Some simple structures for oxides and other ionic compounds

Some simple structures for oxides and other ionic compounds 

As mentioned earlier, the stmctures of ionic compounds can often be seen as close-packing of the large anions, while the normally smaller cations occupy the interstitial voids. (Note that once the stmcture is established, the interstices occupied by cations are not any longer considered interstitial they are part of the ideal (reference) structure.) 

In the NaCl stmcture the anions are cubic close-packed, and the smaller metal ions occupy all the octahedral interstices. Each metal ion is thus surrounded by six anions and vice versa. The NaCl stmcture is illustrated below. Note the similarity with the illustration of the octahedral voids in the fee stmemre shown previously. 

The oxides MgO, CaO, SrO, BaO, CdO, CoO, NiO, FeO, TiO, NbO, and VO possess this crystal stmcture. The last six of these oxides may be highly nonstoichiometric and as such their NaCl stmctures highly defective. 

In the sulfides FeS and NiS, the stmcture is similar, but the close-packing of anions is hexagonal rather than cubic. 

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