Bivalent nature of Fe and W ions in the antiferromagnetic Sr2FeWO6 material produced by the combustion technique

The synthesis of the double perovskite Sr2FeWO6 through the direct combustion technique is reported. A complete characterization of the crystalline structure is performed from Synchrotron X-ray measurements and subsequent Rietveld analysis. The results evidence that this material crystallizes in a monoclinic structure, P21/n space group, with strong distortions in the Fe-O6 and W-O6 octahedra, which is consistent with Glazer's notation a-b+a-. A superficial morphological study through scanning electron microscopy images reveals that this synthesis technique allows obtaining nanometric grains with 160 nm mean size. The semiquantitative compositional analysis through spectroscopy of X-ray energy dispersion by electrons shows that the produced material does not contain other chemical elements besides those expected form the stoichiometric formula. Results of analyzes of spectroscopy of photoelectrons emitted by X-rays suggest that there is 66.9% of the material with oxidation states Fe2+ and W6+ and 33.1% with oxidation states Fe3+ and W5+. This bivalence of Fe and W cations is corroborated by means the M\"ossbauer spectroscopy technique. Finally, the magnetic response in AC susceptibility curves allows us to affirm that at low temperatures the material adopts an antiferromagnetic behavior with TN=32 K, with a tendency to the ferromagnetic state above T=150 K mostly due to the presence of Fe3+ ions in the high spin configuration 3d5, S = 5/2.