A⁵⁷Fe Mössbauer Study ofREBa₂Fe₃O_8+wTriple Perovskites with Varied Oxygen Content (RE=Dy, Er, and Y)

REBa₂Fe₃O_8+wphases (RE=Dy, Er, and Y) with a varied oxygen content (-0.25<w<0.10 in general, widest forRE=Dy, narrowest forRE=Er) have been obtained by quenching from controlled atmospheres. The triple-perovskite-type structure is adopted for these phases, with two crystallographically different iron atoms, tetragonal abovew≈-0.15 and orthorhombic below that oxygen content. The valence and spin states of iron are investigated using⁵⁷Fe Mössbauer spectroscopy. At room temperature, an antiferromagnetic ordering is observed for all values ofw. Six- and five-coordinated Fe³⁺,S=52environments are distinguished in the Mössbauer spectra and their occupancies forw=0 are in the exact structural ratio of these two coordinations. Forw>0, tetravalent iron (S=2) accompanies the extra oxygen atoms at the (originally) square-pyramidal Fe site, whereas divalent iron (S=2) accompanies the oxygen vacancies formed in the octahedral Fe site whenw<0. The latter splits into two components, the intensities of which correspond to the fractions of five- and four-coordinated iron with a random distribution of meridial oxygen vacancies in an octahedron.