Calcium oxide coordination number

CaMo508 provides a link to calcium channels in biological systems. This mixed oxide contains Mo/O columns, of which groups of four define channels in which the Ca2+ cations reside. There is no clearly defined coordination number for these cations Ca-0 distances range from 2.39 A upwards, there being ten oxygens at fairly regularly spaced distances up to 3.04 A from the Ca2+ (117). 

Many ionic compounds of AX2 stoichiometry possess the CaF2 (fluorite), or Na20 (antifluorite) structures shown in Figure 3.15. Fluorite is similar to CsCl, but with every other eight coordinate cation removed. Each fluoride anion is tetrahedrally coordinated by calcium ions. This structure is adopted by several fluorides and oxides. In the antifluorite structure, the coordination numbers are the inverse. Most oxides and other chalcogenides of the alkali metals (e.g. Na2Se, K2Se) possess the antifluorite structure, but so do some more covalent compounds, such as the silicides of Mg, Ge, Sn, and Pb. 

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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