Silicon dioxide coordination number

In Fig. 1 the space structure, the interatomic distance and coordination number distribution function for the most regular diamond-like silicon dioxide structure is presented. This cluster was constructed as 3x3x3 extended crystobalite cells and it containings 708 atoms. This cluster has been used as a reference for highly regular structures. 

Fig. 1. The space structure, the interatomic distance and coordination number distribution function for the most regular diamond-like silicon dioxide structure.

Of substances MX.j, silicon dioxide (radius ratio 0.29) forms crystals with tetrahedral coordination of four oxygen ions about each silicon ion, magnesium fluoride (radius ratio 0.48) and stannic oxide (radius ratio 0.51) form crystals with octahedral coordination of six anions around each cation (the rutile structure, Figure 18-2), and calcium fluoride (radius ratio 0.73) forms crystals with cubic coordination of eight anions around each cation (the fluorite structure. Figure 18-3). The ligancy (coordination number) increases with increase in the radius ratio, as indicated in Figure 18-1. 

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