An investigation of the Fe and Mo oxidation states in Sr2Fe2−xMoxO6 (0.25 ⩽ x ⩽ 1.0) double perovskites by X-ray absorption spectroscopy

► The unit cell of Sr2Fe2−xMoxO6 increased when x changed from 0.50 to 0.65. ► The increase in unit cell results from a cubic to tetragonal transformation. ► Fe K- and Mo K-edge XANES show reduction of Fe3+ to Fe2+/3+ and Mo+6 to Mo5+/6+. ► Reduction of Fe3+ drives the cubic to tetragonal unit cell transformation.