Effects of defects on electrical transport properties of anatase TiO<sub>2</sub> polycrystalline under high pressure: AC impedance measurement

Abstract

The electrical transport properties of anatase TiO<sub>2</sub> polycrystalline have been systematically investigated by using high pressure <i>in-situ</i> impedance spectroscopy measurements. The anomalous behaviors of resistance, parameter factor and relaxation frequency of grain and grain boundary can be found at 6.4, 11.5 and 24.6 GPa. The results indicate that the first two discontinuous points (6.4 and 11.5 GPa) correspond to the phase transitions of TiO<sub>2</sub> from anatase to α-PbO<sub>2</sub> and then to baddeleyite, respectively. Above 24.6 GPa, TiO<sub>2</sub> completely transforms into the baddeleyite phase. Based on the change of grain resistance and grain boundary resistance under pressure, intrinsic defects play a crucial effect in the electrical transport properties of TiO<sub>2</sub> at high pressures. At 6.4 GPa, the occurrence of phase transition gives rise to the variation of defects’ role, from a deep energy level defect (as a recombination centre) changes into a shallow energy level defect (providing carriers for the conduction and valence bands). In addition, the position of defect in energy band changes with pressure increasing. The phase transition of TiO<sub>2</sub> at 6.4 GPa is the rearrangement of TiO<sub>6</sub> octahedron, while the other one at 11.5 GPa can be attributed to the migration of oxygen Schottky defects from inner to surface. Combining the packing factor and relaxation frequency, the electrical transport properties of TiO<sub>2</sub> under pressure are revealed, the packing factor and the relaxation frequency are closely related to the mobility and the carrier concentration, respectively. The activation energy of grain and grain boundary decrease with the pressure elevating, indicating that the transport of carriers in grain and grain boundary become easier under pressure, and the former is smoother than the latter owing to the activation energy of grain being smaller than that of grain boundary in the same pressure range. Moreover, the relaxation frequency ratio of TiO<sub>2</sub> grain and TiO<sub>2</sub> grain boundary decreases with pressure increasing, and the grain boundary effect under high pressure is not obvious.