New rare-earth (Y, Yb) bismuth(iii) germanates. An initial study of a promising series

Abstract

Colored crystals of the middle term (a = 1) of the Bi_2−aR_aGeO₅ series (R = Y (yellow), Yb (brown)) have been grown. The structure of these materials has been established by single-crystal X-ray diffraction. They are isostructural and crystallize in the orthorhombic system, space group Pbca (No. 61), Z = 8, with unit-cell parameters a = 5.341(8) and 5.315(2), b = 15.232(2) and 15.225(2), c = 11.084(3) and 11.025(2) Å, for R = Y and Yb respectively. The structure is formed by [GeRO₅]_∞ layers containing both (RO₇)₆ units and GeO₄ tetrahedra. Between these layers zig-zag chains of ψ-BiO₅ octahedra run in the a direction. The stereoelectronic effect of the Bi³⁺ lone pair leads to this apparent zig-zag, and therefore to the kind of packing observed in this new type of structure. The downward deviation in the molar magnetic susceptibility of BiYbGeO₅ from the Curie–Weiss behavior, observed at low temperatures, is attributed to the effect of the crystal field splitting of the ²F_7/2 ground term of Yb³⁺. Energies of the maximum phonon, 799 and 797 cm⁻¹ for Y and Yb materials, respectively, have been determined from room temperature Raman scattering spectra of polycrystalline samples.