Stability and mixed ionic–electronic conductivity of (Sr,La)(Ti,Fe)O3−δ perovskites

Doping of cation-stoichiometric and A-site-deficient perovskites LaxSr1−xTi1−yFeyO3−δ and LaxSr0.90−xTi0.60Fe0.40O3−δ (0≤x≤0.45, 0.40≤y≤0.80) with lanthanum leads to a greater stability with respect to reduction in H2-containing atmospheres and the reaction with yttria-stabilised zirconia (YSZ). Partial oxygen ionic and p-type electronic conductivities in air as well as thermal expansion decrease with lanthanum content, whilst the effect on the n-type conduction in reducing environments is opposite. The ion transference numbers of (Sr,La)(Ti,Fe)O3−δ, determined by faradaic efficiency measurements in air at 800–1000 °C, vary from 1×10−4 to 4×10−2, increasing when temperature increases. Deficiency of the A sublattice and increasing iron concentration in the B sublattice result in a higher thermal expansion and greater reducibility; the reactivity with YSZ can be suppressed by the creation of A-site vacancies. The ionic and p-type electronic conductivities were both found to decrease with A-site deficiency and to increase with Fe content. The average thermal expansion coefficients of (Sr,La)(Ti,Fe)O3−δ in air at 100–850 °C are in the range (10.6–21.5)×10−6 K−1.