Synthesis and Conductivities of the Apatite-type Systems, La9.33+xSi6-yMyO26+z (M = Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)(10-x)Si6O26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La9.33Si6O26 with Co, Fe, Mn on the Si site, according to the formula La9.33+x/3Si6-xMxO26 (M = Co, Fe, Mn). Substitution limits observed were x <= 1.5 (Co), x <= 1.25 (Fe), x <= 0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La8Mn2Si6O26. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O-2) (0.2-10(-5) atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O-2) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O-26 appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La9.33Si6O26 with potential SOFC cathode materials (La1-xSrxMO3; M = Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface.