Coordination numbers of cations

To determine the structure type from equation (4) it is necessary to calculate the ratio of coordination number of cation and anion (Ki/K2) and taking into consideration the ratio of cation and anion radii values (in the model of rigid spheres), the structure itself can be determined. 

Since in this equation the expressions with equal n cancel separately we may omit the bond number. By using rc and ra for the mean total coordination number of cations and anions, respectively, the equation becomes more similar to the "chemical condition (3) ... 

In monohydrates H2O molecules form a decreasingly important part of the coordination group of the cations as the coordination number of cation increases ... 

Value of — r Predicted coordination number of cation Predicted coordination geometry of cation... 

Problem 5.8 In the crystal structures of the alkali halides the coordination numbers of cations and anions are necessarily equal. Assume that you have a crystal of composition AaB),. Assume that each... 

TABLE 56 Coordination numbers of cations and molar volumes (VLnci, cmVmol) of lanthanide dichlorides data from Astakhova and Goryushkin (1992)... 

Fluorite is a mineral composed of calcium fluoride, CaF2- This mineral gives its name to the fluorite structural type. Its structure (Figure 14.11a) has cubic symmetry, the space group is Fm-3m (a = 5.471 A [31]). The structure can be described as a face-centred cubic (fee) array of cations in which all the fourfold coordinated interstices are filled with anions and the sixfold coordinated ones are empty (Figure 14.11a). The coordination number of cations is eight (cube) and that of anions is four (tetrahedron). 

Most oxides and silicates have crystalline structmes obeying Pauling s rules for ionic crystals, where the ions of small size (generally cations) enter the interstices of big ions (generally anions). The three main rales relate to the coordination number of cations, the coordination number of anions and the coordination number of polyhedra. 

From a structural standpoint, the coordination number of cations on the surface of an oxide synthesized by dry techniques (CVD, electric arc, solid state reactions) is necessarily lower than that of ions within the solid. Upon exposure to ambient atmosphere, satisfying the coordination number of the surface ion is the driving force behind the chemisorption of water which causes its dissociation and combination (Figure 6.1). 

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