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Accurate Site Occupancies for Light Atoms from Powder X‐ray Data? Oxygen/Vacancy Ordering in 6H‐BaFe 0.67 Ti 0.33 O 3−δ (δ = 0.08 and 0.32)
Author(s) -
Grey Ian E.,
Cranswick Lachlan M. D.,
Li Christina
Publication year - 1998
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889898003690
Subject(s) - powder diffraction , crystallography , octahedron , neutron diffraction , tetrahedron , oxygen , oxygen atom , materials science , chemistry , crystal structure , analytical chemistry (journal) , molecule , organic chemistry , chromatography
The accuracy of site‐occupancy results for light atoms obtained from powder X‐ray data refinements was investigated for the case of the oxygen‐deficient 6H‐hexagonal ferrites, BaFe 0.67 Ti 0.33 O 3−δ (δ = 0.08 and 0.32). Comparative Rietveld refinements were made for powder neutron diffraction (PND) data and for multiple powder X‐ray diffraction (PXRD) data‐sets collected on separate preparations using both fixed step counting time (FCT) and variable step counting time (VCT) strategies. The oxygen vacancies were found to be confined to the O1 sites which form the shared face of M 2 O 9 binuclear octahedral units. The O1 site‐occupation fractions (SOF) obtained from the averaged FCT refinements, the averaged VCT refinements and the PND refinements were all within three e.s.d.'s of the values calculated from chemical analyses. The deleterious effect of high peak overlap on the accuracy of the SOF results was demonstrated. The separate application of PND and PXRD refinements gave information on relative local displacements of titanium and iron atoms in the M 2 O 9 dimers due to oxygen removal. Anisotropic displacement refinements and Fourier maps were used to gain further information on local atomic displacements associated with anion vacancies. The results are consistent with the formation of Fe 2 O 7 corner‐shared tetrahedral dimers, as well as (Fe, Ti) 2 O 9 and (Fe, Ti) 2 O 8 groups, as the oxygen vacancies undergo local ordering in response to the local distribution of the two types of metal atoms. The use of a low‐angle truncation procedure in the PXRD refinements was shown to be effective in removing the influence of atomic valence states on the site‐occupation refinement.