Defect chemistry and electrical properties of titanium dioxide. 2. Effect of aliovalent ions

The present work considers the effect of aliovalent ions (donors and acceptors) on the concentration of electronic charge carriers (electrons and electron holes) in titanium dioxide at elevated temperatures (873-1373 K) and the related electrical properties. The data are derived in the form of diagrams representing (i) the concentration of electronic charge carriers and (ii) electrical conductivity as a function of the effective concentration of acceptors. These diagrams may be used to predict the effect of acceptors and donors on the electrical conductivity for both n- and p-type TiO2. It is shown that the concentration of aliovalent ions, added intentionally (dopants) or unintentionally (impurities), has a substantial effect on the semiconducting properties already at the level of several parts per million and, therefore, cannot be ignored. This effect increases with the increasing oxygen activity. The determined critical concentrations of acceptors at 1073 K decreases from 10-3 at p(O2) = 10-10 Pa to 10-6 at p(O2) = 75 kPa. These data indicate that well-defined TiO2 specimens must include an analysis of impurities showing the concentration of aliovalent ions at the level of several parts per million.