Electrical, electrochemical, and thermomechanical properties of perovskite-type (La_1−x Sr _x )_1−y Mn_0.5Ti_0.5O_3−δ (x = 0.15–0.75, y = 0–0.05)

Abstract

Strontium additions in (La_1−x Sr _x )_1−y Mn_0.5Ti_0.5O_3−δ (x = 0.15–0.75, y = 0–0.05) 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 permeation limited by surface exchange increase with increasing x. The oxygen partial pressure dependencies of the conductivity 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 Mn³⁺ and Ti⁴⁺ states. Due to low oxygen nonstoichiometry essentially constant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti⁴⁺ cations and oxygen anions, the tracer diffusion coefficients measured by the ¹⁸O/¹⁶O 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 La_0.5Sr_0.5Mn_0.5Ti_0.5O_3−δ electrodes with Ce_0.8Gd_0.2O_2-δ interlayers, applied onto LaGaO₃-based solid electrolyte, are lower compared to (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ when no metallic current-collecting layers are introduced. However, the polarization resistance is still high, ~2 Ω × cm² in humidified 10 % H₂–90 % N₂ atmosphere at 1073 K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H₂S traces in H₂-containing gas mixtures did not result in detectable decomposition of the perovskite phases.