METALLOTHERMIC REDUCTION OF THE TRIBROMIDES AND TIBROIODIDES OF THULIUM AND YTTERBIUM WITH ALKALI-METALS

Metallothermic reduction of the trihalides MX3 (M = Tm, Yb; X = Br, I) with equimolar amounts of alkali metal (A = Li-Cs; 700 - 800-degrees-C, 7 d, tantalum capsules) results in the formation of the dihalides MX2 (MI2 with the CdI2-type of structure, MBr2 with the SrI2- or alpha-PbO2-type of structure) in the case of lithium and sodium. With A = K, Rb, Cs variants of the perovskite-type structure with the composition AMX3 are obtained. They crystallize with the undistorted cubic CaTiO3-type (CsYbBr3), with the tetragonal NaNbO3-II-type (CsTmBr3) or with the orthorhombic GdFeO3-type (CsMI3, RbMX3 and KMBr3). In KTmI3 and KYbI3, corner- and edge-connected [MI6] octahedra form layers which are connected via K+ in analogy to a stuffed PuBr3- or the FeUS3-type structure (orthorhombic, Cmcm). Single crystals of Rb4YbI6 are obtained as a by-product upon the reduction of YbI3 with rubidium. According to their K4CdCl6-type structure (trigonal, R3cBAR), isolated [YbI6] octahedra are the main structural feature.