Evidence for valence fluctuation of Fe in Sr2FeMoO6−w double perovskite

In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by 57Fe Mossbauer spectroscopy. From 5 K to room temperature the Mossbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.