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

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=1163–1187 pm, β≈103°, Z=2). They are obtained by the reaction of the sesquioxides M2O3 (or the combination of Tb4O7 and Tb 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 °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 image{[(M2)5Cl9]6+} layers are alternatingly piled with anionic ones of the kind image{[(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°) and two bent (142°) Si–O–Si bridges. This particular cyclo-[Si4O12]8− situation could be confirmed by theoretical and experimental infrared-spectroscopic investigations.