Preparation, crystal structure and magnetic behavior of new double perovskites Sr2 B'UO6 with B'=Mn, Fe, Ni, Zn

Sr2 B'UO6 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 P21/n), and contains alternating B'O6 and UO6 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 μB/f.u.) and B'=Ni(3.51 μB/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. - Graphical abstract: The title double perovskites are monoclinic, space group P21/n, and the magnetic properties suggest the possibility of a partial charge disproportionation B'2++U 6+↔B'3++U 5+, accounting for plausible ferrimagnetic interactions between B' and U sublattices