Crystal Structure, Magnetism, and Infrared Reflectivity of Double Perovskites Sr$_{2}$B$^{´}$UO$_{6}$ and Sr$_{2}$B$^{´}$UO$_{6-d}$, B$^{´}$=Mn, Fe, Co, Ni, Zn

Double perovskites Sr$_{2}$B$^{´}$UO$_{6}$ where B'=Mn, Fe, Co, Ni, Zn have been prepared as a polycrystalline powder by solid- state reaction. and studied by X-ray diffraction and magnetic measurements. At 300K, they present the same monoclinic distorted crystal structure, space group P2$_{1}$ /n. The perovskite lattice consists of a completely ordered array of B$^ {´}$O$_{6}$ and UO$_{6}$ octahedra exhibiting a slight tilting of the type a$^{-}$b$^{+}$a$^{-}$. Magnetic measurements show antiferromagnetism for the phases with B$^{´}$=Co, Ni and Mn at low temperature. The effective magnetic moment at T $\geq$ T$_ {Neel}$ (5.22l B/f.u. for Co phase and 3.26 B/f.u. for Ni phase) suggests an unquenched orbital contribution. The value for the Mn phase (5.74 B/f.u.) is consistent with that expected for high-spin Mn$^{2+}$ (5.91 B/f.u.). The topotactic reduction of the stoichiometric sample leads to oxygen deficient disordered perovskites, SrB$^{´}$$_{0.5}$U$_{0.5}$O$_{3-d}$ having an orthorhombic distorted structure, space group Pbnm, at 300K. We will also comment on the temperature dependent infrared reflectivity of Sr$_{2}$CoUO$_{6}$ and isomorphous compounds.