14 сентября, 2015 
Pokraskin Of the three modifications ofTiO2, rutile is the thermodynamically most stable one. Nevertheless, the lattice energies of the other phases are similar and hence are stable over long periods. Above 700 °C, the monotropic conversion of anatase to rutile takes place rapidly. Brookite is difficult to produce, and therefore has no value in the TiO2 pigment industry.
Industrial Inorganic Pigments. Edited by G. Buxbaum and G. Pfaff Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30363-4
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Tab. 2.1: World production ofTiO2 pigment.
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In all three TiO2 modifications, one titanium atom in the lattice is surrounded octahedrally by six oxygen atoms, and each oxygen atom is surrounded by three titanium atoms in a trigonal arrangement. The three modifications correspond to different ways of linking the octahedra at their corners and edges. Crystal lattice constants and densities are given in Table 2.2.
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Tab. 2.2: Crystallographic data for TiO2 modifications
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Rutile and anatase crystallize in the tetragonal system, brookite in the rhombic system. The melting point of TiO2 is at about 1800 °C. Above 1000 °C, the oxygen partial pressure increases continuously as oxygen is liberated and lower oxides of titanium are formed. This is accompanied by changes in color and electrical conductivity. Above 400 °C, a significant yellow color develops, caused by thermal expansion of the lattice; this is reversible. Rutile has the highest density and the most compact atomic structure, and is thus the hardest modification (Mohs hardness 6.5-7.0). Anatase is considerably softer (Mohs hardness 5.5).
Titanium dioxide is a light-sensitive semiconductor, and absorbs electromagnetic radiation in the near-UV region. The energy difference between the valence and the conductivity bands in the solid state is 3.05 eV for rutile and 3.29 eV for anatase, corresponding to an absorption band at <415 nm for rutile and < 385 nm for anatase.
Absorption of light energy causes an electron to be excited from the valence band to the conductivity band. This electron and the electron hole are mobile, and can move on the surface of the solid where they take part in redox reactions.
2.1.1.2
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