Ab initio constrained crystal-chemical Rietveld refinement of Ca10(VxP1-xO4)6F2 apatites

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DOIResolve DOI: http://doi.org/10.1107/S0108768106045538
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TypeArticle
Journal titleActa Crystallographica Section B: Structural Science
ISSN0108-7681
VolumeB63
Pages3748; # of pages: 12
Subjectapatite; crystal chemistry; ab initio calculations; X-ray powder diffraction; Rietveld refinement; crystal-chemical constraints
AbstractExtraction of reliable bond distances and angles for Ca10(VxP1-xO4)6F2 apatites using standard Rietveld refinement with Cu K(alpha) 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舑metal舑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舑Ca舑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.
Publication date
LanguageEnglish
AffiliationNRC Institute for Chemical Process and Environmental Technology (ICPET-ITPCE); NRC Institute for Research in Construction (IRC-IRC); National Research Council Canada
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public
Peer reviewedYes
NRC number51704
NPARC number14146327
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Record identifier477da056-12cf-42e7-964c-e0ecd2df1d20
Record created2010-01-14
Record modified2016-05-09
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