Crystal Structure and Magnetism of the Double Perovskite Sr₃Fe₂MoO₉:A Neutron Diffraction Study

Abstract

Abstract The Sr 3 Fe 2 MoO 9 double perovskite has been prepared in polycrystalline form by heat treatment, in air, from previously decomposed citrate precursors. This material has been studied by X‐ray (XRD), neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure is tetragonal, space group I 4/ m , with a = b = 5.5608(2) Å, c = 7.8471(4) Å. It is convenient to write the crystallographic formula as Sr 2 Fe(Fe 1/3 Mo 2/3 )O 6 , according to the usual nomenclature for double perovskites A 2 B'B''O 6 . The structure contains alternating FeO 6 and (Fe,Mo)O 6 octahedra, tilted by 3.9° in the basal ab plane. Magnetic measurements indicate a weak ferromagnetic behavior below T C = 280 K. Ideally, the Sr 3 Fe 2 MoO 9 double perovskite only contains Fe 3+ and Mo 6+ cations, in such a way that superexchange interactions between neighboring Fe 3+ spins are the only nominal mechanism accounting for the magnetism of this material. Our main finding is that this intrinsically “disordered” sample, containing a random distribution of Fe and Mo at the B'' positions, exhibits a strong magnetic scattering on the low‐angle Bragg positions, originating from naturally occurring groups of Fe 3+ cations in which strong antiferromagnetic (AFM) Fe–O–Fe superexchange interactions are promoted, similar to those existing in the LaFeO 3 perovskite. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)