Recent Studies on Rutile (TiO2)

A review is made of the work on reduced and ``doped'' rutile performed since the appearance of Grant's survey article in the Reviews of Modern Physics (1958). Measurements of electrical and optical properties, and of electron spin resonance spectra are discussed. A model of electronic bound states and conduction levels is suggested that is compatible with the results of these experiments. There is strong evidence that the defects in reduced rutile are interstitial Ti3+ ions. At very low temperatures, nearly all electrons are self-trapped on cation sites (polarons). As the temperature increases, some of these trapped electrons will be excited into the conduction band. The activation energy for this process is approximately 0.007 ev below 50°K, and about one order of magnitude higher around room temperature. It is concluded that conduction takes place in a narrow 3d band associated with Ti ions; the effective mass at the bottom of this band is ∼25m0. If one assumes that the polaron binding energy can be described with a hydrogenic model, one calculates an effective dielectric constant close to the static value. This result is at variance with the commonly accepted ideas concerning electron lattice coupling.