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

摘要

YBa2Fe3OSdivided byw has been investigated by neutron powder diffraction as function of temperature and oxygen nonstoichiometry close to the limits of the homogeneity range, -0.24 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 YBa2Cu3O6divided byw 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 (1 (1) over bar0/110/002), having orthorhombic symmetry when the nuclear structure is tetragonal and monoclinic symmetry when the nuclear structure is orthorhombic. The iron moments are coupled anti ferromagnetically in all three directions, the Neel temperature is almost constant as a function of w (T-N approximate to 660 K), and so is also the low-temperature saturation moment mu(AF) approximate to 4.0mu(B). (C) 2003 Elsevier Science (USA). All rights reserved. [References: 32]