A FE-57 MOSSBAUER STUDY OF REBA2FE3O8-CONTENT (RE = DY, ER, AND Y)(W TRIPLE PEROVSKITES WITH VARIED OXYGEN)

REBa2Fe3O8+w phases (RE = Dy, Er, and Y) with a varied oxygen content (-0.25 < w < 0.10 in general, widest for RE = Dy, narrowest for RE = E r) have been obtained by quenching from controlled atmospheres. The tr iple-perovskite-type structure is adopted for these phases, with two c rystallographically different iron atoms, tetragonal above w approxima te to -0.15 and orthorhombic below that oxygen content, The valence an d spin states of iron are investigated using Fe-57 Mossbauer spectrosc opy. At room temperature, an antiferromagnetic ordering is observed fo r all values of w, Six- and five-coordinated Fe3+, S = 5/2 environment s are distinguished in the Mossbauer spectra and their occupancies for w = 0 are in the exact structural ratio of these two coordinations, F or w > 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 sit e when w < 0, The latter splits into two components, the intensities o f which correspond to the fractions of five- and four-coordinated iron with a random distribution of meridial oxygen vacancies in an octahed ron. (C) 1998 Academic Press.