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Title:
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Ab initio constrained crystal-chemical Rietveld refinement of Ca10(VxP1-xO4)6F2 apatites.
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Author:
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Mercier, Patrick H. J.; Dong, Zhi Li.; Baikie, Thomas.; Page, Yvon Le; White, T. J.; Whitfield, Pamela S.; Mitchel, Lyndon D.
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Copyright year:
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2007 |
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Abstract:
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Extraction of reliable bond distances and angles for
Ca10(VxP1-O4)6F2 apatites using standard Rietveld refinement
with Cu Kα X-ray powder data was significantly
impaired by large imprecision for the O-atom coordinates.
An initial attempt to apply crystal-chemical Rietveld refinements
to the same compounds was partly successful, and exposed the problematic determination of two oxygen–metal–
oxygen angles. Ab initio modeling with VASP in space groups P63/m, P21/m and Pm showed that both these angular
parameters exhibited a linear dependence with the vanadium content. Stable crystal-chemical Rietveld refinements in
agreement with quantum results were obtained by fixing these angles at the values from ab initio simulations. Residuals were comparable with the less precise standard refinements.
The larger vanadium ion is accommodated primarily by
uniform expansion and rotation of BO4 tetrahedra combined
with a rotation of the Ca–Ca–Ca triangular units. It is
proposed that the reduction of symmetry for the vanadium
end-member is necessary to avoid considerable departures from formal valences at the AII and B sites in P63/m. The
complementarity of quantum methods and structural analysis
by powder diffraction in cases with problematic least-squares extraction of the crystal chemistry is discussed. |
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Subject:
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DRNTU::Engineering::Materials::Ecomaterials. |
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Type:
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Journal Article |
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Series/ Journal Title:
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Acta crystallographica B |
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School:
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School of Materials Science and Engineering |
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Rights:
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© 2007 International Union of Crystallography. This paper was published in Acta Crystallographica B and is made available as an electronic reprint (preprint) with permission of International Union of Crystallography. The paper can be found at the following DOI: http://dx.doi.org/10.1107/S0108768106045538. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law. |
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Version:
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Published version |
