Evidence of local defects in the oxygen excess apatite La9.67(SiO4)6O2.5 from high resolution neutron powder diffraction

From neutron diffraction data collected at 3 K on a powder of La9.67(SiO4)6O2.5 composition and a careful examination of the average structure, a model was proposed to explain the oxygen over-stoichiometry in the apatite structure. This model leads to realistic distances to neighbouring atoms. Moreover, it accounts perfectly for the maximum oxygen content observed in these materials. Up to 0.5 oxygen atom located at the vicinity of the 2a site (0, 0, View the MathML source14) would be shifted to a new interstitial position in the channel at (−0.01, 0.04, 0.06), creating a Frenkel defect, with the possibility of a maximum occupancy in this site equal to twice the Frenkel defect numbers. This structural model is in good agreement with the oxygen diffusion pathways recently proposed by Bechade et al. (2009) using computer modeling techniques. It supports preferred oxygen diffusion pathways via interstitial oxygen atoms and vacant sites along [0 0 1], close to the centre of the La(2)-O channels.