Vibrational dynamics and oxygen diffusion in a nanoporous oxide ion conductor 12CaO·7Al₂O₃ studied by¹⁸O labeling and micro-Raman spectroscopy

Nanoporous oxide 12CaO·7Al₂O₃ (C12A7) is a fast oxide-ion conductor that may encage various types of anionic oxygen species such as O^2-, O₂^-, O₂^2-, and O^- in its crystallographic cages. The C12A7 single crystals thermally annealed in ¹⁸O₂ were examined by micro-Raman spectroscopy. The symmetric properties and the origin of Raman bands were analyzed by the isotope shift and the depolarization on the bases of the crystal symmetry of C12A7, and the Raman band due to the stretching mode of extraframework O₂^2- was identified. Spatially resolved measurements of the isotope shift of the Raman bands in a partially ¹⁸O-substituted sample indicated that the local fraction of ¹⁸O is equal between the framework and extraframework sites, demonstrating that the substitution of O^2- ions in the C12A7 framework is as fast as the formation of extraframework oxygen species by thermal oxygen loading. It strongly suggests that the extraframework oxygen species diffuse by replacing oxygens in the framework sites, rather than simply permeating through intercage openings.