On the new ferrimagnetic Sr3Fe2UO9 double perovskite with TC above room temperature: a neutron diffraction study

Abstract

Sr₃Fe₂UO₉ double perovskite has been prepared in polycrystalline form by a solid state reaction, in air. This material has been studied by X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure is tetragonal, space group I4/m, with a = b = 5.6994(5) Å, c = 8.039(1) Å. Its crystallographic formula can be rewritten as Sr₂Fe(Fe_1/3U_2/3)O₆, according to the usual nomenclature for double perovskites A₂B′B″O₆. The structure contains alternating FeO₆ and (Fe,U)O₆ octahedra, almost fully ordered, tilted in antiphase by 7.6° in the basal ab plane. Magnetic measurements indicate strong ferrimagnetic behaviour below T_C ∼ 330 K, with a maximum magnetization of 2 μ_B fu⁻¹, not fully saturated for H = 50 kOe. Ideally, the Sr₃Fe₂UO₉ double perovskite only contains Fe³⁺ and U⁶⁺ cations, in such a way that superexchange interactions between neighbouring Fe³⁺ spins are the only nominal mechanism accounting for the magnetism of this material. At 3K, the ordered magnetic moments at Fe and (Fe,U) sites are 3.9(1) μ_B and −0.7(1) μ_B, respectively. By comparison with other members of the Sr₃Fe₂B″O₉ double perovskite series (B″ = Mo, W), we establish a direct correlation between the degree of antisite disordering (implying the distribution of B″ over both kinds of BO₆ octahedra), and the strength of the global ferromagnetic interactions, giving superior T_C's and ordered magnetic moments for the most structurally disordered members.