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

Sr2FeMoO6 double perovskite systems are widely studied because of their interesting and technologically relevant physical properties. Sr2FeMoO6 is just a single composition in the Sr2Fe2-xMoxO6 solid-solution, and it is important to understand how the composition impacts the transition-metal valence states. Variations in the lattice parameters of these materials were studied using powder X-ray diffraction and it was found that a large change in the lattice constant occurs between Sr2Fe1.50Mo0.50O6 and Sr2Fe1.35Mo0.65O6 that likely coincides with a transition from a cubic to a tetragonal unit cell, in agreement with previous studies. Fe K- and Mo K-edge X-ray absorption near-edge spectra were also collected to investigate how the oxidation state and coordination environment change with composition. When the Mo content is low, Fe adopts a 3+ oxidation state and Mo adopts a 6+ oxidation state. As the Mo content is increased, the Fe and Mo cations are both partially reduced, resulting in a mixture of Fe3+ and Fe2+ and Mo5+ and Mo6+. The reduction of the metal centers apparently drives the change in unit cell. The influence of preparation method on the oxidation states of Fe and Mo was also investigated by annealing the materials under vacuum. The results reported here show that the oxidation states of Fe and Mo are strongly impacted by both composition and preparation method, which may account for the wide variety of oxidation state and magnetic properties that have been reported previously. (C) 2012 Elsevier B.V. All rights reserved.