The crystal structure determination of the crystalline layered silicic acid H-RUB-18

Abstract

The layer silicate H-RUB-18 (chemical compo-sition: Si₄O₇(OH)₂) was prepared by ion exchange from the parent material Na-RUB-18. The high degree of structural disorder typical for this material precluded a classical structure analysis based on diffraction data only. Nevertheless, a detailed picture of the structure was obtained by a step by step process using a combination of complementary methods: Crystal chemical considerations and solid state NMR spectroscopy allowed to establish a rough model of the structure which was optimized by forcefield molecular dynamics (MD) simulations. A subsequent Rietveld refinement in space group I4₁/amd confirmed the model consisting of a sequence of pseudo tetragonal silicate layers with an intra-layer repeat unit a = 7.38 Å and inter-layer distances of c = 7.44 Å. The main type and degree of disorder was analyzed by the simulation of various stacking sequences of rigid silicate layers which served as the building blocks. The random displacement of consecutive layers is accompanied by a slight distortion of the layers as indicated by an additional density functional theory (DFT) energy minimization calculation in triclinic symmetry. The given displacement and distortion allow for the formation of hydrogen bonds between the terminal OH groups necessary for the stabilization of the structure. The concept of a structure analysis presented in this paper may be suitable as a guide line to be applied to many other disordered materials.