Ion transport properties and Seebeck coefficient of Fe-doped La(Sr)Al(Mg)O3−δ

Iron incorporation into lanthanum aluminate-based solid electrolyte leads to increasing oxygen ionic and p-type electronic conductivities. The oxygen ion transference numbers of perovskite-type La0.90Sr0.10Al0.85−xFexMg0.15O3−δ (x=0.2-0.4), determined by faradaic efficiency measurements in air, vary from 6.9×10−4 to 1.6×10−2 at 1073-1223 K, increasing with temperature and decreasing when iron content increases. In order to study the behavior of transition metal cations dissolved in a lattice of cations with stable oxidation state, the total conductivity and Seebeck coefficient of La0.90Sr0.10Al0.65Fe0.20Mg0.15O3−δ were measured at 1073-1223 K in the oxygen partial pressure range from 10−20 to 0.5 atm. The p-type electronic conduction dominating under oxidizing conditions, with hole mobility activation energy of 15 kJ mol−1, occurs via a small-polaron mechanism. Reducing oxygen pressure results in increasing ionic conduction, which becomes predominant at p(O2) lower than 10−10 atm. The activation energy for ionic transport, 87-107 kJ mol−1, decreases with reducing p(O2) due to a decreasing contribution of the vacancy formation enthalpy. Contrary to the Fe-substituted aluminates, dense ceramics of La(Sr)Al(Mg,M)O3−δ (M = Co, Ni) cannot be obtained under atmospheric oxygen pressure due to La2MO4-based phase segregation in the course of sintering at temperatures above 1400 K.