This thesis presents relationships between ionic conductivity and crystal structure of hexagonal perovskite revealed from a new perspective: order and disorder. It describes three state-of-art research results in the new area of structure of ionic conductors, hexagonal perovskite-related structures, based on a new viewpoint of structural order/disorder. The first is occupational disorder of oxygen atoms and ionic conductivity in Ba3MNbO8.5 (M = Mo, W) investigated by high-temperature neutron diffraction and the maximum-entropy method. Second is the hidden chemical order in disordered Ba7Nb4MoO20·0.15H2O that was discovered by a combined technique of resonant X-ray diffraction and solid-state nuclear magnetic resonance. This combined technique was named as RXRD/NMR method. The third is the discovery of a rubidium-containing oxide-ion conductor with large positional disorder of oxygen atoms. This volume gives readers two outlooks. One is proposed guidelines for the search and development of ionic conductors with hexagonal perovskite-related structures. Another is future application of analyses for order/disorder used in this study, especially the RXRD/NMR method.

Yuta Yasui obtained his B.Sc. (2018), M. Sc. (2020) and D. Sc. (2023) from the Tokyo Institute of Technology under Professor Masatomo Yashima. He won the Poster Award in the 13th Pacific Rim Conference of Ceramic Societies (PACRIM13). He specialized in advanced techniques for crystal structure analyses such as maximum-entropy method and resonant X-ray diffraction.