Anisotropic Conduction in Nonstoichiometric Rutile (TiO2)

Measurements are reported for the anisotropic conductivity in rutile for different nonstoichiometric compositions. This anisotropy, $\frac{{\rho }_a}{{\rho }_c}$, has a peak in excess of 10 000 to 1 and is a maximum at the nonstoichiometric composition corresponding to ${\rho }_c={10}^3$ ohm cm. Based on these data, it is proposed that for very nonstoichiometric material, impurity band conduction predominates, and as the material is made more stoichiometric, a transition to conventional conduction via the conduction band occurs. This transition occurs at very different nonstoichiometric compositions for the two crystal directions. Thus, a large anisotropy is observed in the intermediate region.