UBC Theses and Dissertations
Diffusion and surface trapping of 8Li in rutile TiO2 and the comparison on 8Li and 9Li spin relaxation using β-NMR Chatzichristos, Christos Aris
It is well established that the properties of many materials change as their thickness is shrunk to the nanoscale, often yielding novel features at the near-surface region that are absent in the bulk. Even though there are several techniques that can study either the bulk or the surface of these materials, there are very few that can scan the near-surface region of crystals and thin films versus depth. Beta-detected NMR (b-NMR) is capable of this and therefore has been established as a powerful tool for material science. This thesis aims to further develop the capabilities of b-NMR. The first part of this thesis demonstrates that by comparing the spin-lattice relaxation rates (SLR) of two radioactive Li isotopes (⁸,⁹Li) it is possible to distinguish whether the source of SLR in a given situation is driven by magnetic or electric interactions. This is an important development for b-NMR, since there are instances where it is problematic to distinguish whether the measured relaxation is due to magnetic or electric fluctuations. Using this method, it was found that the SLR in Pt is (almost) purely magnetic in origin, whereas the spin relaxation in SrTiO₃ is driven (almost) entirely by electric quadrupolar interactions. The second part of this thesis traces the development of a-radiotracer, that uses the progeny a-particles from the decay of ⁸Li, in order to directly measure the nanoscale diffusivity of Li⁺ in Li-ion battery materials. To develop this technique, Monte Carlo simulations of the experimental configuration were carried out, a new apparatus and a new a-detector were designed and used for experiments on rutile TiO₂. In rutile, the measurements revealed that Li+ gets trapped at the (001) surface, a result that helps explain the suppressed intercalation of Li⁺ in bulk rutile. Moreover, the diffusion rate of Li⁺ in rutile was found to follow a bi-Arrhenius relationship, with a high-T activation energy in agreement with other reported measurements and a low-T component of similar magnitude with the theoretically calculated diffusion barrier as well as the activation energy of the Li-polaron complex found with b-NMR below 100 K.