Oxygen ionic and electronic conductivity of La0.3Sr0.7Fe(Al)O3-δ perovskites

Substitution of iron with aluminum in perovskite-type La0.3Sr0.7Fe1-chiAlchiO3-delta (x = 0-0.4) decreases the unit cell volume, thermal expansion, and partial oxygen ionic and electronic conductivities. At oxygen partial pressures from 10(-10)-10(-8) to 0.5 atm, the total conductivity is predominantly p-type electronic. Temperature-activated character and relatively low values of hole mobility, estimated from the total conductivity and Seebeck coefficient data, suggest a small polaron mechanism. The oxygen ion transference numbers of La0.3Sr0.7Fe1-chiAlchiO3-delta (x = 0.2-0.4) in air, determined by the faradaic efficiency measurements, vary from 7 x 10(-4) to 1 x 10(-2) at 1023-1223 K, increasing with temperature and aluminum content. As for other (La,Sr)FeO3-based phases, reducing oxygen pressure below 10(-14)-10(-10) atm results in dominating ionic and n-type electronic contributions to the total conductivity. The activation energies for ionic transport in La0.3Sr0.7(Fe,Al)O3-delta, 95-102 kJ mol(-1), are essentially independent on the cation composition and oxygen chemical potential, while the ionic conductivity may slightly increase on reducing P(02). The level of oxygen ionic conduction in Al-containing ferrites was found lower than in Ga-substituted analogues. The average thermal expansion coefficients of La0.3Sr0.7Fe1-chiAlchiO3-delta ceramics, calculated from dilatometric data in air, are the in range (12.1-12.9) x 10(-6) K-1 at 350-700 K and increase up to (23.2-27.4) x 10(-6) K-1 at 700-1300 K. (C) 2004 Elsevier SAS. All rights reserved.