Effect of the dopant nature on the conductivity of oxygen overstoichiometric oxyapatites with controlled microstructures

To characterise the effect of the dopant on the conduction of lanthanum silicate apatite materials, La9.67❒0.33Si6O26.5, La9Ba1Si6O26.5, La10Si5.5Mg0.5O26.5, La9.67❒0.33Si5.5Ge0.5O26.5 and La9.83❒0.17Si5.5Al0.5O26.5 ceramics with same oxygen content and similar microstructures were prepared. If one excepts the Mg-doped sample, whose conductivity at 700 °C was slightly higher (1.7 × 10− 2 S cm− 1), all compositions exhibit similar conductivities in the 1.1 × 10− 2 S cm− 1–1.3 × 10− 2 S cm− 1 range at 700 °C. This shows that, the nature of the dopant on the conduction properties has a minor effect and that the key parameter governing the performances of these materials is the oxygen excess content. The structure of all compounds was refined in the P63/m space group. At room temperature, longer La(2)–La(2) distances were observed for the Mg containing sample. The structure of La9Ba1Si6O26.5 was also refined at variable temperatures. Whereas the volume of SiO4 tetrahedrons remained almost constant when temperature increases, the volume of La(2)–O7 polyhedrons and the La(2)–La(2) distance increased. The La(2)–La(2) distance characterizes the size of the La(2) channels; higher conductivities are characterised by larger channels which further confirmed the key role of the oxide ions located in these channels in the conduction process.