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Anatase & Brookite

TiO₂ forms in the three different structures, each containing its own similar, but different structural and optical properties. The most abundant and most extensively studied is the rutile phase (click link for more information). The other two polymorphs are anatase and brookite. Anatase more commonly used form in laboratory research and is the least abundant of all three forms, while the brookite phase, until recently, was less researched in detail. All have been studied for their photocatalytic and photoelectrochemical applications. The difference in these three crystal structures can be attributed to various pressures and heats applied from rock formations in the Earth. At lower temperatures the anatase and brookite phases are more stable, but both will revert to the rutile phase when subjected to high temperatures (915^oC for the anatase phase and 750^oC for the brookite phase). Although rutile is the most abundant of the three phases, many quarries and mines containing only the anatase or brookite form exist. Brookite was first discovered in 1849 in Magnet Cove, a site of large deposits of the mineral. It was originally dubbed ‘arkansite’ for the state it was discovered in, Arkansas [1].

Figure 1: Anatase Structure viewing down the (100) axis

Figure 2: Anatase Structure view down (001) axis

Figure 3: Brookite Unit Cell structure

Figure 4: Extended Brookite Structure

The structure of anatase is similar to that of rutile. It has tetragonal symmetry and belongs to the same point group, but contains an inversion center perpendicular to the [001] axis that gives it body centered symmetry with a space group I4₁/amd. Note that there is a diamond glide along c in the ab plane (See Figures 1 and 2). As in rutile, the TiO₆ octahedra are linked at the corners in an alternating pattern. There is a distortion in the octahedra that causes longer bond lengths along the axial Ti-O bonds than those in the horizontal plane. This distortion is slightly larger for anatase [2]. The brookite structure is more complicated and has a larger cell volume than the other two. It is also the least dense of the three forms (in g/cm³). The unit cell is composed of eight formula units of TiO₂ and is formed by edge sharing TiO₆ octahedra, similar to rutile and anatase (See Figures 3 and 4). Brookite belongs to the Orthorhombic crystal system its space group is Pbca. By definition, the brookite structure is of lower symmetry than its TiO₂ countermorphs, the dimensions of the unit cell are unequal (See Table 1). Also the Ti-O bond lengths vary more so than in the rutile or anatase phases, as do the O-Ti-O bond angles [2].

The optical properties of each phase are also similar, but have some slight difference. The absorption band gap for the rutile, anatase, and brookite phases were calculated to be 1.78eV, 2.04eV, and 2.20eV, respectively. In addition to the slight increase in the band gap, the anatase form also has a slightly higher Fermi level (0.1eV). In thin films it has been reported that the anatase structure has higher mobility for charge carriers versus the rutile structure [3]. For photocatalytic processes, anatase is the preferred structure, although all three forms have shown to be photocatalytic (See rutile). The electronic structure of brookite is similar to anatase, based on minor differences in the local crystal environment [2].

    Table 1.        Rutile      Anatase     Brookite 
                    TiO2        TiO2        TiO2   
    ________________________________________
    
    Form.Wt.        79.890      79.890      79.890      
    Z               2           4           8         
    CrystalSystem   Tet         Tet         Orth        
    PointGroup      4/mmm       4/mmm       mmm      
    SpaceGroup      P42/mnm     I41/amd     Pbca
    
    UnitCell
    a(Å)            4.5845      3.7842      9.184
    b(Å)                                    5.447
    c(Å)            2.9533      9.5146      5.145
    Vol             62.07       136.25      257.38

References

[1]. M. Howard, Brookite, Rutile Paramorphs after Brookite, and Rutile Twins from Magnet Cove, Arkansas. Rocks and Minerals. Heldref Publications. (1999)

[2]. S. Mo, W. Ching, Phys. Rev. B 51, 19, 13023 (1995)

[3]. H. Tang, et. al. Appl. Phys. 75, 2042 (1994)