Partial oxygen ordering in cubic perovskite REBa2Fe3O8+w (RE = Gd, Eu, Sm,Nd)

Single-phase samples of cubic REBa2Fe3O8+w with RE = Gd, Eu, Sm, Nd were sy nthesized and equilibrated at 900 degrees C in atmospheres with controlled partial pressures of oxygen. The oxygen content parameter w ranged from app roximately -0.30, which is the lower decomposition limit, to between w = 0. 17 for RE = Gd and w = 0.37 for RE = Nd, achieved in O-2 without crossing t he upper limit. According to Fe-57 Mossbauer spectroscopy, all samples are antiferromagnets at room temperature, with iron in high-spin states (S = 2 for Fe2+ and Fe4+; S = 5/2 for Fe3+). The contents of divalent or, alternat ively, tetravalent iron states are consistent with the stoichiometry of the samples. At the stoichiometric composition (w = 0), all Mossbauer componen ts correspond to trivalent iron, differing only in the coordination geometr ies of their oxygen neighborhoods. The sum-up of the observed coordination numbers shows that the oxygen disorder in these cubic (by X-ray diffraction ) phases is a linear combination of the two limiting cases of oxygen vacanc y distribution: binomial (random) and ordered (one vacancy per every third pseudocubic cell). This corresponds to a gradual change from the long-range order seen in triple-perovskite-type phases (RE = Er to Dy) via a short-ra nge order seen in the present systems (RE = Gd to Nd) to a fully random dis order (RE = La). Eventual variations in re affect the coordination statisti cs in details, but change the overall picture very little. (C) 1999 Academi c Press.