Nonstoichiometry and defect structure of the perovskite-type oxides La1−xSrxFeO3−°

In order to elucidate the defect structure of the perovskite-type oxide solid solution La_1?xSr_xFeO_3?δ (x = 0.0, 0.1, 0.25, 0.4, and 0.6), the nonstoichiometry, δ, was measured as a function of oxygen partial pressure, P_O2, at temperatures up to 1200°C by means of the thermogravimetric method. Below 200°C and in an atmosphere of P_O2 ≥ 0.13 atm, δ in La_1?xSr_xFeO_3?δ was found to be close to 0. With decreasing log P_O2, δ increased and asymptotically reached $\text{x}\text{2}$. The value corresponding to $\text{δ =}\text{x}\text{2}$ was about ?10 at 1000°C. With further decrease in log P_O2, δ slightly increased. For LaFeO_3?δ, the observed δ values were as small as <0.015. It was found that the relation between δ and log P_O2 is interpreted on the basis of the defect equilibrium among Sr′_La (or V?_La for the case of LaFeO_3?δ), V^··_O, Fe′_Fe, and Fe^·_Fe. Calculations were made for the equilibrium constants K_ox of the reaction

$\text{1}\text{2}\text{O}{}_2\text{(g) + V}{}^{\mathrm{··}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= O}{}^{\mathrm{x}}{}_{\mathrm{o}}\text{+ 2Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe}}$

and K_i for the reaction

$\text{2Fe}{}^{\mathrm{x}}{}_{\mathrm{Fe}}\text{= Fe}{}^{\mathrm{\prime }}{}_{\mathrm{Fe}}\text{+ Fe}{}^{\mathrm{·}}{}_{\mathrm{Fe·}}$

Using these constants, the defect concentrations were calculated as functions of P_O2, temperature, and composition x. The present results are discussed with respect to previously reported results of conductivity measurements.