Order-disorder at Fe sites in SrFe2/3B"1/3O3 (B" = Mo, W, Te, U) tetragonal double perovskites

We have prepared SrFe2/3B''O-1/3(3) (B" = Mo, U, Te, and W) double perovskites in polycrystalline form by ceramic methods. Phases with B" = U, Te and W have been studied by X-ray powder diffraction and the results have been compared with neutron diffraction data available for B" = Mo. At room temperature, the stoichiometric samples crystallize in the tetragonal crystal system (space group I4/m, Z = 4). Cell parameters when B" = U, Te and W are a = 5.6936(1) Angstrom, c = 8.0637(1) Angstrom; a = 5.5776(1) Angstrom, c = 7.9144(3) Angstrom and a = 5.5707(3) Angstrom, c = 7.9081(5) Angstrom, respectively. The Mossbauer spectra at room temperature for all compounds show hyperfine parameters belonging to two Fe3+ sites located at lattice positions with different degrees of distortion. This is in agreement with diffraction data that indicate that the series of compounds display different degrees of Fe-site disorder, which increases in the following sequence: Mo<U<Te<W. (C) 2003 Elsevier Inc. All rights reserved.