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

Colored crystals of the middle term (a = 1) of the Bi2-aRaGeO5 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) Angstrom, for R = Y and Yb respectively. The structure is formed by [GeRO5](infinity) layers containing both (RO7)(6) units and GeO4 tetrahedra. Between these layers zig-zag chains of psi-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 F-2(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.