begin mathsize 48px style r subscript s m a l l end subscript over r subscript b i g end subscript end style

rsmaller= Radius of the smaller ion (usually the Cation) ; rbigger = Radius of the bigger ion (usually the Anion)    

Geometric requirement for close-packing of ions of different sizes (assumed spherical) is expressed in terms of radius ration. Usually rsmall corresponds to the radius of the Cation and rbig corresponds to radius of the Anion. There is a specific range of radius ratio associated to a specific structure of the crystal. However, there are many exceptions to this gross generalizations, i.e. BeS , LiBr , LiI, BaO, KCl, RbCl etc.

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