Hydrogenoxosilicates of the lanthanides

Abstract

Abstract Single crystals of Gd[SiO3(OH)] were obtained in attempts to synthesize a gadolinium fluoride oxosilicate by fusing a mixture of GdF3, Gd2O3 and SiO2 in a 1:4:9 molar ratio at 700 °C for 10 days in an evacuated silica ampoule. The title compound crystallizes in the space group Pnma (a = 861.86 (7) pm, b = 387.95 (3) pm, c = 908.79 (8) pm) with four formula units per unit cell. Since the absence of fluorine in the structure was confirmed by electron microprobe analysis, the crystals must contain one hydroxy group as ligand within the ortho-oxosilicate tetrahedra. The latter are arranged as fake strands of apically vertex connected trigonal bipyramids (≡ face-shared tetrahedra) along [0 1 0] in which the waist triangle is formed by O2− anions and the axial ligands are considered to be OH− anions, due to their larger Si4+ O2− bond length (d(Si4+ OH−) = 167 pm versus d(Si4+ O2−) = 159–164 pm for the remaining three distances). The Si4+ cations are situated in the center of each partial tetrahedron of the trigonal bipyramids. They refine statistically sub-occupied (sof(Si) ≈ 50%) so that there is no need for two of them being placed next to each other (d(Si4+⋯Si4+) = 59 pm) in the middle of two face-shared tetrahedral entities and discrete [SiO3(OH)]3− anions emerge. The crystallographically uniform Gd3+ cations, that are situated between the mentioned { [ Si O 3 / 1 ( OH ) 2 / 2 ] 3 − } ∞ 1 strands, show coordination numbers of nine, with figures consisting of seven O2− (d(Gd3+ O2−) = 236–269 pm) and two OH− anions (d(Gd3+ OH−) = 274 pm) in a rather irregular, but evenly spherical polyhedron.