Electrical properties of pure and doped rutile ceramics

• Main Author: Dang, Yun
• Other Authors: West, Anthony
• Published: University of Sheffield 2018
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.755273

Rutile, TiO2, has drawn significant attention due to its unique properties. In this project, the structural and electrical properties of undoped and Cr/Al/Ga/Zn-doped rutile have been investigated. Undoped rutile is increasingly oxygen-deficient on heating in air above ~700 °C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The lattice parameters of quenched samples show an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increase with a small oxygen loss, δ; chemical expansion of the lattice occurs and is attributed to increase in average Ti-O bond lengths. At higher δ, lattice parameters start to decrease, giving a chemical contraction effect attributed to partial collapse of columns of edge-sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear, CS, plane formation. Oxygen-deficient rutile samples quenched from above 700 °C are n-type, whereas samples annealed and measured at 450-500 °C are p-type and believed to be slightly oxygen-rich. Holes are located on oxide ions at or near the sample surface and arise from redox electron transfer between under-bonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600 °C show cross-over between n- and p-type behavior. Cr-doped rutile, Ti1-xCrxO2-x/2-δ, samples show a volume expansion at low Cr content, which is associated with the larger size of Cr3+ and the increase in average M-O bond length. At relatively higher Cr content, samples show an unusual volume contraction effect, which is also believed to be attributed to the formation of shear planes. The electrical properties of Cr-doped rutile are very dependent on composition x. Impedance results show that at low dopant concentration (0.001 ≤ x ≤ 0.01), samples are two-phase mixtures. The phase identified by R2C2 is more resistive than R1C1 and has larger activation energy. R2C2 represents a phase with very small Cr3+ dopant concentration, i.e. x = 0.001 ± 0.0005, and R1C1 represents a phase with relatively higher dopant content, i.e. x = 0.008 ± 0.002. In addition, since Cr content of both phases are different, R2C2 may have random point defects, such as oxygen vacancies. With relatively higher dopant content, oxygen vacancies collapse to form shear planes, i.e. defects present in the phase R1C1 may be random shear planes. At low x, Cr-doped rutile samples show mixed conduction, with p-type electronic conduction and oxide ion conduction. The location of holes is thought to be related with O2- ions as well. At higher x, a p-n transition is observed; samples are semiconducting and have small activation energy (~0.2 eV). Such semiconductivity is attributed to intrinsic hopping of charge carriers associated with Cr-Cr or Cr-Ti. Since orbital overlap may occur, charge carriers could hop between Cr3+ and Ti4+ and thus, Cr4+ and Ti3+ are generated and samples become semiconducting. Moreover, if oxygen vacancies collapse to form shear planes, the distance between Cr-Cr or Cr-Ti in face-sharing octahedra in the shear planes are smaller, and hopping of charge carriers becomes easier and the conductivity increases. Moreover, polaron hopping is an alternative description of the amount of semiconductivity. Electrical properties of Al-doped TiO2 are dependent on cooling condition. Samples show a p-n transition with increasing quench temperature. The p-type conductivity is driven by surface absorption of oxygen and associated with O- at the sample surface. Ga-doped TiO2 shows an n-p transition with increasing dopant concentration, whereas such electronic conduction is quite weak. Maybe with Ga3+ doping in TiO2, ionic compensation is the main compensation mechanism. With Zn2+ doping, it is clear that the homogeneity of rutile ceramics improves in some degree. However, Zn-doped samples still show evidence of a constriction resistance or a dipole orientation-related impedance. Zn-doped rutile samples show n-type conduction.