Brookite structures

Maloney, J., Titanate Pigments Gol-ored Rutile, Priderite and Pseudo-brookite Structured Pigments, in High Performance Pigments (Ed. H. Smith), Wiley-VGH, Weinheim, 2002... 

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. 

Titanium dioxide exists in nature as three different polymorphs rutile, anatase and brookite. This material has been extensively studied during the last few decades due to its interesting physical properties and numerous technological applications. Rutile and anatase (a popular white pigment) are widely used in photocataly s and as sensors. Both of them have had new structural and electronic applications suggested recently (see for a review). 

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. 

Table 1. Calculated properties of ratile, anatase, brookite, and columbite phases. Relative deviation from experimental values is shown in brackets. Structural experimental data are from 19,20,21,9 respectively. Bulk modulus of ratile extrapolated to 0 K is from 2.

NakayamaS however, has suggested that, for rutile, which is tetragonal in structure, the strong bond between metal and oxide results from the favourable spacing between titanium ions in the rutile lattice and those in the metal structure. This explanation, however, does not account for the fact that other oxides of titanium, such as brookite, which is orthorhombic, and anatase, which is tetragonal, are also protective . 

The elucidation of the factors determining the relative stability of alternative crystalline structures of a substance would be of the greatest significance in the development of the theory of the solid state. Why, for example, do some of the alkali halides crystallize with the sodium chloride structure and some with the cesium chloride structure Why does titanium dioxide under different conditions assume the different structures of rutile, brookite and anatase Why does aluminum fluosilicate, AljSiCV F2, crystallize with the structure of topaz and not with some other structure These questions are answered formally by the statement that in each case the structure with the minimum free energy is stable. This answer, however, is not satisfying what is desired in our atomistic and quantum theoretical era is the explanation of this minimum free energy in terms of atoms or ions and their properties. 

The first was not the structure of brookite. The second, however, had the same space-group symmetry as brookite (Ft,6), and the predicted dimensions of the unit of structure agreed within 0.5% with those observed. Structure factors calculated for over fifty forms with the use of the predicted values of the nine parameters determining the atomic arrangement accounted satisfactorily for the observed intensities of reflections on rotation photographs. This extensive agreement is so striking as to permit the structure proposed for brookite (shown in Fig. 3) to be accepted with confidence. 

The Principles Determining the Structure of Complex Ionic Crystals.—The success of the coordination method in predicting structures for brookite and topaz has led to the proposal of a set of principles governing the structure of a rather extensive class of complex ionic crystals. 

Brookite is also based upon a double hexagonal close-packed arrangement of the oxygen ions. The dimensions of the unit of structure differ so much on account of distortion from those for the ideal arrangement, however, that the existence of dose-packing was recognized only after the structure had been determined. 

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... 

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... 

The fact that brookite also was found experimentally to have the symmetry of F 5 and to have 8 TiO in the unit suggests strongly that our second structure is to be attributed to this crystal. There is also approximate agreement (within 7%) in the dimensions of the unit. It... 

The structure found for brookite is that shown in Fig. 5. Each titanium atom is surrounded by six oxygen atoms and each oxygen by three titanium atoms. The various interatomic distances, given in Table VII, are similar to those in rutile and anatase for example, the average titanium-oxygen distance is 1,95 A. The oxygen-oxygen distances and the distortion of the basic octahedra have been discussed in the previous sections in the course of derivation of the structure. 

Brookite, Ti02, is found with the use of Laue and spectral photographs to have an orthorhombic unit of structure with d100 = 9,166 A, d l == 5,436 A, and do01 = 5,135 A, containing 8 Ti02, and to have the symmetry of space-group V . A structure predicted by means of the... 

Linus Pauling and J. H. Sturdivant, The cristal structure of brookite. 

---