Zachariasen rule

Zachariasen suggested four rules for the formation of an oxide glass  

Each oxygen ion should be linked to not more than two cations. 

The CN of oxygen ions about the central cation must be small, 

Oxygen polyhedra share corners, not edges or faces. 

At least three corners of each polyhedron should be shared. 

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Zachariasen Rules. In 1932, W. H. Zachariasen considered the conditions for constructing a random network like the one shown in Fignre 1.48 and proposed four rules for the formation of oxide glasses ... 

This volates the Zachariasen rule that an oxygen atom cannot be linked to more than two cations. For two A104 units are required because the AI2O3 unit provides 1.5 oxygen/tetrahedron. Additional oxygen must be provided by or A" ... 

In addition to the Zachariasen and radius ratio rules, for oxides the electronegativity of the predominant cation should be between 1.7 and 2.1 (7). If the cation electronegativity is too high, the compound tends to form molecules or discrete polyatomic ions rather than a connected network. For example, CrO satisfies the radius ratio rule, but the highly electronegative Cr ions promote the formation of discrete dichromate(VI) ions, Cr202 , in the presence of other oxides. 

Burdett and McLarnan (1984) went on to test the valence sum rule by assessing a further prediction implied in the rule, namely that anions of the same electronegativity should be found in sites of equal bond sum. This was confirmed by calculating that, when hypothetical anions with equal parameters midway between O and N are used, the most stable structure is the W-Pmc2, ) type, where all the sites are of equal bond-strength sum. Also investigated was the Zachariasen-Baur extension of the second rule (Baur, 1970), stating that oversaturated anions should... 

Since there are no known structures of such compounds with plutonium, it is useful to utilize Zachariasen s rules (123 tor estimating actinide-oxygen bond lengths which would be required in a particular coordination geometry. 

ZACHARIASEN S RULES FOR GLASS FORMATION IN SIMPLE OXIDES... 

In 1932 W.H. Zachariasen proposed a set of rules that is usually satisfied when an oxide forms a glass. His analysis was based on the following considerations ... 

Zachariasen summarized his findings as four rules and emphasized how the structure of SiOi glass differs from the crystalline form shown here as the (111) plane of cristobalite. 

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