The Sphere-packing Principle for Compounds

The geometric principles for the packing of spheres do not only apply to pure elements. As might be expected, the sphere packings discussed in the preceding chapter are also frequently encountered when similar atoms are combined, especially among the numerous alloys and intermetallic compounds. Furthermore, the same principles also apply to many compounds consisting of elements which differ widely. 

As already mentioned it is frequently difficult to predict in which crystal structure a particular compound will crystallize even in the equilibrium state at low temperatures [68]. Important decision criteria involve the electronegativity, the polarizability, preferred co-ordination spheres and in particular the ionic radius. Textbooks of crystallography and structural chemistry are recommended to the reader for the details . Here attention will only be drawn to the fruitful ordering principle of the close-packing of spheres . 

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