Ring double

For both Linde A and X the cation has the most marked influence on absorption band frequencies in the regions 440-490 and 540-590 cm-1. These bands have been attributed to the Si(Al)-0 bending mode and double ring (double 4-ring for Linde A, double 6-ring for Linde X) vibrations, respectively (1). The Linde A double 4-ring vibration was found to consist of two components. The more intense lower frequency component, however, was more sensitive to the exchangeable cation. 

The reason for the deviation of the absorption band of KA from the straight line as found for the double 4-ring vibration is not clear. It may be caused by an overlap of this band with the adjacent higher frequency band (see Figure 1). 

Thallium and Silver Zeolite-A The zeolite-A structure is constructed from simple aluminosilicate cubeoctahedra, connected to form a three-dimensional, cubic network by so-called double-4 rings (Figure 1). This arrangement leads to the formation of large a-cages whose internal diameter is approximately 11 A. 

The PBUs are built from smaller units composed of a limited number of T-atoms, by applying a simple operation to the smaller unit, e.g. translation, rotation.[4] The zeolite framework types can be analysed in terms of these component PBUs. The infinite PBUs, such as (multiple) chains, tubes, and layers, and the finite PBUs, such as (double) 4-rings, (double) 6-rings, and cages are far from unique. However, they are common to several zeolite framework types and allow an easy description of the frameworks to be made. Infinite PBUs and finite PBUs can be used to build the zeolite frameworks (see details in the database of zeolite structures Schemes for Building Zeolite Structure Models).141 Here only an example from AFI is presented to show the building of zeolites. 

Figure 2.9 Connection mode and unit-cell content in AFI along c in perspective view (left) and in parallel projection (top right). For clarity, only 7 V2 repeat units of the PBUs along c are drawn. A double crankshaft chain (consisting of 2-fold-connected double 4-rings), and 3-fold-connected double 6-rings are indicated in bold. Reproduced from www.iza-structure.org...

Figure 2.38 Arrangement of the two structural building units of polymorph C along the 42 screw axis, cage [54] formed by four 5-rings and D4R cage formed by double 4-rings. Reproduced with permission from [107], Copyright (2001) Wiley-VCH...

Table VI shows the influence of dilution in a Na-(CH3)4N system containing (CH3)4NBr. The smaller precursors are formed in the presence of higher concentrations of TMA ions. Therefore, it seems that TMA ions stabilize not only the precursors of the A-type structure, presumably double 4-rings, but also, at higher concentrations, single 4-rings and 6-rings. The same number of TMA ions may be required for stabilization of either precursor this explains the concentration effect.

Na-Pl. The structure proposed for Na-Pl by Barrer (2) has the space group Im3m with a0 = 10.0A. The framework is formed by joining double 4-rings of (Al,Si)04 tetrahedra (cubes) on their comers so that every cube is connected to 8 other cubes. 

Figure 19 Crystal structure of the ACO zeolite (a) and its corresponding topology (b) showing connectivity of tetrahedral atoms that could be constructed from periodic double 4-ring (D4R) as the zero-dimensional periodic building unit (PerBU), in bold.

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