present for Na, K, and Ba end-members; however, with increased substitution of K{sup +} for Na{sup +}, the axis connecting the split-site maxima rotated from an orientation parallel to the b-axis to along the a-axis. Substitution of Ba{sup 2+} for Na{sup +} did not result in rotation, but splitting of the interlayer site was more pronounced.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 929891
- Report Number(s):
- BNL-80473-2008-JA; AMMIAY; TRN: US0806669
- Journal Information:
- American Mineralogist, Vol. 92; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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