The structural characterization of hydrous layer silicates using X-ray- and neutron powder diffraction studies and complementary experiments with solid state NMR is presented for a variety of materials of the RUB-18 family. The parent Na-RUB-18 shows interesting temperature dependent proton dynamics and proton conduction which is studied with NMR and neutron diffraction experiments. The silicic acid H-RUB-18 crystallises without intercalate water and with considerable distortion of the original structure. Acetone and alanine intercalated in H-RUB-18 occupy the channel-like void of the interlayer space by re-arranging the original stacking motive upon uptake. The article is a summary on work on this family of materials presenting new results in perspective with results already available in the literature.

The structure of CaTaOAlO 4 (CTAO) has been determined using X-ray powder diffraction and density functional methods in combination with 27 Al MAS NMR spectroscopy. A structural phase transition occurs near room temperature in CTAO as indicated by heat-capacity measurements, lattice strain data and infrared spectroscopy. Rietveld analysis of the powder diffraction data does not indicate deviation from monoclinic symmetry C 2/ c . But the observed quadrupolar coupling of the Al atom is reproduced by the electronic structure calculations only in a structure with space-group symmetry P 2 1 / n , distinguished by two different Ta coordination environments. The atomic coordinates of this low-temperature structure of CTAO are obtained by computational force relaxation within the experimental unit cell determined at 170 K.

In order to obtain a suitable model system for investigating the formation of nanodefects in a zeolite framework during the in situ synthesis of phthalocyanines, metal-free phthalocyanine was encapsulated within the pores of a NaY zeolite by in situ cyclotetramerization of adsorbed 1,2-dicyanobenzene. Such defects will be regarded as nanodefects, which are created by removal of only a few atoms or ions from framework positions. In contrast to scanning electron microscopy, powder X-ray diffraction, nitrogen adsorption, 27Al, and 29Si MAS NMR, all showing no significant changes, the occurrence of an absorption at λ≈280 nm in the diffuse reflectance UV spectrum was found to be characteristic for nanodefects in the zeolite framework. This absorption occurs due to the change of the coordination of a small percentage of framework aluminum centers from tetrahedral to octahedral.

Two excess-boron olenite samples which had been synthesized from a reaction mixture of 0.625 Na2O . 4.5 Al2O3 . 6.0 SiO2 . 3.0 B2O3 + excess H2O at 600degreesC / 25 kbar (sample 1) and 650degreesC / 20 kbar (sample 2) were structurally analyzed by a Rietveld refinement. The investigated tourmalines possess space group symmetry R3m with lattice parameters of a = 15.5996(8) Angstrom, C = 7.0224(6) Angstrom for sample 1 and a = 15.6329(8) Angstrom, c = 7.0365(6) Angstrom for sample 2. Si-29 MAS NMR spectroscopy showed no octahedral Si, so that the octahedral Y and Z sites are exclusively occupied by Al3+ ions. The average T-O distances are 1.573 Angstrom and 1.590 Angstrom for samples 1 and 2, respectively, indicating that B3+ ions (B-O = 1.470 Angstrom) partly replace Si4+ ions (Si-O = 1.620 Angstrom) at the T-position. The refined occupancy factors give a chemical composition of Na0.8Al2.9Al6[Si3.8B2.2O18](B3O9)(OH,O)(4) for sample 1 and NaO0.7Al2.9Al6[Si4.2B1.8O18](B3O9)(OH,O)(4) for sample 2, approximately confirming an earlier chemical analysis of sample 1. The difference in charge which is generated by the partial replacement of Si4+ ions by B3+ ions is compensated by protons leading to OH-contents near 4. It is interesting to note that sample 1 synthesized at a higher pressure and lower temperature contains a larger amount of tetrahedral boron.

Solid-state H-1 and Si-29 MAS NMR spectroscopies were applied to the hydrous layer silicate Na-RUB-18 to investigate the hydrogen-bond networks formed within the crystals as a function of temperature. Two different proton environments can be differentiated. At temperatures below 230 K, the weak hydrogen bonds formed by the intercalate water and the strong hydrogen bonds formed by the silanol groups do not interact. At temperatures up to 360 K an increasing number of hydrogen bonds localized at silanol groups breaks up. At this temperature, 1D infinite hydrogen-bond networks are formed in the material, resulting in proton conductivity. Molecular dynamics (MD) simulations yielded increasing vibrations of these hydrogen bonds as an explanation for the breaking. From H-1 MAS NMR experiments with modified samples, it becomes obvious that only one-half of all water molecules participate in the network. Further, it is shown that a minimum coherence of the 1D network is necessary to stabilize the high-temperature state.

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.