Framework 'interstitial' oxygen in La10(GeO4)5-(GeO5)O2 apatite electrolyte

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(GeO4)(6)O-3', it has been shown that this compound is more correctly described as an La-10(GeO4)(5)- GeO5) 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 GeO4 tetrahedra to GeO5 distorted trigonal bipyramids. This structural modification is consistent with known crystal chemistry and may occur generally in oxide apatites.