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

Abstract

By selection of appropriately sized rare earth elements and suitable reaction atmosphere, a new double-perovskite-type iron oxide REBaFe ₂ O _{5+ w} (RE=Nd and Sm) has been synthesized, with, ideally, all Fe atoms in square-pyramidal coordinations when w=0. Like in the related triple-perovskite-type YBa ₂ Fe ₃ O _{8+ w ′} , the added oxygen atoms w are accommodated in the RE layer. The homogeneity 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 °C without the upper homogeneity limit being crossed. The most reduced REBaFe ₂ O _{5+ w} phases oxidize very easily, even at room temperature. The crystal structure, as seen at room temperature after quenching from ca. 1000 °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 as an example) which are orthorhombic. For samples with w=0.5 low temperature (ca. 500 °C) annealing leads to ordering of the added oxygens within the rare earth layer. This ordering produces equal concentrations of square pyramidal and octahedral Fe ³⁺ . The ordered structure, a type which has not previously been observed, belongs to space group Pmna (no. 53) with a≈4a _p , b≈a _p and c≈2a _p , where a _p is the primitive cubic perovskite cell edge (a _p ≈3.9 Å). Structural refinements were obtained by applying the Rietveld method to synchrotron X-ray powder diffraction data.