Silicon hexagonal prisms

The distribution of cations in a hydrated zeolite is mainly controlled by their sizes and can be described by a statistical model. In the dehydrated state, most of the cations are located on the intraframework sites their occupancies are governed by mutual repulsions and cation—framework interactions [1]. By which, the environments of the framework silicon atoms and their corresponding ssi NMR spectra are affected [2,3]. The chemical shift and lineshape of Si NMR have been found to depend on the nature and the distribution of cations in the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3 ions from the supercages to the small sodalite and/or D6R cavities by... 

Fig. 13.21 The structures, elucidated by X-ray diffraction, of (a) [Si802o], determined for the salt [Me4N]g[Si802o]-65H20 [M. Wiebcke et al. (1993) Microporous Materials, vol. 2, p. 55], and (b) [Sii203o], determined for the salt Ki2[a-cyclodextrin]2[Sii203o]-36H20 [K. Benner et al. (1997) Angew. Chem., Int. Ed. Engl., vol. 36, p. 743]. The silicon atoms in (a) and (b) define a cube and hexagonal prism respectively. Colour code Si, purple O, red.

Refractive indices for several polytypes of silicon carbide have been measured [8-12]. The 6H polytype of SiC has been measured in the most detail [8]. For the hexagonal form the c axis is assumed to be perpendicular to the surface. Thus, a normal incidence wave can be used to measure transmission and/or reflection. This normal incidence wave is often called the ordinary ray. A prism with its sides perpendicular to the c-axis is used to determine both n0 and ne in the normal way (n0 = ordinary ray, ne = extraordinary ray). A short summary of the data is presented here for additional information please refer to [8,9]. 

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