Neutron powder diffraction was used to investigate the detailed structure of the novel nanoporous oxide 12CaO center dot 7Al(2)O(3) (C12A7) over a wide temperature range. The position of the free oxygen ion captured in the cage structure of C12A7 was confirmed to be displaced from the center of the cage as well as from the S-4 axis running through the two calcium ions at the top and the bottom of the cage. The calcium ions were also found to move off of the S4 axis and shift toward the center of the cage when the free oxygen ion is present inside the cage. Examination of the structure revealed three types of aluminum-oxygen bonds which are characterized as short, medium and long. The distance between the free oxygen ion and the oxygen ion forming the longest bond with an aluminum ion becomes shorter as the temperature increases. suggesting promotion of exchange of the oxygen ions between the two sites. A possible migration path of the oxygen ion is examined from the obtained structural configuration. (C) 2008 Elsevier B.V. All rights reserved.