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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, BaFe0.67Ti0.33O3−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 M2O9 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 M2O9 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 Fe2O7 corner-shared tetrahedral dimers, as well as (Fe, Ti)2O9 and (Fe, Ti)2O8 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.

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