Helix-Layered-Silicate

The powder patterns were recorded with monochromatized CuKa1 radiation from capillary samples to avoid preferred orientation of the plate-like crystals. The patterns are limited to a diffraction angle of 37 °2theta since usually only little information can be deduced from higher angle reflections.

Because HLSs often show characteristic anisotropic peak broadening, this database presents for better comparison predominantly experimental XRD powder patterns and not the calculated ones. Only in those cases where no material was available the XRD patterns were calculated based on the structure data and assuming CuKa1 radiation.

The fact that the powder XRD patterns of the HLSs show anisotropic broadening of the peak halfwidths is mainly due to a certain degree of disorder of the layer stacking and to a much lesser extent to the specific morphology (thin plateletts). Typically, all reflections representing the atomic arrangement within the layer are sharp. These are the HK0-reflections, if we assume that the stacking direction is parallel to the c axis. Usuallly, all 00L-reflections representing the interlayer distance are moderately sharp (in spite of the very small crystal size along this direction), indicating that the inter-layer distances are about identical. In contrast, all other H0L, 0KL and HKL reflections with H, K, L ≠ 0 are broad. These reflections include also information on the orientation of layers relative to each other which is not perfect because only weak bonding interactions exist between neighboring layers.

Please note, that H-RUB-18 and MCM-22(p) are highly disordered materials, the listed values for the diffraction angles and intensities are not very reliable.

To complement the XRD powder pattern all possible reflections (according to the space group symmetry) are listed up to a diffraction angle of 37° 2θ. The peaks are characterized by their HKL indices, the multiplicity M, the diffraction angle (2θ) with respect to CuKa1 radiation (λ = 1.5406 Å), the relative intensity (observed or calculated) and the d-value. Note, that in the case of HLSs the peak height is usually not proportional to the peak intensity because of significant anisotropy of halfwidths!

The fractional atomic coordinates may be used, e.g., to draw a structure plot of the compound or to calculate interatomic distances.

The structure of the HLS is presented in two projections representing instructive pictures of the arrangement of layers, cations and water molecules.

The silicate layer is displayed with silicon atoms shown as small purple spheres, bridging oxygen atoms as red spheres and terminal oxygen atoms (OH- or O-- groups) as blue spheres. Si-O bonds are drawn as black lines, and hydrogen bridges as light blue lines. The inter-layer region may contain water molecules (light blue spheres), carbon atoms as white spheres, nitrogen atoms as small green spheres, sodium ions as yellow spheres. TMA cations are displayed as large green spheres since these cations are typically rotationally disordered (only the position of the nitrogen atom is approx. fixed). Hydrogen atoms are not included in the drawings since their position remained unknown in nearly all cases. White lines represent N-C and C-C bonds.

The FTIR spectra of the HLS are displayed in a range between 400 and 1300 cm^-1. The spectra were recorded using the ATR technique and were not further corrected. In the given range the FTIR spectra are dominated by the lattice vibrations of the silicate layer. Absorption bands due to vibrations of organic cations are also present but they are usually quite weak. The spectra are included here to serve as a rough “finger print” of the layer-type. Probably because the interactions between silicate layer and “low-charge-density-cation” are weak, the FTIR spectra of different HLSs with the same layer type show similar characteristics.

Download of the Crystallographic Information File (CIF).