By means of the radioactive-tracer sectioning technique, the tracer diffusion of the impurity ions, 46Sc, 51Cr,54Mn, 59Fe, 60Co, 63Ni and 95Zr, in rutile single crystals was measured as a function of crystal orientation, temperature, oxygen partial pressure and Al impurity content (figures 1 to 4). The diffusion coefficients were very sensitive to the electric charge of the impurity ions. Divalent impurities (e.g., Co and Ni) diffused extremely rapidly in TiO2, compared to cation self-diffusion, and exhibited an extreme anisotropy in diffusion behavior, divalent-impurity diffusion parallel to the c-axis was much higher than it was, perpendicular to the c-axis. Trivalent impurity ions (Sc and Cr) and tetravalent impurity ions (Zr) diffused similarly to cation self-diffusion, both as functions of temperature and oxygen partial pressure. The divalent impurity ions Co and Ni apparently diffused as interstitial ions along open channels parallel to the c-axis. The results suggested that Sc, Cr and Zr ions diffused via an interstitialcy mechanism involving the simultaneous and cooperative migration of tetravalent interstitial titanium ions and tracer-impurity ions. Iron ions diffused both as divalent and as trivalent ions. The impurity diffusion as a function of oxygen partial pressure and Al-impurity content was consistent with calculations of point-defect concentrations in rutile.

Tracer Impurity Diffusion in Single-Crystal Rutile (TiO2-x). Sasaki, J., Peterson, N.L., Hoshino, K.: Journal of Physics and Chemistry of Solids, 1985, 46[11], 1267-83

Figure 2

Diffusivity of Cr in rutile as a function of temperature

a: parallel to c-axis, b: perpendicular to c-axis