Silicon ionic crystal radius

Nonbonded radii enable us to rationalize aspects of crystal chemistry that cannot be understood in terms of the ionic model. For instance, it is difficult to understand why silicon is four-coordinated in most oxides. This is unlikely to be because of the radius ratio of the classical ionic model 81 is too small to have four oxygens around it. Six-coordinated silicon is known in certain cases (e.g. SiP207) silicon is octahedrally coordinated in fluorides, although the ionic radii of and F are similar. The reason... 

When the ratio of the radii increases, then a new type of crystal structure occurs, in which each divalent ion is surrounded by six univalent ions, and each univalent by three divalent ones. This arrangement, shown in Figure 14, is called the rutile type, named after the mineral Ti02. While CaF2 still has the structure with 8 4 coordination, the rutile structure is observed in MgF2 because of the smaller radius of the Mg2+ ion. After the rutile type there is a further reduction of the coordination to 4 2 this type of structure occurs in BeF2 and Si02 in the different modifications of silica, each silicon ion is surrounded by 4 O2" ions, and each O2 ion is between two Si4+ ions. The ionic ratio r+/r can also be decreased if,... 

Before the discussion of particular dependencies which are encountered in clinker solid solutions it will be useful to remind general principles of solid solutions formation. In crystal chemistry significant importance have the interionic distances in the lattice, which is the basis for ionic radius determination . The radii ratio of neighbouring ions in the lattice is deciding of the numbers of the closest neighbours, then of so called coordination number of ion. For example silicon ion is as a mle surrounded by four oxygen ions (CN=4), located in the tetrahedron comers . 

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