Thermochemistry of La1-xSrxFeO3-δ solid solutions (0.0 ≤ x ≤ 1.0, 0.0 ≤ δ ≤ 0.5)

A series of compounds with the general formula La1-xSrxFeO3-delta have been synthesized in the complete solid solution range 0.0 <= x <= 1.0 and 0.0 <= delta <= 0.5 with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO(.)B(2)O(3) at 702 degrees C was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction 2Fe(Fe)(x) + V-O(center dot center dot) + 0.5O(2)(g) = 2Fe(Fe)(center dot) + O-O(X) is independent of oxygen nonstoichiometry in each La1-xSrxFeO3-delta 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 LaFeO3- SrFeO2.5 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 SrFeO2.5 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 LaFeO3-SrFeO3 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.