Abstracts

Defects have gained increasing attention for their capability in tailoring the properties and/or exploring new functionalities in oxide materials. For instance, the colossal permittivity (CP, >103) was successfully achieved by introducing the acceptor/donor co-doping in rutile TiO2. The resultant dielectric properties can be either affected by the defect-induced polarisation or totally dominated by the defects. The defect configurations and its related dielectric properties vary with the different properties of dopant ions, e.g. ionic size, electronegativity, valence state, and so on. More interestingly, the different dopant ions affect their nearby environment differently in structurally strong correlated solid-state system, which may lead to special defect complexes. The research on the dielectric properties of defect-engineered rutile TiO2 is still at early stage and more work need to be done in this field for comprehensive understanding the defect chemistry and dielectric polarization behaviours. This dissertation, therefore, aims to use defect engineering to control the dielectric behaviour of rutile TiO2. Stimulated by the excellent CP behaviour achieved in In+Nb co-doped rutile TiO2, investigation was carried out on the CP behaviour of Ga and Nb co-doped rutile TiO2 i.e. Ga0.5xNb0.5xTi1-xO2 (x = 0.1%, 0.5%, 5%, 10%), where Ga3+ is from the same group as In3+ but with a much smaller ionic radius. Colossal permittivity up to 104 ~ 105 with an acceptably low dielectric loss (tan = 0.05 ~ 0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect and dielectric characterizations suggest that the CP in this system is not dominated by the EPDD. Instead, multiple polarisation mechanisms including defect dipoles, polaron-like electron hopping/transport and a surface barrier layer capacitor effect together make contributions to the CP behaviour observed in this new material. This work provides a comprehensive guide for the design of new CP materials by considering the choice of the acceptor dopant. CP materials have many important applications in electronics but their development has generally been obscured due to the difficulty in achieving a relatively low dielectric loss. A new ionic co-doping material system, i.e. In+Ta co-doped TiO2 with nominal compositions In0.5xTa0.5xTi1-xO2 (x = 0.5%, 5%, and 10%), was also systematically investigated. This system manifests high dielectric permittivity and low dielectric loss based on the electron-pinned defect-dipole. The dielectric loss can be reduced down to e.g. 0.002 at 1k Hz, giving high performance, less temperature-independent dielectric property i.e. r >104 meanwhile tan <0.02 in a broad temperature range of 50-400 K. Density functional theory computation