Cs3NdSi8O19 und Cs6Nd2Si21O48: Zwei cäsiumhaltige Oxosilicate des Neodyms im Vergleich Cs3NdSi8O19 and Cs6Nd2Si21O48: Two Caesium-containing Oxosilicates of Neodymium in Comparison

In the quaternary system Cs / Nd / Si / O, two new representatives, the phyllo-oxosilicate Cs3NdSi8O19 and the tecto-oxosilicate Cs6Nd2Si21O48, were synthesized by CsF-flux-supported solid-state reactions between Nd2O3 and SiO2. The first one, Cs3NdSi8O19 (orthorhombic, Cmcm (no. 63), a = 705.74(5), b = 2712.85(19), c = 1163.72(8) pm, Z = 4), is not isotypic to the related scandium compound Cs3ScSi8O19. The [SiO4]4− tetrahedra (d(Si4+–O2−) = 156 – 163 pm) in the structure of Cs3NdSi8O19 are connected via common corners to form corrugated, loop-branched double layers containing fourand eight-membered rings in the (010) plane and eight-membered rings along [001]. Each of the eight-membered ellipses emerging along [100] is additionally loopbranched by two four-membered chains. The oxosilicate double layers are cross-linked by vertexsharing via otherwise isolated [NdO6]9− octahedra (d(Nd3+–O2−) = 232 – 234 pm) to build up a three-dimensional framework. Also in between the oxosilicate double layers, the (Cs1)+ cations are located on the 8 f site. Each of the octagonal channels along [001] hosts one (Cs3)+ and two (Cs2)+ cations, which both reside at only partially occupied sites (8g and 8 f , respectively) and disorder, because otherwise too short Cs+ · · · Cs+ distances would occur. The second compound, Cs6Nd2Si21O48, crystallizes also in an orthorhombic space group (Pmmn (no. 59), a = 2189.24(15), b = 731.92(5), c = 1593.61(11) pm, Z = 2). Starting from a loop-branched single layer containing fiveand eight-membered rings, a three-dimensional framework of vertex-shared [SiO4]4− tetrahedra (d(Si4+–O2−) = 149 – 164 pm) built up, in which the Si–O distances range from 149 to 164 pm within a broad range. In certain cavities, one kind of Nd3+, but four kinds of Cs+ cations (here, all sites with full occupation) are embedded. Also surrounded by only six O2− anions just like in the first case, the Nd3+ cations (d(Nd3+–O2−) = 233 – 237 pm) exhibit an unusually small, but not unknown coordination sphere for this relatively large lanthanoid(III) cation.