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

Abstract

Abstract Sr 2 CaIrO 6 and Sr 2 MgIrO 6 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 P2 1 / n ), and the unit‐cell parameters are related to the ideal simple perovskite cell as a ≈ √2 a 0 , b ≈ √2 a 0 , c ≈ 2 a 0 , β ≈ 90°, where a 0 is the edge of the simple cubic ABO 3 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 BO 6 octahedra. Magnetic measurements indicate an antiferromagnetic ordering below T N = 58 K for Sr 2 CaIrO 6 and 74 K for Sr 2 MgIrO 6 . The low‐temperature magnetic structure of Sr 2 CaIrO 6 was determined by NPD and selected from the possible magnetic solutions compatible with the P 2 1 / 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 T N with an ordered magnetic moment of 1.33(2) μ B for the Ir 6+ cations. Sr 2 MgIrO 6 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 Ir 6+ and Ir 5+ for the Mg perovskite, in agreement with the NPD data, which indicate a significant oxygen understoichiometry for this material.