The lanthanoid(III) chloride cyclo-tetrasilicates M6Cl10[Si4O12] (M = Sm, Gd-Dy): Synthesis, structure and IR investigations

摘要

The chloride derivatized lanthanoid(III) cyclo-tetrasilicates of the composition M6Cl10[Si4O12] (M = Sm, Gd-Dy) crystallize monoclinically in space group C2/m (a = 1062-1065, b = 1036-1052, c = 1163-1187pm, beta approximate to 103 degrees, Z = 2). They are obtained by the reaction of the sesquioxides M2O3 (or the combination of Tb4O7 and Th in 3:2-molar ratio for the terbium case), the corresponding trichlorides MCl3, and SiO2 (silica gel) in stoichiometric ratios with double the amount of MCl3 as flux in evacuated silica tubes (7d at 850 degrees C) as transparent, pseudo-octagonal, pillar-shaped single crystals with the colour of the respective lanthanoid trication M3+. Their crystal structure can be considered as a layered arrangement in which cationic(infinity)(2){[(M2)(5)Cl-9](6+)} layers are alternatingly piled with anionic ones of the kind(infinity)(2){[(M1)Cl[Si4O12]](6-)}. In the latter, the (M1)(3+) cations show a slightly distorted hexagonal bipyramidal environment built up by two Cl- and six O2- anions (CN = 8), whereas the (M2)(3+) cations exhibit a coordination number of only seven (five Cl- and two O2- anions in the shape of a distorted pentagonal bipyramid). The cyclo-tetrasilicate units consist of four corner-linked [SiO4](4-) tetrahedra in all-ecliptical conformation each, fused to eight-membered rings, which contain two almost linear (178 degrees) and two bent (142 degrees) Si-O-Si bridges. This particular cyclo-[Si4O12](8-) Situation could be confirmed by theoretical and experimental infrared-spectroscopic investigations. (c) 2006 Elsevier Inc. All rights reserved.