Nonbonded interactions in ionic crystals

A natural question that arises is whether there is any correlation between Si—O—Si (T——T) angles (0) and Si—O (T— X) bond lengths, /. Geometrically, the Si Si(T T) distance, d, and I are related by the expression, / = (d/2) cosec (0/2). A correlation between / and 0 should exist when cations are in contact d is constant). When 0 is large, no Si Si contact exists and therefore there should be no correlation between d and /. A correlation would exist between / and d when 0 145°, thus indicating that tetrahedral structures are determined by T— X and T T interactions rather than X X interactions. 

Nonbonded radii enable us to rationalize aspects of crystal chemistry that cannot be understood in terms of the ionic model. For instance, it is difficult to understand why silicon is four-coordinated in most oxides. This is unlikely to be because of the radius ratio of the classical ionic model 81 is too small to have four oxygens around it. Six-coordinated silicon is known in certain cases (e.g. SiP207) silicon is octahedrally coordinated in fluorides, although the ionic radii of and F are similar. The reason 

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