Interstitial oxygen in oxygen-stoichiometric apatites

Several oxy-apatite materials La10-xSrx( TO4)(6)O3-0.5x ( T = Ge, Si; (10-x) = 9.00, 8.80, 8.65 and 8.00) and La-9.33( Si1-xGexO4)(6)O-2( 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)(6)O-2. Time-of-flight neutron data have been collected for La-9.33(Si0.5Ge0.5O4)(6)O-2, La8Sr2(SiO4)(6)O-2 and La8Sr2(GeO4)(6)O-2. 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)(6)O-2 and La-9.33(Si0.5Ge0.5O4)(6)O-2 contain interstitial oxygen, associated to vacancies at the oxygen channels. The amount of interstitial oxygen is negligible in La8Sr2( SiO4)(6)O-2 and La8Sr2(GeO4)(6)O-2. 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.