Oxygen Non-Stoichiometry and Electrical Conductivity of La_0.2Sr_0.8Fe_0.8B_0.2O_{3 − δ}, B = Fe, Ti, Ta

The oxygen non-stoichiometry was determined by coulometric titration for the perovskite oxides La_0.2Sr_0.8FeO_{3 − δ} and La_0.2Sr_0.8Fe_0.8B_0.2O_{3 − δ} (B = Ti⁴⁺ and Ta⁵⁺) in the temperature range 600 °C ⩽ T ⩽ 900 °C and the oxygen partial pressure range: 1⋅10^-15≤po₂≤0.209 atm. The non-stoichiometry (δ) is observed to decrease with B-site substitution of Fe. The data can be well fitted with simple defect chemistry models. At low oxygen non-stoichiometry all compositions show a deviation from a localized electrons defect model. The standard and partial molar thermodynamic quantities were obtained and a gradual transition from localized to itinerant electrons with decreasing non-stoichiometry is proposed from the δ-dependency of the configurational entropy. The absolute value of the enthalpy of oxidation decreases upon B-site substitution of Fe proposing a decreased thermodynamic stability for the substituted materials. The electrical conductivity was measured at T = 900 °C in the oxygen partial pressure range: 1⋅10^-17≤po₂≤0.209 atm. The electrical conductivity and charge carrier mobility decrease upon 20% substitution of Fe roughly by a factor of 2, but do not show a significant dependence on the nature of the B-site dopant.