Transparent single crystals of rutile that were free of grain-boundaries, and contained a small amount of Al2O3, were grown by using the float-zone method. It was found that the optimum Al addition was 0.4at%. On the other hand, Al-free pure rutile crystals were dark blue and comprised many low-angle grain boundaries. It was deduced that Al3+ ions played 2 roles in the rutile crystals. One role was to pin dislocations during cooling due to the difference in ionic radii between Ti4+ and Al3+. Because of this, the formation of low-angle grain boundaries did not occur. The other role was to form O vacancies, via which O ions could easily migrate during cooling to room temperature after crystal growth. At temperatures ranging from 600 to 900C the conductivity of Al-doped crystals was larger, by an order of magnitude, than that of pure crystals. This indicated that the diffusion rate of O ions was much higher in Al-doped crystals than in pure crystals.

Floating Zone Growth and Characterization of Aluminum-Doped Rutile Single Crystals. Hatta, K., Higuchi, M., Takahashi, J., Kodaira, K.: Journal of Crystal Growth, 1996, 163[3], 279-84