The problem of packing ions of different charge is greatly simplified when (as is often the case) the anion is substantially larger than the cation. A combination of efficient space filling, without excessive repulsive contributions, is achieved when the anions are approximately close packed and the cations fit into interstitial holes. The proportion of interstitial holes filled is dictated by the chemical composition (i.e. stoichiometry) .
In the simple ionic model the coordination number of the cation should be as high as possible provided that cation-anion contact is maintained (to optimise the Coulomb attraction) and anion-anion contact avoided (to minimise the electrostatic repulsion). Thus, as the cation/ anion radius ratio is increased a switch from tetrahedral to octahedral, and then to cubic interstitial site filling is anticipated. As you will observe in the exercises other factors also operate in elementary structures. Directed forces (see below) may dictate a lower cation coordination number than expected. Secondary, non-directed forces (e.g. Van der Waals forces) may cause structural changes especially where large polarisable anions (or cations in "anti-structures") are involved e.g. in layer structures.
You are provided with an unknown Structure
Construct a model of the structure and answer the following questions. (Refer to your earlier exercises and the references supplied.)
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