Interstitial oxygen in oxygen-stoichiometric apatites

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Abstract
Several oxy-apatite materials La10−xSrx(TO4)6O3−0.5x (T = Ge, Si; 10−x = 9.00, 8.80, 8.65 and 8.00) and La9.33(Si1−xGexO4)6O2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La9.33(SiO4)6O2. Time-of-flight neutron data have been collected for La9.33(Si0.5Ge0.5O4)6O2, La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. The room-temperature structures have been derived from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data. High temperature structures have been obtained only from Rietveld refinements of neutron powder diffraction data. The refinements show that La9.33(SiO4)6O2 and La9.33(Si0.5Ge0.5O4)6O2 contain interstitial oxygen, associated to vacancies at the oxygen channels. The amount of interstitial oxygen is negligible in La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. Hence, the novelty of this work is to explain the high oxide conductivity of the lanthanum-deficient samples which it is due to the presence of interstitial oxygens. Lanthanum stoichiometric samples do not have interstitial oxygens and, so, their conductivities are much lower.