Crystal structures, structural disorders and diffusion paths of ionic conductors from diffraction experiments

The present paper is a review of our recent works on the crystal structures, structural disorders and diffusion paths of mobile ions in various bulk ionic and mixed conductors, which was presented in the Keynote Lecture of SSI-16 international conference. The reported studies were conducted based on the nuclear/electron density distributions obtained by a combined technique including Rietveld refinement, the maximum entropy method (MEM) and the MEM-based pattern fitting (MPF) of the neutron/synchrotron powder diffraction data measured at high temperatures. Diffusion paths along the <100> directions are observed in various ionic conductors with fluorite-type structures such as δ-Bi1.4Yb0.6O3, Ce0.93Y0.07O1.96, Y0.785Ta0.215O1.715 and α-CuI. The diffusion paths of oxide ions in the cubic perovskite-type (La0.8Sr0.2)(Ga0.8 Mg0.15Co0.05)O3 − δ exhibit an arc shape away from the (Ga0.8Mg0.15Co0.05) cation, forming a three-dimensional network of curved diffusion paths. Double perovskite-type La0.64(Ti0.92Nb0.08)O2.99 shows a similar diffusion path near the (004) planes, forming a two-dimensional network of curved diffusion paths. The curved feature of the diffusion paths is observed in various oxide-ion conductors such as δ-Bi1.4Yb0.6O3, (La0.8Sr0.2)(Ga0.8 Mg0.15Co0.05)O3 − δ and La0.64(Ti0.92Nb0.08)O2.99, as well as in the copper-cation conductor α-CuI and lithium-cation conductor La0.62Li0.16TiO3.