Preparation, crystal structure and magnetic behavior of new double perovskites Sr 2B′ UO 6 with B′=Mn, Fe, Ni, Zn

Abstract

Sr 2 B′ UO 6 double perovskites with B′=Mn, Fe, Ni, Zn have been prepared in polycrystalline form by solid-state reaction, in air or reducing conditions. These new materials have been studied by X-ray diffraction (XRD), magnetic susceptibility and magnetization measurements. The room-temperature crystal structure is monoclinic (space group P2 1/ n), and contains alternating B′O 6 and UO 6 octahedra sharing corners, tilted along the three pseudocubic axes according to the Glazer notation a − a − b +. The magnetic measurements show a spontaneous magnetic ordering below T N=21 K for B′=Mn, Ni, and T C=150 K for B′=Fe. From a Curie–Weiss fit, the effective paramagnetic moment for B′=Mn (5.74 μ B/f.u.) and B′=Ni(3.51 μ B/f.u.) are significantly different from the corresponding spin-only moments for the divalent cations, suggesting the possibility of a partial charge disproportionation B′ 2++ U 6+⇔ B′ 3++ U 5+, also accounting for plausible ferrimagnetic interactions between B′ and U sublattices. The strong curvature of the reciprocal susceptibility for B′=Fe precludes a Curie–Weiss fit but also suggests the presence of ferrimagnetic interactions in this compound. This charge disproportionation effect is also supported by the observed B′ O distances, which are closer to the expected values for high-spin, trivalent Mn, Fe and Ni cations.