Brookite crystal structures

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

Three forms of titanium dioxide, Ti02, are known. Of these the crystal structures of the two tetragonal forms, rutile and anatase, have been thoroughly investigated2) in each case only one parameter is involved, and the atomic arrangement has been accurately determined. The third form, brookite, is orthorhombic, with axial ratios... 

There are three crystal structures of titanium dioxide rutile, anatase, and brookite. The most active phase is rutile, which has a tetragonal structure [133], as shown in Figure 8.5 [134],... 

Rutile and anatase are used in photocatalysis and as catalyst supports and we describe some important structural features later. The titanium-oxygen system plays a key role in understanding stoichiometric variations. The highest oxide, Ti02, is polymorphic and the four known crystal structures are rutile, anatase, brookite and a high-pressure form similar to a-Pb02. 

Experimental studies of single crystal TiOi surfaces have utilized almost exclusively the rutile crystal structure, which is more stable than the anatase or brookite structures. Most of the work reviewed here was carried out on the (110) or (001) crystal planes, so a short introduction to the structures and coordination environments of those planes is presented first. 

Let us finally turn to a brief discussion of the third term, Ti02 (i.e., titanium dioxide). Ti02 has three different crystal structures [18] rutile, anatase, and brookite only the former two of them are commonly used in photocatalysis. Like for many other metal oxides (also for titanium oxide) have the respective structural, optical, and electronic properties... 

In anatase, the arrangement of the aifions and cations is similar and the crystal is again tetragonal, but now each octahedron is somewhat distorted and shares four of its edges with other octahedra. In brookite, the structure is even more complicated with octahedra sharing both edges and corners. So the trend rutile-anatase-brookite is to ever decreasing symmetry. 

Figure 34. The crystal structures of orthorhombic brookite (space group Pbca tetragonal anatase (74]and tetragonal rutile The edge- and eomer-sharing TiO octahedra and the...

Titanium dioxide occurs in nature in three crystalline forms, rutile, anatase, and brookite. The crystal structures are shown in Fig. 1 [24]. Some of the physical properties are given in Table 1. Much of the fundamental work on Ti02 has been carried out with rutile single crystals because these are relatively straightforward to produce. Anatase single crystals are more difficult to produce, and some work has been carried out with natural crystals but it is possible to grow such crystals with the use of the vapor-phase transport method [25]. Photocatalytic reactions... 

Meagher, E.P. Lager, G.A. (1979) Polyhedral thermal expansion in the TiO, polymorphs refinement of the crystal structures of rutile and brookite at high temperature. Can. Mineral., 17,77-85. 

FIGU RE 9.11 Polyhedral models of TiOj polymorphs (a) rutile, (b) anatase, and (c) brookite. (Reprinted with kind permission from Dr. K. Nomura, Intermediate Temperature Solid Conductors—Crystal Structure Gallery—AIST, http //staff.aist.go.jp/nomura-k/english/ itscgallary-e.htm.)... 

Titanium is relatively abundant and widely distributed in the earth s crust In igneous rocks titanium forms the acidic component of basic magmas and the basic component of acidic magmas. In the first case titanates are present, the most important of which are ilmenite FeTiOj and perovskite CaTiOj. In the second case oxides are formed. Among different minerals with the formula TiOj, rutile (Figure M28) is the most important Other variants, with other crystal structures but the same composition TiOj, are brookite and anatase. 

Luehen, H. Elsenhans, U. Stamm, U. (1987). Low-Symmetric Coordination Polyhedra -Pseudosymmetry and Idealization, Acta Cryst. A, Vol.43, pp. 187-194 Meogher, E.P. Lager, G.A (1979). Polyhedral Hermale Xpansion in that Ion, Polymorphs Refinement of the Crystal Structures Rutile and Brookite at High Temperature, Canadian Mineralogist, Vol.l7, pp. 77-85... 

The specific electronic and ionic properties of Ti02 " strongly depend on the crystal structure (anatase, rutile, and brookite). As anatase shows the highest electron mobility, this is the most desired crystal structure for many electron-conducting applications such as solar cells or photo-catalytic electrodes. 

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