Silicon crystal, electron distribution

Figure Al.3.22. Spatial distributions or charge densities for carbon and silicon crystals in the diamond structure. The density is only for the valence electrons the core electrons are omitted. This charge density is from an ab initio pseudopotential calculation [27].

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Si Al ratio of the outer layers (ca. 10 A) of the zeolite crystals. The extent to which the surface composition differs from the bulk composition appears to depend on preparation conditions, and all three possible situations (silicon rich surface, silicon deficient surface and surface composition equal to bulk composition) have been reported (refs. 12-14). Variations in aluminium distribution have also been probed by high resolution scanning electron microscopy (ref. 15) and energy dispersive X-ray analysis (ref. 16). 

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