Framework structure surrounding

Figure Representation of the pseudo-cell of zeolite A, (Space Group Pm3m) illustrating the framework structure surrounding a large cage. The cation positions are shown in bold face lettering using the formalism of Mortier (6). The extent of projection of cations in sites B,C and G is exaggerated for clarity.

Figure 6.17 An example of a zeolite framework structure (faujasite). The aluminosilicate framework surrounds void spaces which contain the charge balancing cations. Access to these voids is restricted by the size of the apertures between pores. (Adapted from [204].)... 

The size, location, and structure of platinum clusters in H-mordenite were modeled by molecular mechanics energy minimization and molecular dynamics simulation techniques [96G1]. It was suggested that the relative stability of monoatomic platinum sites in aluminosilicate mordenites is related to the specific aluminum insertion in T sites of the framework structure. The structural features of the platinum cluster confined to the 12-ring main channel are almost independent of the total Pt content and strongly dependent upon the surrounding zeolite structural field. 

Pillaring of metal sulfides has been achieved [91], and even metal-rich framework structures can be prepared [92]. Even for these materials template removal is not possible, they are still an interesting approach to nano-sized semiconductors, namely so-called anti-dot lattices where the semiconducting lattice surrounds template-filled voids of lower conductivity [86]. Template removal is probably not the essential problem for the design of a nanostructur-ed material, since often the goals of unusual size-dependent optical or electronic properties might be achieved with the composite of template and framework. 

In addition to four atoms in a row, a four-atom deiocaiized framework can have a centrai atom surrounded by three other atoms with which it interacts. An exampie is the carbonate anion, whose Lewis structure dispiays resonance ... 

A molecule is composed of positively charged nuclei surrounded by electrons. The stability of a molecule is due to a balance among the mutual repulsions of nuclear pairs, attractions of nuclear-electron pairs, and repulsions of electron pairs as modified by the interactions of their spins. Both the nuclei and the electrons are in constant motion relative to the center of mass of the molecule. However, the nuclear masses are much greater than the electronic mass and, as a result, the nuclei move much more slowly than the electrons. Thus, the basic molecular structure is a stable framework of nuclei undergoing rotational and vibrational motions surrounded by a cloud of electrons described by the electronic probability density. 

As we can see from the last entry in this table, we have deduced only a rule. In InBi there are Bi-Bi contacts and it has metallic properties. Further examples that do not fulfill the rule are LiPb (Pb atoms surrounded only by Li) and K8Ge46. In the latter, all Ge atoms have four covalent bonds they form a wide-meshed framework that encloses the K+ ions (Fig. 16.26, p. 188) the electrons donated by the potassium atoms are not taken over by the germanium, and instead they form a band. In a way, this is a kind of a solid solution, with germanium as solvent for K+ and solvated electrons. K8Ge46 has metallic properties. In the sense of the 8-A rule the metallic electrons can be captured in K8Ga8Ge38, which has the same structure, all the electrons of the potassium are required for the framework, and it is a semiconductor. In spite of the exceptions, the concept has turned out to be very fruitful, especially in the context of understanding the Zintl phases. 

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