Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen

Abstract

Oxygen-stoichiometric La_9.33□_0.67(Si₆O₂₄)O₂ and oxygen-excess La_8.65Sr_1.35(Ge₆O₂₄)O_2.32 and La_8.65Sr_1.35(Si₆O₂₄)O_2.32 oxy-apatites have been structurally characterized at low temperatures by the Rietveld method. Oxygen-interstitial distribution has been studied at 15 K for La_9.33□_0.67(Si₆O₂₄)O₂ and La_8.65Sr_1.35(Ge₆O₂₄)O_2.32 by time-of-flight neutron powder diffraction and at 4 K for La_8.65Sr_1.35(Si₆O₂₄)O_2.32 by constant-wavelength neutron powder diffraction. The low temperature structural study was undertaken in order to distinguish between the effects of static disorder, originated mainly from the presence of interstitial oxygens, and the anisotropic thermal vibrations. At such low temperatures, the influence of the anisotropic thermal vibrations is minimised. This structural study has firmly established the existence of interstitial oxygens in these materials, which may be useful as electrolytes for solid oxide fuel cells.