A high temperature powder neutron diffraction structural study of the apatite-type oxide ion conductor, La9.67Si6O26.5

High-resolution neutron powder diffraction studies of the oxide ion conductor La9.67Si6O26.5 are reported for temperatures ranging between 25 and 900 degrees C. The best fit to the data was obtained for space group P6(3) and there was no evidence for any change in symmetry over the temperature range studied. Interstitial oxide ions are identified lying in sites similar to those predicted by previous computer modelling studies, and in agreement with structural studies on related materials. Furthermore, occupancy of these sites is enhanced by Frenkel-type disorder from neighbouring positions. The results thus add further weight to the interpretation that, in these apatite-type systems, the silicate substructure is important for the accommodation of interstitial oxide ions and their migration. (C) 2008 Elsevier Ltd. All rights reserved.