Influence of Extended Defects on the Electrical Properties of TiO2 (Rutile)

TiO2 is a promising material for many technological applications such as solar cells, water splitting, memory devices and Li-ion batteries. Even though the functionalities are diverse depending on their applications, the point defects in TiO2 play a decisive role in all these technologies. The point defect chemistry of TiO2 was intensively studied in the last few decades and so far modification of point defect concentrations was attained by either aliovalent doping or by nano size effects. Both methods are widely applied in the field of solid state ionics, however, with limitations such as limited solubility of dopants grain growth effects already at moderate temperatures. Another adjusting screw for altering point defect concentrations is by incorporating mircostructural modifications in the material such as one dimensional line defects (dislocations), or two dimensional planar defects (grain boundaries). In ionic solids, these extended defects should be charged, and in order to maintain global charge-neutrality, they will locally modify the point defect concentration in space charge zones. The influence of dislocations on point defect concentrations in oxides, and thus the electrical properties, has not been studied much in TiO2 for far. The present thesis gives such a detailed investigation using TiO2 in the rutile structure as model material. Dislocations were created in TiO2 single crystals as well as polycrystalline materials by uniaxial compression at elevated temperature. Based on the creep deformation map for TiO2, a suitable combination of temperature and pressure was chosen to activate dislocation creep. The dislocation formation and migration strongly depends on the material properties; in case of TiO2 various factors – such as comparable cation (Ti interstitials) and anion (oxygen vacancies) mobilities, comparably low melting temperature and shear modulus when compared to other wide band gap oxides – allows for their formation at moderate conditions (typically at 1000 C, 40 MPa for single crystals and 925 C, 400 MPa for polycrystalline materials). In the temperature range (350 C to 550 C) in which the conductivity measurements have preformed, dislocations are typically immobile and therefore measured properties well reproducible. Dislocations generated by this process are characterized by transmission electron microscopy and it is found that the dislocations favorably lie on {110} slip planes. Based on the slip planes, electrical measurement axis is chosen to be [001] and [110], directions parallel and perpendicular to the dislocations respectively. Electrical properties are also studied as a function of dislocation density, temperature and oxygen partial pressure. With increasing density of dislocations the conductivity type of TiO2 at moderate temperatures (550 C) and high oxygen partial pressures (1 bar to 10 bar) changes from usual hole conductivity (p-type semiconductor) to predominant ionic conductivity. Further, to discriminate between oxygen vacancy and titanium interstitial transport in the partial ionic conductivity, oxygen isotope exchange and SIMS analysis were performed. The comparison of the results from these characterization techniques shows that the enhanced ionic conductivity is due to negatively charged dislocation cores and adjacent space charge accumulation zones of positive carriers. This interpretation is further supported by the fact that the concentrations of ionic defects with their higher charge are influenced to a much greater degree than electronic defects. Similar effects of dislocations on the electrical properties are observed for polycrystalline TiO2. Dislocation creation have a persistent effect on the electrical properties with the ionic conductivity of the samples increased more strongly than the electron hole conductivity. The partial ionic conductivity of the sample is measured by Hebb-Wagner type electrodes, and an unusually high ionic transference number is observed for TiO2 at high oxygen partial pressures. Further, effects of acceptor dopant (0.1 mol % Y) on the dislocation generation is studied by either homogeneously doping or by selectively decorating grain boundaries. No dislocations were observed in homogeneously doped samples due to solid solution strengthening, which therefore resulted in a regular defect chemistry as expected for an acceptor doped TiO2. In case of decorated samples, very similar to the undoped samples, dislocations are generated throughout the sample and once again a oxygen partial pressure independent ionic conductivity is observed, typically in the range of 1 bar to 10 bar. The effect of dislocations is very persistent even a high temperature treatment at 1300 C for 5 h did not anneal much of the dislocation density. Hence, the observed changes in electrical properties are very stable and reproducible over a wide temperature range. Grain boundary cores of perovskite and fluorite structured oxides with large band gaps such as SrTiO3, CeO2, ZrO2 (Y stabilized) are typically positively charged due to the presence of excess anion vacancies. Positive grain boundary cores impede the transport of positively charged defects such as holes, cation interstitials and anion vacancies by formation of depletion layers. However, in case of TiO2 also negative grain boundary or dislocation core charges may form. For this reason, grain boundaries of TiO2 bicrystals with symmetric tilt boundaries are investigated in the second part of the study. Two main orientations viz. Σ5 (210)[001] and 6 [001] symmetric tilt boundaries are investigated with a focus on the boundary electrical properties. High-resolution transmission electron microscopy revealed that the symmetric tilt grain boundaries correspond to a periodic array of dislocations with a spacing according to Frank’s rule. It is also observed that the electrical conductivity in two boundary orientations are similar and the boundaries are not blocking for the transport of holes. This indicates that the symmetric tilt boundaries in TiO2 are not similar to other wide band gap oxides, but are usually positively charged. To summarize, it is quite obvious from this work on TiO2 that dislocations can be used as a means of modifying defect transport of ionic solids locally and globally, hence, allowing additional degrees of freedom for tuning the ionic/electronic properties of various functional oxides.