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

Oxygen-stoichiometric La-9.33 rectangle(0.67)(Si6O24)O-2 and oxygen-excess La8.65Sr1.35(Ge6O24)O-2.32 and La8.65Sr1.35(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 rectangle(0.67)(Si6O24)O-2 and La8.65Sr1.35(Ge6O24)O-2.32 by time-of-flight neutron powder diffraction and at 4 K for La8.65Sr1.35(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 in. uence 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.