Defect equilibrium and charge transport in R0.25Sr0.75FeO3−δ (R = La, Nd, Y, Ho): The effect of cation type

Abstract

The perovskite-type oxides of R0.25Sr0.75FeO3−δ series for R = La, Nd, Y, Ho were subjected to X-ray and electron diffraction studies, thermogravimetric and dilatometric measurements. Coulometric titration and four-probe DC techniques were used to obtain data on oxygen content and electrical conductivity of the oxides in the range of oxygen partial pressure from 10−19 to 0.5 atm at 750–950 °C. A decrease in the size of R-cation in R0.25Sr0.75FeO3−δ was found to result in a decrease in the oxygen content in the oxides, thermal expansion, and electrical conductivity. In case of Y and Ho, an extra factor, in addition to the cation radius, affecting the oxide properties, is argued to be the limited oxygen coordination, which promotes local ordering in the respective oxides. This statement was confirmed by electron diffraction study, which detected incommensurate diffuse scattering as the evidence of partial local ordering of cations in the A sublattice in combination with the ordering of oxygen vacancies. This partial local ordering affects strongly functional properties of Y and Ho containing oxides. In particular, oxygen-ion conductivity at 950 °C decreases from 0.328 ± 0.005 to 0.055 ± 0.003 S/cm in response to the substitution of La in R0.25Sr0.75FeO3−δ by Y.