Shinji Kawasaki

Molecular dynamics analysis of ionic conduction mechanism in yttria-stabilized zirconia

Abstract Molecular dynamics simulation was performed to calculate the ionic conductivity of yttria-stabilized zirconia (YSZ), by varying Y 2 O 3 concentration to 5.9, 8.0, 10.2 and 12.5 mol%. The calculated conductivity exhibited a maximum at 8 mol%, which agreed with previously reported experimental data. In order to clarify the reason for this maximum conductivity, the position occupied by oxygen vacancies was determined. It was found that oxygen vacancies are trapped at the second neighbor position to Y ions. It was also found that the number of oxygen sites which trap the vacancy increased with increasing Y 2 O 3 concentration. Therefore, the trade-off effect between the increase in the number of vacancies and the decrease of vacancy mobility was considered to be the reason for the maximum ionic conductivity occurring at a Y 2 O 3 concentration of 8 mol%.

Materials design of perovskite-based oxygen ion conductor by molecular dynamics method

Abstract Molecular dynamics simulation was performed in order to design a new perovskite oxygen ion conductor. The results suggested that (Sm1−xAx)AlO3−x/2 (A=Ca or Sr) exhibits relatively high conductivity. Based on this result, (Sm1−xCax)AlO3−x/2 (x=0.1–0.22) compounds were synthesized to investigate the properties of oxygen ion conduction. The conductivity obtained quantitatively agreed with the calculated value at each Ca concentration. In this system, (Sm0.8Ca0.2)AlO2.9 had the highest conductivity of 0.037 S/cm at 800 °C with an ionic transference number of 0.96.

Difference in spin state and covalence between La1−xSrxCoO3 and La2−xSrxLi0.5Co0.5O4

We present a comparative study of the spin states and electronic properties of La1-xSrxCoO3 and La2-xSrxLi0.5Co0.5O4 using X-ray absorption near-edge structure spectroscopy at both the O-K and Co-L-2.3 thresholds. In the La2-xSrxLi0.5Co0.5O4 system the CoO6 octahedra are isolated, the holes induced by Sr doping are trapped in the isolated Co(IV)O-6 octahedra, and a low-spin state is found for the Co ions, which does not change upon Sr doping. In the La1-xSrxCoO3 system, the interconnected CoO6 octahedra, with a 180degrees Co-O-Co bond angle, give rise to a transition from low-spin to intermediate-spin state with a ferromagnetic alignment of the Co spins. The double-exchange, ferromagnetic coupling between Co ions mediated by the 180degrees bond angle is responsible for suppressing the low spin-state. We find that the branching ratio of spectral intensities at the L-2 and L-3 thresholds in the Co-L-2.3 X-ray absorption spectra is sensitive to the spin state of the Co ions allowing its direct spectroscopic determination