Titanium valence-band orbitals

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

In order to gain a better understanding of the differences between these compounds, the UV-Visible absorption properties have been discussed on the basis of a schematic band diagram. The electronic band stmctures of titanium oxides and titanium hydroxyfluorides exhibit similar features. The conduction band is characterized by the 3d states of titanium and the valence band is composed of the 2p orbitals of O and/or F atoms. 

Fourth, the difference in localization between cl electrons and s electrons becomes less for larger quantum numbers (the difference is less for the 4d series and especially so for the 5d series, in comparison to the 3d series) and is also less far to the left in each series. Both trends are expected intuitively from consideration of Ihe potentials and the orbits and they will be apparent in the electronic structure. In titanium, at the left end of the transition series, the 3d states will be much more like s and p valence states and will contribute to the bonding. Even at the far left of the 5/ series, the / levels will behave as valence states and will form bands in metallic thorium (Freeman and Koelling, 1974). These two trends—a decrease in the extent of localization as we move left in each series and as we move down in the periodic table—constitute the two most important trends in the transition series. They will show up in different ways in different systems, but have the same origin. 

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