4 Diffusion Experiment in Lithium Ionic Conductors with the Radiotracer of 8

Radioactive nuclides have been used in materials science for many decades. Besides their classical application as tracers for diffusion studies, nuclear techniques (i.e. Mössbauer Spectroscopy, Perturbed Angular Correlation, β-Nuclear Magnetic Resonance, Emission Channeling, etc.) are now being routinely used to gain microscopic information on the structural and dynamical properties of the bulk of materials via hyperfine interactions or emitted particles themselves (Wichert & Diecher, 2001). These nuclear techniques were primarily developed in nuclear physics for detecting particles or γ-radiations emitted during the decay of the radioactive nuclides. More recently these techniques have also been applied to study complex bio-molecules, surfaces, and interfaces (Prandolini, 2006). With the advent of most versatile ‘radioactive isotope beam (RIB) factory’ represented by the on-line isotope separator (ISOL)–based RIB facility (see Fig. 1), the possibilities for such investigations have been greatly expanded during the last decade (Cornell, 2003). At the tandem accelerator facility of Japan Atomic Energy Agency (JAEA)-Tokai, a RIB facility, TRIAC (Watanabe et al., 2007)-Tokai Radioactive Ion Accelerator Complexis operating since 2005. In the facility, short-lived radioactive nuclei produced by proton or heavy ion induced nuclear reactions can be accelerated up to the energy necessary for experiments. The energy is variable in the range from 0.1 to 1.1 MeV/nucleon, which is especially efficient for studies of the bulk of materials by using the RIBs as tracers. It allows us to implant (incorporate) the RIBs into specimens at a proper depth, avoiding the difficulties caused by the surface (e.g. diffusion barrier like oxide layers that often hampers the incorporation of those radioactive isotope probes into the materials of interest). In the facility, the separation and the implantation of radioactive probes are integrated into one device, as shown in Fig.1. Although the main concerns of the facility are nuclear physics experiments, as an effort to effectively use the available radioactive isotope beams at the TRIAC for materials studies, we have developed a diffusion tracing method by using the short-lived radioactive nuclei of 8Li as diffusion tracers. The method has been successfully applied to measure diffusion coefficients in a typical defectmediated lithium ionic conductor (refer to Chandra, 1981 for ionic conductors). We found that the present method is very efficient for the micro-diffusion, where the diffusion length is about 1μm per second.