Interstitial oxide positions in oxygen-excess oxy-apatites

Several oxy-apatites materials, La8.65Sr1.35(Si6O24)O2.32, La8.65Sr1.35(Ge6O24)O2.32, La9Sr1(Si5.5Al0.5O24)O2.25, La9.67□0.33(Si5.5Al0.5O24)O2.25, La8.5Sr1.5(Si5.5Al0.5O24)O2 and La9.5□0.5(Si5.5Al0.5O24)O2 have been prepared as crystalline phases. The impedance study showed that all samples are oxide ion conductors and their conductivities are similar to those previously reported for related oxy-apatites. A thorough study on the oxygen sublattices for oxygen excess samples has been carried out using neutron powder diffraction data by the Rietveld method. The structural study shows the presence of interstitial oxide anions close to the periphery apatite channels. However, the interstitial oxide position in cation stoichiometric silicates, f. i. La8.65Sr1.35(Si6O24)O2.32, is different from that in germanates, f. i. La8.65Sr1.35(Ge6O24)O2.32. This is likely due to the different structural flexibility of the two tetrahedral groups and it explains the previously reported evidence of higher oxygen contents for germanates oxy-apatites. The structural characteristics of these oxide anion conductors are discussed.