Disorder and diffusion path of mobile ions in ionic conductors

Department of Materials Science and Engineering, Tokyo Institute of Technology
â—‹Masatomo Yashima

Solid materials that exhibit high ionic conductivity have attracted considerable attention owing to their many applications in solid oxide fuel cells, sensors, catalysts and batteries. The development of better electrolyte materials requires a better understanding of the mechanism of ionic conduction, and crucial to this is a comprehension of the crystal structure at high temperatures where the materials work efficiently. Here we review our recent works on the positional disorder and conduction path of oxide ions in Bi2O3, Bi1.4Yb0.6O3, CeO2, Ce0.93Yb0.07O1.96, and (La0.8Sr0.2)(Ga0.8Mg0.15Co0.05)O2.8 at high temperatures. We describe the conduction path of Li cations in Li-doped lanthanum titanate perovskite La0.62Li0.16TiO3 at room temperature. These were studied through the nuclear density distribution obtained by a combined technique including a Rietveld refinement and a maximum-entropy method (MEM)-based pattern fitting of the neutron-powder-diffraction data. Neutron-diffraction data were collected using the HERMES and HRPD at JRR-3M, JAEA. We found that the mobile ions in fluorite-type ionic conductors have a complicated disorder spreading over a wide area and shift to the <111> directions from the ideal fluorite site at high temperatures. We have demonstrated that the conduction path of oxide ions in the Lanthanum gallate-based compound was not along the straight line between the ideal positions, but exhibited an arc shape away from the B-site cation (Ga0.8Mg0.15Co0.05). At 77K, the Li cations in La0.62Li0.16TiO3 are located at the 2c site (Cmmm space group) on the (002) La deficient layer, while at room temperature, they are spread over a wide area and migrate following the 2c-4f-2c or 2c-2d-2c tie line on the (002) layer.