Bi-Arrhenius diffusion and surface trapping of $^{8}$Li$^{+}$ in rutile TiO$_2$

We report measurements of the diffusion rate of isolated ion-implanted Li-8(+) within similar to 120 nm of the surface of oriented single-crystal rutile TiO2 using a radiotracer technique. The alpha particles from the( 8)Li decay provide a sensitive monitor of the distance from the surface and how the depth profile of Li-8 evolves with time. The main findings are that the implanted Li+ diffuses and traps at the (001) surface. The T dependence of the diffusivity is described by a bi-Arrhenius expression with activation energies of 0.3341 (21) eV above 200 K, whereas at lower temperatures it has a much smaller barrier of 0.0313(15) eV. We consider possible origins for the surface trapping, as well the nature of the low-T barrier.