The present brief review describes the application of neutron powder diffractometry and maximum-entropy method to the studies of crystal structure and diffusional pathways of mobile ions in ionic conducting ceramic materials. La
0.62Li
0.16TiO
3 and Li
0.6FePO
4 exhibit two- and one-dimensional networks of Li cation diffusional pathways, respectively. In the fluorite-structured ionic conductors such as ceria solid solution Ce
0.93Y
0.07O
1.96, bismuth oxide solid solution δ-Bi
1.4Yb
0.6O
3 and copper iodide CuI, a similar curved diffusion pathway along the ‹100› directions is observed. In the cubic
ABO
3 perovskite-type ionic conductor, lanthanum gallate solid solution, the mobile ions diffuse along a curved line keeping the interatomic distance between the
B cation and O
2- anion. We have experimentally confirmed that the anisotropic thermal motions of the apex O2 atom and the interstitial O3 atoms are essential for the high oxygen permeability of the K
2NiF
4-type mixed conductor. Diffusion paths of proton are visualized along
c axis in hexagonal hydroxyapatite.
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