Neutron powder diffraction study of nuclear and magnetic structures of oxidized and reduced YBa2Fe3O8+w

YBa2Fe3O8+w has been investigated by neutron powder diffraction as function of temperature and oxygen nonstoichiometry close to the limits of the homogeneity range, -0.24<w<0.12. The nonstoichiometry in this triple perovskite is achieved either by accommodation of extra oxygen atoms (w>0) in the structural layers of Y, or by creating oxygen vacancies (w<0) between the BaO layers in the ab plane of the octahedrally coordinated Fe. Moderate amounts of these defects do not alter the long-range crystallographic symmetry, which is best described in space group P4/mmm at all temperatures. However, in the most reduced samples, oxygen vacancies order and the nuclear structure becomes orthorhombic (Pmmm), thus showing a mirror-like similarity to the behavior of YBa2Cu3O6+w′ upon oxidation. The effects of nonstoichiometry on these related crystal structures are discussed in terms of bond-valence sums. The cooperative magnetic structure for all compositions is based on a larger cell related to the nuclear cell by the transformation matrix (11̄0/110/002), having orthorhombic symmetry when the nuclear structure is tetragonal and monoclinic symmetry when the nuclear structure is orthorhombic. The iron moments are coupled antiferromagnetically in all three directions, the Néel temperature is almost constant as a function of w (TN≈660 K), and so is also the low-temperature saturation moment μAF≈4.0μB.