Titanium dioxide coordination number

During the investigation of the structure of brookite, the orthorhombic form of titanium dioxide, another method of predicting a possible structure for ionic compounds was developed. This method, which is described in detail in Section III of this paper, depends on the assumption of a coordination structure. It leads to a number of possible simple structures, for each of which the size of the unit of structure, the space-group symmetry, and the positions of all ions are fixed. In some cases, but not all, these structures correspond to closepacking of the large ions when they do, the method further indicates... 

Rutile structure (Fig. 4-15). Titanium dioxide occurs naturally as ana-tase, brookite, and rutile, all of which contain octahedral TiC>68 units. The coordination number of the central Ti4+ is very obviously six, and a little thought confirms that the same is true of the Ti4+ ions at the corners. That the coordination number of the O2- ions is three is seen from the nearest... 

The rutile structure. Titanium dioxide crystallizes tn three crystal forms at utmospheric pressure anatase, brookite, and rutile (Fig. 4.4a). Only the last (tetragonal P42/mnin) will be considered here. The coordination numbers are 6 for the cation (six oxide anions arranged approximately octahedrally about the titanium ions) and 3 for the anion (three tiianium ions trigonally about the oxide ions). The rutile structure is also found in the dioxides of Cr, Mn, Ge, Ru, Rh, Sn, Os. Ir, Pt. and Pb. 

The situation becomes complicated for stoichiometries other than 1 1. In crystals of an AB compound, the coordination number of A is twice that of B and the structure is determined by the coordination number of the smaller ion. The radius ratio rule is clear-cut only with simple ions, and polymorphism is much more prevalent when complex ions are involved. For example, the commonly employed pigment titanium dioxide is known to exist in two polymorphs, anatase and rutile, whose structures are each built up from different arrangements of octahedral coordination. As shown in Fig. 3, the coordination polyhedron of rutile is essentially ideal, while for anatase there are very distorted octahedra of oxygen atoms about each titanium ion. Although rutile is... 

Figure 3 shows that the titanium CT band observed in Ti0x/Al203 sample occurs at lower wavelength (270 nm) and is shifted to 260 nm after calcination at 773 K. We attribute this shift to a decrease of the polymerization degree of titanium on y-alumina after dehydroxilation, indicating a better dispersion of titanium dioxide at the surface. Moreover, we note that a coordination number of titanium must be six as it is known, the tetrahedral titanium is characterized by a CT band at 205 nm [13]. 

The rutile structure of titanium dioxide, Ti02, is not close-packed. The radius ratio of 0.59 falls in the octahedral hole/C.N. = 6 region, and the drawing of the structure in Figure 7.22c shows this to be the case. Note that this is not a cubic unit cell but rather is tetragonal (see Figure 7.15). The coordination number of the oxides is 3, which is consistent with the rule stated earlier. The correlation between the known structure and radius-ratio calculations is 75% for the rutile structure. 

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