Key role of the charge distribution in the properties of the Ba2FeMo1−xRexO6 double perovskites

The solid solution Ba2FeMo1−xRexO6 has been synthesized for 0⩽x⩽1 and characterized by X-ray diffraction. The magnetic properties of this series are remarkable: TC goes to a maximum reaching 370 K for x=0.50. The Mossbauer study shows the important role of the charge distribution in those properties. The main iron state is close to Fe2+ which corresponds to 80%, whereas the 20% remaining corresponds to a mixed valent Fe(3−δ)+ state. This suggests degeneracy and hybridization of the Fe3+–Mo5+/Fe2+–Mo6+ and Fe3+–Re5+/Fe2+–Re6+ states forming a narrow band which would be responsible for the double exchange type mechanism in these materials. The optimization of TC for x=0.50 is discussed in terms of redox reaction between Mo6+/Mo5+ and Re6+/Re5+ couples. The transport properties of these ceramics are also presented.