Abstract
The oxide ion conduction mechanism was clarified for high purity RE 9.33(SiO 4) 6O 2 and Sr 2RE 8(SiO 4) 6O 2 (RE = La and Nd) by Rietveld and maximum entropy method (MEM) analysis using neutron powder diffraction data collected at room temperature. All the compounds had an apatite-type structure in the space group P6 3/m. Neither site splitting nor interstitial sites of the oxide ion was observed. RE 9.33(SiO 4) 6O 2 had cation vacancies only at the 4 f site. In Sr 2RE 8(SiO 4) 6O 2, the 4 f sites were fully occupied by strontium and rare earths with a molar ratio of 1 : 1. Also, in RE 9.33(SiO 4) 6O 2, the oxide ion at the hexagonal channel site had a large displacement along the c-axis. This large displacement is induced by cooperative rotation of SiO 4 tetrahedra around rare earths at the 4 f site through oxide ion polyhedra around another rare earth at the 6 h site. The displacement, enhanced by a vacancy in the 4 f site, is directly related to oxide ion conduction in RE 9.33(SiO 4) 6O 2.
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