Interstitial oxygen in oxygen-stoichiometric apatites

Abstract

Several oxy-apatite materials La_10−xSr_x(TO₄)₆O_3−0.5x (T = Ge, Si; 10−x = 9.00, 8.80, 8.65 and 8.00) and La_9.33(Si_1−xGe_xO₄)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 La_9.33(SiO₄)₆O₂. Time-of-flight neutron data have been collected for La_9.33(Si_0.5Ge_0.5O₄)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 La_9.33(SiO₄)₆O₂ and La_9.33(Si_0.5Ge_0.5O₄)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.