Masahito Yahagi

Simulation Study on the Measurements of Diffusion Coefficients in Solid Materials by Short-lived Radiotracer Beams

We have examined, by a computer simulation, an on-line measurement of diffusion coefficients by using a short-lived alpha particle emitter, 8Li (half life of 0.84 s), as a radiotracer. The energy spectra of alpha particles emitted from diffusing 8Li primarily implanted in the sample of LiAl are simulated as a measure of the diffusion of 8Li in the sample. As a possible time sequence for the measurement, a time cycle of 6 s, i.e. the implantation of 8Li for 1.5 s and subsequent diffusion for 4.5 s, is supposed. The sample is primarily set on a given temperature for the measurement. The time-dependent yields of alpha particles during the time cycle reveal the possibility to measure the diffusion coefficient with an accuracy of 10% if larger than 1×10-9 cm2/s, by the comparison with the experimental spectra measured at the temperature, i.e. at a certain diffusion coefficient.

Anomalous heat capacity and defect structure in β-LiGa

Abstract Order–disorder transition of the Li vacancies in an intermetallic compound β-LiGa has been presented based on the electrical and heat capacity measurements. Anomalous heat capacity has been observed at T c =233 K, with excess enthalpy of 72.5 J/mol. Abrupt change in the electrical resistivity has been observed at the same temperature of T c . The observed anomalous behavior was compared with that of β-LiAl in connection with the defect structure near the Li deficient β-phase.

A Study on Thermal Expansion of LiAl1-xInx System

The measurement of the thermal expansion coefficient for LiAl 1- x In x system is performed and it is found that the variation of these measured values with x has a maximum at half the composition ( x =0.5). This phenomenon seems to rise in the deformation due to the arrangement of aluminum and indium atoms with different atomic radii. Gruneisen constants of LiIn crystal are estimated to be 2.66 and 3.22 from the thermal expansion coefficient and specific heat.

Simulation of radiotracer method for diffusion studies using short-lived radioactive nuclear beams

Abstract We have studied, by a computer simulation, the feasibility of the radiotracer method for diffusion studies using short-lived radioactive nuclear beams as tracers. We have considered two typical examples of radiotracers that will be available in KEK-JAERI RNB facility, 18F (half-life: 1.8 h, β+-emitter) and 8Li (half-life: 0.83 s, α-emitter). The sample is set on a given temperature and irradiated by the tracer beams for a time-duration. In the case of 18F, the serial sectioning technique by the ion-beam-sputtering device is assumed to obtain the concentration-depth profile of the tracer diffused during the irradiation time. In the case of 8Li, the simulation reveals that the time-dependent yields of α particles can be used as a measure of the diffusivity of the tracer in a non-destructive way. The present method could be applied to measure rather large diffusion coefficients of various radiotracers with a short measuring time.

Diffusion Experiment in Lithium Ionic Conductors with the Radiotracer of 8Li:from Micro‐ to Nano‐diffusion

We have developed a radiotracer method for diffusion studies in lithium ionic conductors, by using, as the tracer, the short‐lived α‐emitting radioisotope of 8Li from TRIAC (Tokai Radioactive Ion Accelerator Complex). In the method, we measured α‐particles coming out of the sample of interest and have found that the time‐dependent yields of α‐particle from the diffusing 8Li primarily implanted is a good measure of the Li diffusion in the sample. The method has been successfully applied to measure the lithium diffusion coefficients in a typical defect‐mediated lithium ionic conductor of LiGa, well demonstrating that the method is very efficient to measure the diffusion in the micro‐meter regime per second. Further development, as an extension of the present method, was proposed to measure the diffusion on the nanoscale in lithium ionic conductors.

NMR Study on Defect Structure in β-LiGa

The mechanism of superionic conductivity in β-LiGa has been investigated by 7 Li and 71 Ga NMR spin–lattice relaxation rates 1/ T 1 in Li excess 50.0 at. %, Li deficient 44.0 and 47.0 at. % samples. The activation energies of Li + ionic diffusions through free Li vacancy V Li and through bound Li vacancy trapped by antisite Li Ga have been estimated to be 0.13 and 0.064 eV by applying Bloembergen–Purcell–Pound (BPP) type equation to 1/ T 1 in 50.0 at. % Li sample. In Li deficient samples, the same activation energy, 0.11–0.13 eV, for Li + ionic diffusion through free V Li has been obtained at high temperatures, and evidence for the ordering of V Li has been also detected by anomalies at 200–240 K in the temperature dependence of 1/ T 1 for 7 Li. However, the motional narrowing observed in 7 Li NMR line width above 110–130 K indicates that disorder in V Li can still remain even below its ordering temperature in 44.0 and 47.0 at. % Li samples.

Li+ ionic diffusion and vacancy ordering in β-LiGa

7Li and 71Ga NMR measurements have been performed to study the Li+ ionic motion and vacancy ordering in the lithium semimetal β-LiGa. The temperature dependence of the spin–lattice relaxation rate, T−11 of the 7Li nuclei in the 50 atom% Li sample shows an asymmetric broad peak around 175 K and is interpreted in terms of fast Li+ ionic diffusion. The activation energy of hopping is estimated as 0.11 eV using a non-Debye type relaxation model. In the temperature dependence of T−11 of the 7Li nuclei in 44 and 47 atom% Li samples, steep peaks are observed at 225 and 195 K, respectively. The origin of these anomalous peaks is attributed to the order–disorder transformation of Li+ vacancies. The temperature dependence of T−11 of the 71Ga nuclei measured above 200 K is interpreted in terms of the relaxation originating from the fluctuation of the electric field gradient at the 71Ga nuclei due to mobile Li+ ions. The activation energy for the Li+ ionic diffusion estimated from T−11 of the 71Ga nuclei is comparable with that obtained from T−11 of the 7Li nuclei.

7Li Spin–Lattice Relaxation at Low Temperatures in a Superionic Conductor β-LiGa

In order to investigate the Li + ionic diffusion and the electronic states in a mixed conductor β-LiGa with high Li + ionic diffusibility and electron/hole conductivity, 7 Li NMR linewidth and spin–lattice relaxation measurements have been performed in 44.0, 47.0, and 50.0 at. % Li β-LiGa samples at 10.03 MHz in the temperature range between 10 and 320 K. The onset temperature T MN =70 K of the motional narrowing in 50.0 at. % sample has been determined from the temperature dependence of the linewidth. The Li + ionic diffusion is found to contribute to the spin–lattice relaxation rate 1/ T 1 down to ∼0.5 T MN even below T MN where the motional narrowing does not occur. The high diffusibility of Li + ions has been proved from a microscopic point of view. At low temperatures, the relations 1/ T 1 T =3.5×10 -4 , 3.8×10 -4 , and 5.1×10 -4 s -1 K -1 are observed in 44.0, 47.0, and 50.0 at. % Li samples, respectively. The density of states of conduction electrons at the Fermi level in these compounds becomes hi...

On-Line Diffusion Tracing in Li Ionic Conductors by the Short-Lived Radioactive Beam of 8Li

A non-destructive and on-line diffusion tracing in Li ionic conductors has been demonstrated. As the tracer, the pulsed beam of the short-lived α-emitting radioisotope of 8Li was implanted into a typical Li ionic conductor LiGa. By analyzing the time-dependent yields of the α-particles measured in coincident with the repetition cycle of the beam, the tracer diffusion coefficients were extracted with a good accuracy. The ordering of the Li vacancies in the Li-deficient β phase of LiGa was observed for the first time in terms of the Li diffusion by the present method.