Oxygen Nonstoichiometry, Mixed Conductivity, and Mossbauer Spectra of Ln0.5A0.5FeO3-δ (Ln = La-Sm, A = Sr, Ba): Effects of Cation Size

Increasing the difference of the Ln(3+) and A(2+) cation radii in perovskite-type Ln(0.5)A(0.5)FeO(3-delta)) (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 Mossbauer 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 Ba2+ doping and on decreasing Ln(3+) size in Ln(0.5)Sr(0.5)FeO(3-delta) series. The n-type electronic conduction and low-p(O-2) stability at 1223 K are Substantially unaffected by the cation radius mismatch.