title: Framework 'interstitial' oxygen in La(10)(GeO(4))(5-)(GeO(5))O(2) apatite electrolyte.
abstract: Oxygen conduction at low temperatures in apatites make these materials potentially useful as electrolytes in solid-oxide fuel cells, but our understanding of the defect structures enabling ion migration is incomplete. While conduction along [001] channels is dominant, considerable inter-tunnel mobility has been recognized. Using neutron powder diffraction of stoichiometric 'La(10)(GeO(4))(6)O(3)', it has been shown that this compound is more correctly described as an La(10)(GeO(4))(5-)(GeO(5))O(2) apatite, in which high concentrations of interstitial oxygen reside within the channel walls. It is suggested that these framework interstitial O atoms provide a reservoir of ions that can migrate into the conducting channels of apatite, via a mechanism of inter-tunnel oxygen diffusion that transiently converts GeO(4) tetrahedra to GeO(5) distorted trigonal bipyramids. This structural modification is consistent with known crystal chemistry and may occur generally in oxide apatites.
authors.affiliation: Nanyang Technological University, School of Materials Science and Engineering, 50 Nanyang Avenue, 639798 Singapore.
authors.forename: Stevin S
authors.initials: SS
authors.lastname: Pramana
authors.forename: Wim T
authors.initials: WT
authors.lastname: Klooster
authors.forename: T J
authors.initials: TJ
authors.lastname: White