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The oxygen-nonstoichiometry locus in YBa2Fe3O(8+w) changes with the sign of w. The singularity at w = 0 is studied by Rietveld refinement of combined high-resolution, high-intensity neutron and synchrotron X-ray diffraction data collected at room temperature on red-colored stoichiometric YBa2Fe3O8. A residual disorder of oxygen atoms is identified in a concentration of 0.03 vacancy/interstitials per formula, with corresponding anisotropy in thermal displacements of oxygen neighbors. The disorder improves accommodation of trivalent iron over the two unequal coordination polyhedra and is not identical with the charged Frenkel disorder induced thermally. The symmetry of the cooperative magnetic order is Ib'am, with opposite moments of equal magnitude (4.07 micro(B) at 295 K) at the two iron sites.
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Karen et al. (2005) studied this question.
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