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

Abstract

Sr2CaIrO6 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.