Systematic studies on effects of cationic ordering on structural and magnetic properties in Sr2 FeMoO6

Sr2 FeMoO6 samples with various degrees of Fe, Mo ordering were prepared from a sol-gel-derived precursor with a sample-encapsulation technique. The order parameter ξ for Fe and Mo, determined by Rietveld refinement, ranged from 0.56 to 0.95. Structural data suggest that the equilibration reaction FeIII + MoV = FeII + MoVI is shifted toward the left by the trapping of minority-spin electrons in Mo-rich trap states for ξ<0.7 and that it is shifted toward the right at a transition from polaronic to itinerant minority-spin electrons at ξ0.9. In accord with a first-order polaronic-itinerant electronic transition, well-ordered itinerant-electron regions appear to be nucleated and to grow in a partially ordered matrix until they meet at antiphase boundaries. This heterogeneous development is at odds with models of homogeneous site disorder with a constant oxidation state of the Fe and Mo atoms.