Electrical, electrochemical, and thermomechanical properties of perovskite-type (La1−x Sr x )1−y Mn0.5Ti0.5O3−δ (x = 0.15–0.75, y = 0–0.05)

Abstract

Strontium additions in (La1−xSrx)1−yMn0.5Ti0.5O3−δ (x00.15-0.75, y00-0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeationlimited bysurface exchangeincrease with increas- ing x. The oxygen partial pressure dependencies of the con- ductivity and Seebeck coefficient studied at 973-1223 K in the p(O2) range from 10 −19 to 0.5 atm suggest a dominant role of electron hole hopping and relatively stable Mn 3+ and Ti 4+ states. Due to low oxygen nonstoichiometry essentially con- stant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti 4+ cations and oxygen anions, the tracer diffusion coefficients measured by the 18 O/ 16 O isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum-strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8- 15.0×10 −6 K −1 at 300-1370 K and oxygen pressures from 10 −21 to 0.21 atm. The anodic overpotentials of porous La0.5Sr0.5Mn0.5Ti0.5O3−δ electrodes with Ce0.8Gd0.2O2-δ inter- layers, applied ontoLaGaO3-basedsolid electrolyte, are lower compared to (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ when no metal- lic current-collecting layers are introduced. However, the po- larization resistance is still high, ~2 Ω×cm 2 in humidified 10 % H2-90 % N2 atmosphere at 1073 K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H2 St races in H2-containing gas mix- tures did not result in detectable decomposition of the perov- skite phases.