Oxygen Nonstoichiometry, Mixed Conductivity, and Mössbauer Spectra of Ln_0.5A_0.5FeO_3−δ (Ln = La−Sm, A = Sr, Ba): Effects of Cation Size

Abstract

Increasing the difference of the Ln 3+ and A 2+ cation radii in perovskite-type Ln 0.5 A 0.5 FeO 3−δ (Ln = La, Pr, Nd, Sm; A = Sr, Ba) results in higher oxygen deficiency and lower oxygen-ionic and p-type electronic conductivities, determined using the oxygen permeation and total conductivity measurements at 973−1223 K. The relationships between the anion transport and A-site cation size mismatch remain essentially similar in air and under reducing conditions when most iron cations become trivalent, thus confirming critical influence of oxygen-vacancy trapping processes induced by the lattice strain. At low temperatures, analogous correlation is also observed for quadrupole splittings derived from the Mössbauer spectra of oxygen-stoichiometric Ln 0.5 A 0.5 FeO 3 . Contrary to the ionic conductivity variations, the role of surface exchange kinetics as a permeation-limiting factor, evaluated from the membrane thickness dependence of oxygen fluxes, tends to decrease on Ba 2+ doping and on decreasing Ln 3+ size in Ln 0.5 Sr 0.5 FeO 3−δ series. The n-type electronic conduction and low-p(O 2 ) stability at 1223 K are substantially unaffected by the cation radius mismatch.