Semiconductor-Insulator Transition in Undoped Rutile, TiO2, Ceramics

The electrical conductivity of undoped rutile ceramics is very dependent on sample processing conditions, especially the temperature and atmosphere during sintering and the subsequent cooling rate. Samples become increasingly semiconducting when quenched from temperatures above 700 degrees C without the need for a reducing atmosphere. Thus, samples quenched from 1400 degrees C in air have conductivity 1x10-2Scm-1with activation energy 0.01(1)eV over the temperature range 10100K, whereas similar samples that are slow cooled or annealed in air at 300 degrees C500 degrees C are insulating with activation energy 1.67(2)eV and conductivity, e.g., 1x10-7Scm-1 at 400 degrees C. The very wide range of electrical properties is attributed to variations in oxygen content which are too small to be detected using thermogravimetry. Impedance analysis shows that, depending on cooling rate, partially oxidized samples may be prepared in which samples retain a semiconducting core, but have an oxidized outer layer.