Thermochemistry of La _1-Sr _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-xSr _xFeO _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 ₂C 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 2Fe _Fe ^X + V _O ^•• + 0.5O ₂(g) = 2Fe _Fe ^• + O _O ^X is independent of oxygen nonstoichiometry in each La _1-xSr _xFeO _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. (c) 2005 American Chemical Society.