Floating zone growth and characterization of aluminum-doped rutile single crystals

Transparent and grain-boundary-free rutile (TiO2) single crystals, to which a small amount of Al2O3 was added,:were successfully grown by the floating zone method. The most effective Al addition was 0.4 at%. On the Other hand Al-free, pure rutile crystals were dark-blue and comprised many low-angle grain boundaries. Al3+ ions have two roles in rutile crystals. One is to pin down the migration of dislocations during cooling because of the difference in the ionic radii between Ti4+ and Al3+, so that polygonization, namely formation of low-angle grain boundaries, does not occur. The other role is to form oxygen vacancies, via which oxygen ions can easily migrate in the rutile crystal during cooling to room temperature after the crystal growth. The conductivity of the Al-doped crystal was larger by one order of magnitude than that of the pure crystal at a temperature range of 600-900 degrees C, which indicates that the diffusion rate of oxygen ions is much higher in the Al-doped crystal than in the pure crystal.