Thermochemistry of La_1-xSr_xFeO_3-δ Solid Solutions (0.0 ≤ x ≤ 1.0, 0.0 ≤ δ ≤ 0.5)

A series of compounds with the general formula La 1 - x Sr x FeO 3 - δ have been synthesized in the complete solid solution range 0.0 ≤ x ≤ 1.0 and 0.0 ≤ δ ≤ 0.5 with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B 2 O 3 at 702 °C was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction 2F + + 0.5O 2 (g) = 2F + is independent of oxygen nonstoichiometry in each La 1 - x Sr x FeO 3 - δ series with a given x, and further is approximately constant at −200 ± 50 kJ/mol O 2 for 0 < x ≤ 0.5 and −140 ± 30 kJ/mol O 2 for 0.5 < x < 1.0. The enthalpies of formation from oxides in the LaFeO 3 −SrFeO 2.5 solid solution can be fitted either by a quadratic equation or by two straight line segments intersecting at x = 0.5. The quadratic fit gives a positive interaction parameter, 36.1 ± 4.9 kJ/mol, suggesting a tendency toward phase separation. The two linear segment models two regions of oxygen vacancy presence: dilute (random distribution) for x ≤ 0.5 and concentrated (short-range ordering) for x > 0.5. Extrapolation to the end-member ( x = 1) gives the enthalpy of formation of perovskite-type SrFeO 2.5 and the enthalpy of the hypothetical brownmillerite−perovskite phase transition at room temperature is estimated as 5.5 ± 4.0 kJ/mol. This small value implies extensive short-range order in the perovskite phases with high x . The enthalpies of formation from oxides in the LaFeO 3 −SrFeO 3 solid solution are virtually independent of Sr composition when x ≤ 0.67 and are a few kJ/mol more exothermic when x ≥ 0.7. This is interpreted by phase evolutions from low symmetries to cubic.