We review our recent works on the positional disorder and diffusion path of oxide ions in Bi
2O
3, CeO
2 and (La
0.8Sr
0.2) (Ga
0.8Mg
0.15Co
0.05) O
3-δ. These were studied through the nuclear density distribution obtained by a combined technique including a Rietveld refinement, a maximumentropy method (MEM) and MEM-based pattern fitting of the neutron powder diffraction data measured at high temperatures. Oxide ions in fluorite-type structured δ-Bi
2O
3 and CeO
2 have a complicated disorder spreading over a wide area and shift to the ‹111› directions from the ideal fluorite site at higher temperatures. Diffusion path of oxide ions in (La
0.8Sr
0.2) (Ga
0.8Mg
0.15Co
0.05) O
3-δ perovskite is not along the straight line between the ideal positions, but exhibits an arc shape away from the B-site cation (Ga
0.8Mg
0.15Co
0.05) .
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