Double perovskite Sr2FeMoO6-xNx (x=0.3, 1.0) oxynitrides with anionic ordering

Two new oxynitride double perovskites of composition Sr2FeMoO6-xNx (x=0.3, 1.0) have been synthesized by annealing precursor powders obtained by citrate techniques in flowing ammonia at 750 degrees C and 650 degrees C, respectively. The polycrystalline samples have been characterized by chemical analysis, x-ray and neutron diffraction (NPD), Mossbauer spectroscopy and magnetic measurements. They exhibit a tetragonal structure with a=5.5959(1) angstrom, c=7.9024(2) angstrom, V=247.46(2) angstrom(3) for Sr2FeMo5.7N0.3: and a=5.6202(2) angstrom, c=7.9102(4) angstrom, V=249.85(2) angstrom(3) for Sr2FeMoO5N: space group I4/m, Z=2. The nitridation process seems to extraordinarily improve the long-range Fe/Mo ordering, achieving 95% at moderate temperatures of 750 degrees C. The analysis of high resolution NPD data, based on the contrast existing between the scattering lengths of O and N, shows that both atoms are located at (O,N)2 anion substructure corresponding to the basal ab plane of the perovskite structure, whereas the O1 site is fully occupied by oxygen atoms. The evolution of the < Fe-O > and < Mo-O > distances suggests a shift towards a configuration close to Fe4+(3d(4), S=2):Mo5+(4d(1), S=1/2). The magnetic susceptibility shows a ferrimagnetic transition with a reduced saturation magnetization compared to Sr2FeMoO6, due to the different nature of the magnetic double exchange interactions through Fe-N-Mo-N-Fe paths in contrast to the stronger Fe-O-Mo-O-Fe interactions. Also, the effect observed by low-temperature NPD seems to reduce the ordered Fe moments and enhance the Mo moments, in agreement with the evolution of the oxidation states, thus decreasing the saturation magnetization. (C) 2011 Elsevier Inc. All rights reserved.