Crystal and Magnetic Structure of Sr2MIrO6 (M = Ca, Mg) Double Perovskites – A Neutron Diffraction Study

Abstract

AbstractSr2CaIrO6 and Sr2MgIrO6 double perovskites have been prepared as polycrystalline powders from citrate precursors treated under oxygen pressure conditions. These compounds have been studied by X‐ray and neutron powder diffraction (NPD), magnetic measurements, and X‐ray absorption spectroscopy. The crystal symmetry of both oxides is monoclinic (space group P21/n), and the unit‐cell parameters are related to the ideal simple perovskite cell as a ≈ √2a0, b ≈ √2a0, c ≈ 2a0, β ≈ 90°, where a0 is the edge of the simple cubic ABO3 perovskite. This superstructure is a result of the 1:1 ordering of Ca/Mg and Ir ions over the B site and the distortion of the oxygen sublattice, which leads to a tilted three‐dimensional network of corner‐sharing BO6 octahedra. Magnetic measurements indicate an antiferromagnetic ordering below TN = 58 K for Sr2CaIrO6 and 74 K for Sr2MgIrO6. The low‐temperature magnetic structure of Sr2CaIrO6 was determined by NPD and selected from the possible magnetic solutions compatible with the P21/n space group according to the group‐theory representation. The propagation vector is k = (1/2 1/2 0). A canted antiferromagnetic structure is observed below TN with an ordered magnetic moment of 1.33(2) μB for the Ir6+ cations. Sr2MgIrO6 does not present magnetic reflections. An X‐ray absorption spectroscopy investigation gives an insight into the Ir oxidation states and suggests a hexavalent state for the Ca compound and an admixture of Ir6+ and Ir5+ for the Mg perovskite, in agreement with the NPD data, which indicate a significant oxygen understoichiometry for this material.