Mixed conductivity, Mössbauer spectra and thermal expansion of (La,Sr)(Fe,Ni)O3 − δ perovskites

The incorporation of Sr2+ in perovskite-type La1 − ySryFe1 − xNixO3 − δ increases oxygen deficiency and ionic transport at elevated temperatures, but leads to a lower thermodynamic stability as reflected by narrowing the solid-solution domain at 1373 K down to x ≈ 0.25 at y = 0.10 and x ≈ 0.12 at y = 0.20 in air. The average thermal expansion coefficients of La1 − ySryFe1 − xNixO3 − δ (x = 0.1–0.4, y = 0.1–0.2) ceramics vary in the ranges (12.4–13.4) × 10− 6 K− 1 at 700–1150 K and (14.2–18.0) × 10− 6 K− 1 at 1150–1370 K, rising with strontium and nickel contents. These additives lower also the temperature of orthorhombic → rhombohedral phase transition visible in the dilatometric curves, promote hole delocalization, increase total conductivity and decrease Seebeck coefficient studied in the oxygen partial pressure range from 10− 5 to 0.4 atm at 973–1223 K. The steady-state oxygen permeation fluxes through single-phase La1 − ySryFe1 − xNixO3 − δ (x = 0.1–0.2, y = 0.1–0.2) membranes, with an activation energy of 206–235 kJ/mol, are determined by both surface-exchange and bulk ionic conduction limited by the oxygen-vacancy concentration. Doping with nickel has a weak negative effect on the oxygen transport, probably due to defect cluster formation involving the vacancies and Ni2+. On the contrary, at low temperatures when (La,Sr)(Fe,Ni)O3 − δ becomes almost oxygen-stoichiometric in air, the Mössbauer spectroscopy and thermogravimetric analysis showed that the average oxidation state of nickel is higher than + 3, similar to iron cations.