The defect structure and chemical lattice strain of the double perovskites Sr2BMoO6−δ (B = Mg, Fe)

Abstract

The defect structure of B-site ordered double perovskites Sr₂BMoO_6−δ was analyzed. The defect structure model was proposed and successfully verified using data on oxygen nonstoichiometry of Sr₂MgMoO_6−δ and Sr₂FeMoO_6−δ. As a result, equilibrium constants of the defect reactions involved were estimated. Fe and Mo in Sr₂FeMoO_6−δ were found to be in the mixed oxidation state close to +2.5 and +5.5, respectively. Chemical strain of the Sr₂FeMoO_6−δ double perovskite lattice was studied by in situ high temperature XRD at 1100 °C depending on pO₂. Parameter a of the Sr₂FeMoO_6−δ cubic cell was found to increase with decreasing pO₂ because of lattice chemical expansion. The tetragonal polymorph of Sr₂FeMoO_6−δ was shown to exhibit transversal isotropy with respect to chemical expansion. It was also found that its crystal lattice expands in the ab-plane and simultaneously contracts along the c-axis when the oxygen content in the double perovskite decreases. In order to describe the degree of anisotropy of chemical strain a new phenomenological coefficient was introduced. This coefficient was shown to affect both the magnitude and change direction of an oxide cell volume caused by its reduction/oxidation. Excellent agreement between the chemical expansion along the a-axis calculated for both polymorphs of Sr₂FeMoO_6−δ according to the model recently developed and that measured experimentally was shown. Chemical contraction observed along the c-axis with a decreasing oxygen content in the tetragonal polymorph was also found to coincide completely with that calculated using the approach developed in the present study.