Cluster self-organization of germanate systems: Suprapolyhedral precursor nanoclusters and self-assembly of CAN-Na8(Al6Ge6O24)Ge(OH)6(H2O)2 and CAN-Cs2Na6(Al6Ge6O24)Ge(OH)6) zeolite crystal structures re]20100419

Abstract

Geometrical and topological analysis of zeolite crystal structures having a tetrahedral framework of the cancrinite (CAN) type, namely, (CAN) Na8(Al6Ge6O24)Ge(OH)6(H2O)2 (acentric space group P63, hP64, Na-CAN) and Cs2Na6(Al6Ge6O24)Ge(OH)6 (P63, hP52, CsNa-CAN), is carried out with the use of computer techniques (the TOPOS 4.0 program package). An AT6 hexapolyhedral precursor nanocluster centered with a template cation A (Na, Cs) is identified. The topological type of a two-dimensional (2D) crystalforming T-net 4.6.12, which corresponds to a uninodal semiregular Shubnikov net, is recognized. The full 3D reconstruction of crystal structure self-assembly is performed as follows: precursor nanocluster → primary chain → microlayer → microframework → … framework. The symmetry of an AT6 precursor nanocluster is described by point group 3; the symmetry axis passes through the center of the nanocluster and cation A. The coordination number (CN) of a precursor nanocluster, which characterizes the nanocluster stacking in the macrostructure, is six. In both structures, six Na atoms and a Ge(OH)6 polyhedral species are spacers filling the voids between AT6 precursor nanoclusters. The Ge(OH)6 polyhedral species is characterized by four and two orientationally allowed positions in Na-CAN and CsNa-CAN, respectively. The minimal number of suprapolyhedral AT6 precursor nanoclusters required for the 3D microframework to form is 16; that is, 96 tetrahedra are involved in microframework self-assembly.