Dataset Information

Reinvestigation of the crystal structure of Ca₂Ce₈(SiO₄)₆O₂ apatite by Rietveld refinement.

ABSTRACT: Ca₂Ln₈(SiO₄)₆O₂ apatites with Ln = La, Ce, Pr, Nd, Sm, Eu, Gd and Tb crystallize in space group P6₃/m. The crystal structure of apatite-type Ca₂Ce₈(SiO₄)₆O₂ [dicalcium octa-cerium hexa-kis-(silicate) dioxide], which has been synthesized by calcination, was refined from powder X-ray diffraction data using the Rietveld method. A database survey shows that contrary to the previously published Ca₂Ce₈(SiO₄)₆O₂ structure [Skakle et al. (2000 ?). Powder Diffr.15, 234-238], the cell volume of the structure reported here is consistent with those of other Ln apatites.

SUBMITTER: Massoni N

PROVIDER: S-EPMC6038630 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature

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Reinvestigation of the crystal structure of Ca2Ce8(SiO4)6O2 apatite by Rietveld refinement.

Massoni Nicolas N Hegron Ronan R Campayo Lionel L

Acta crystallographica. Section E, Crystallographic communications 20180612 Pt 7

Ca2Ln8(SiO4)6O2 apatites with Ln = La, Ce, Pr, Nd, Sm, Eu, Gd and Tb crystallize in space group P63/m. The crystal structure of apatite-type Ca2Ce8(SiO4)6O2 [dicalcium octa-cerium hexa-kis-(silicate) dioxide], which has been synthesized by calcination, was refined from powder X-ray diffraction data using the Rietveld method. A database survey shows ...[more]

PMID: 30002893

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Project description:This study aims to solve the site-occupancy issues of any crystalline substance, showing that the structure refinement results should not be limited to a simple description of the crystal geometry but should go beyond that, providing refined site-scattering values consistent with the chemical data. A detailed description of the structure of the amphibole-supergroup minerals is very challenging owing to their complex chemical composition that renders the process of cation partition extremely difficult, particularly because of the occurrence of multivalent elements. Since amphiboles naturally occur under a fibrous morphology and have largely been used to produce asbestos, there is a growing demand for detailed and accurate structural data in order to study the relationships between structure, composition and toxicity. The present study proposes a recommended refinement procedure for both X-ray single-crystal structure refinement (SREF) and Rietveld analysis for tremolite, selected as a test case. The corresponding structural results are compared to estimate the ‘degree of confidence’ of the Rietveld refinement with regard to SREF. In particular, it is shown that the interpretation of the electron density of the tremolite structure by SREF is model dependent. By assuming that the site-scattering values from SREF should be as close as possible to those from electron microprobe analysis, as a crucial constraint for the correct description of the final crystal-chemical model, it is found that it is best satisfied by using partially ionized scattering curves (SCs) for O and Si, and neutral SCs (neutral oxygen curves or NOCs) for other atoms. This combination leads to the best fit to the diffraction data. Moreover, it is found that Rietveld refinement using NOCs produces the best structural results, in excellent agreement with SREF. It is worth noting that, due to the complexity of the diffraction pattern and the fairly large number of freely refinable parameters, refinements with different combinations of SCs produce results almost indistinguishable from a statistical point of view, albeit showing significant differences from a structural point of view.

Dataset Information

Reinvestigation of the crystal structure of Ca₂Ce₈(SiO₄)₆O₂ apatite by Rietveld refinement.

Publications

Reinvestigation of the crystal structure of Ca<sub>2</sub>Ce<sub>8</sub>(SiO<sub>4</sub>)<sub>6</sub>O<sub>2</sub> apatite by Rietveld refinement.

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