Structural and electrical characterization of Sr- and Al- doped apatite type lanthanum silicates prepared by the pechini method

The subject of this work regards the synthesis of La9.83−xSrxSi6−yAlyO26+δ (0≤x,y≤0.5) via a modified Pechini method. The compounds where characterized by XRD and SEM. Pure La9.83Si6O26+δ, La9.38Sr0.45Si6O26+δ and La9.38Sr0.45Si5.70Al0.30O26+δ were prepared after sintering at 1400 °C for 20 h while La9.38Sr0.45Si5.55Al0.45O26+δ and La9.38Sr0.45Si5.50Al0.50O26+δ contained traces (<8%) of La2SiO5 as secondary phase. Rietveld analysis showed that La9.83−xSrxSi6−yAlyO26+δ (0≤x,y≤0.5) compounds crystallize in the P63/m space group. Al doping on Si site exhibits more pronounced effect upon structural parameters in comparison to Sr doping on La site. Interstitial oxygen accommodates a position at the periphery of the hexagonal channels in the vicinity of the SiO4 groups. Ion conduction is close related with the size of the hexagonal channels and the interstitial oxygen content. The ion conductivity is promoted when an optimum balance between the aforementioned magnitudes is reached. The LS and LsSa4530 compounds exhibit the highest values of ionic conductivity at 700 °C with 11 and 14 mS/cm and activation energy of 0.47 and 0.46 eV, respectively.