Oxygen Stoichiometry in Sr 2 FeMoO 6 , the Determination of Fe and Mo Valence States, and the Chemical Phase Diagram of SrO‐Fe 3 O 4 ‐MoO 3

Phase diagram studies of the system SrO‐Fe 3 O 4 ‐MoO 3 were undertaken in Ar/1% H 2 at 1200°C with particular emphasis on the region near Sr 2 FeMoO 6 (SFMO). We find that Sr 2 FeMoO 6 is not a point compound but is part of the Sr 2+ z Fe 2− x Mo x O y solid solution region. The oxygen stoichiometry, y , in Sr 2 Fe 2− x Mo x O y was also determined across the solid solution region. Using the experimental data and by invoking a substitution model based on preferential substitution of similar ionic radii for Mo ions on the Fe ion sites, it was possible, for the first time, to calculate the individual Fe and Mo valence states in the substituted samples. In Ar/1% H 2 at 1200°C, Fe 2+ /Fe 3+ and Mo 5+ /Mo 6+ are present in Sr 2 Fe 2− x Mo x O y . With increasing Mo substitution, the amount of Fe 3+ decreases. In the ideal Sr 2 FeMoO 6 composition, we predict the amounts of Fe 2+ , Fe 3+ , Mo 5+ , and Mo 6+ to be equal. In Ar/1% H 2 at 1200°C, there is a predominance of Mo 5+ in Sr 2 FeMoO y because the oxygen stoichiometry, y , in SFMO has a value of only 5.8. Hence such processing conditions are too reducing. The work predicts the processing conditions for optimizing film and bulk properties, as well as explaining why there is a wide variability of reported sample properties in the literature. To optimize the magnetoresistive properties of Sr 2 FeMoO 6 , it is critical (a) to carefully select the correct annealing atmosphere to obtain oxygen stoichiometric Sr 2 FeMoO 6 and (b) to control cation composition; otherwise nonstoichiometric Sr 2 Fe 2− x Mo x O y will form instead of Sr 2 FeMoO 6 .