Synthesis and structural investigations of the double perovskites REBaFe2O5+w (RE = Nd, Sm)

By selection of appropriately sized rare earth elements and suitable reacti on atmosphere, a new double-perovskite-type iron oxide REBaFe2O5+w (RE = Nd and Sm) has been synthesized, with, ideally, all Fe atoms in square pyrami dal coordinations when w = 0. Like in the related triple-perovskite-type YB a2Fe3O8+w, the added oxygen atoms w are accommodated in the RE layer. The h omogeneity range in w is very wide, extending from 0.02(1) for RE = Sm and 0.050(6) for RE = Nd to w = 0.65 and w = 0.80, respectively, seen in O-2 at 985 degrees C without the upper homogeneity limit being crossed. The most reduced REBaFe2O5+w phases oxidize very easily, even at room temperature. T he crystal structure, as seen at room temperature after quenching from ca. 1000 degrees C, is tetragonal, except for the most reduced compositions for RE = Sm (w < 0.045) and most oxidized compositions for RE = Nd (w = 0.69 a s an example) which are orthorhombic. For samples with w = 0.5 low temperat ure (ca. 500 degrees C) annealing leads to ordering of the added oxygens wi thin the rare earth layer. This ordering produces equal concentrations of s quare pyramidal and octahedral Fe3+. The ordered structure, a type which ha s not previously been observed, belongs to space group Pmna (no. 53) with a approximate to 4a(p), b approximate to a(p) and c approximate to 2a(p), wh ere a(p) is the primitive cubic perovskite cell edge (a(p) approximate to 3 .9 Angstrom). Structural refinements were obtained by applying the Rietveld method to synchrotron X-ray powder diffraction data.