Silicon cationic radius

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. 

Isomorphic substitution The replacement of some of the aluminum and silicon in aluminosilicate minerals by cations of similar ionic charge and radius. This usually occurs as a result of chemical weathering. 

As noted earlier, cations other than silicon may occupy the tetrahedral centers. A major factor in predicting which cations will be found to substitute for silicon is ionic size. In general, cations whose size is about 0.03-0.1 nm are the best candidates. Si " has an ionic radius of about 0.041 nm. Cations such as Fe (ionic radius = 0.07 nm), Al+ (0.05 nm), Ca+ (0.1 nm), and Mg (0.065 nm) are often found in silicate-like structures and meet this requirement. 

The silicon ion Si4+ is an interesting exception. The high charge and small ionic radius make this cation polarizing or electronegative (see Box 4.2), such that its bonds with the oxygen anion O2 in silicate minerals (see Section 4.2) are distorted. This produces an appreciable degree of covalency in the Si-O bond. 

Radius ratio values relative to O2 are given in Table 4.3. The table shows that silicon (Si) exists in four-fold (tetrahedral) coordination with oxygen (O), i.e. it will fit into a tetrahedral site. This explains the existence of the Si04 tetrahedron. Octahedral sites, being larger than tetrahedral sites, accommodate cations of larger radius. However, some cations, for example strontium (Sr2+) and caesium (Cs+) (radius ratio >0.732), are too big to fit into octahedral sites. They exist in eight-fold or 12-fold coordination and usually require minerals to have an open, often cubic, structure. 

It follows that the starting point for any model for the structure of a vitreous material must lie in a determination of the coordination unit for the high field strength cations, which serve as the network forming cations in the material. Traditionally, we estimate the possible coordination number of each cation using the radius ratio, or by consideration of the coordination states usually observed for these cations in other materials. On this basis, we can state that silicon is almost... 

The stmcture of cordierite-like materials is based on a frame of font- and six-member rings composed of aluminum-silicon-oxygen tetrahedral. Mg cations are localized in octahedrons between tetrahedrons of six- and four-member rings. A frame stmcture allows to obtain modified materials with 3d cations instead of Mg having similar ionic radiuses, for example, Mn, Co Ni, Cu, Zn [4]. The presence of transition elements is of interest because these elements may strongly influence on the cordierite properties and catalytic activity. 

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