Temperature Dependence of the EPR Spectra of Niobium-Doped TiO2

Detailed measurements of the temperature dependence of the EPR spectrum and Hall coefficient of niobium-doped rutile have been made. At 4.2 °K and 34 GHz, the EPR spectrum consists of a set of well-resolved hyperfine lines which coalesce and then shift with increasing temperature. Computer calculations of explicit line shapes have been performed based on a model of random hopping between donor sites. Direct impurity-impurity and impurity-conduction-band-impurity hopping are discussed. The hopping frequency is found to vary exponentially with temperature. The temperature dependence is proportional to that of the conduction-electron concentration and is attributed to exchange scattering of the donor electrons with the conduction electrons. The exchange-scattering cross section is found to equal ${\sigma }_{\mathrm{ex}}=3\mathrm{}\times {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$.