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| - Thermochemistry of La1-xSrxFeO3-δ Solid Solutions(0.0 ≤ x ≤ 1.0, 0.0 ≤ δ ≤ 0.5)
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| - The formation enthalpies in the solid solution LaFeO3−SrFeO2.5 can be fitted by a two linear segment model intersecting at x = 0.5. Oxygen vacancies are present in two regions: dilute (random distribution) for x ≤ 0.5 and concentrated (short-range ordering) for x> 0.5. Extrapolation to x = 1 gives the enthalpy of formation of perovskite-type SrFeO2.5 and the enthalpy of the phase transition from brownmillerite to perovskite at 298 K is estimated as 5.5 ± 4.0 kJ/mol.
- A series of compounds with the general formula La1-xSrxFeO3-δ have been synthesized in the completesolid solution range 0.0 ≤ x ≤ 1.0 and 0.0 ≤ δ ≤ 0.5 with a variety of heat treatments. High-temperaturedrop solution calorimetry in molten 2PbO·B2O3 at 702 °C was performed to determine their enthalpiesof formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction 2F++ 0.5O2(g) = 2F+is independent of oxygen nonstoichiometry in each La1-xSrxFeO3-δseries with a given x, and further is approximately constant at −200 ± 50 kJ/mol O2 for 0 < x ≤ 0.5 and−140 ± 30 kJ/mol O2 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 segmentsintersecting 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 oxygenvacancy presence: dilute (random distribution) for x ≤ 0.5 and concentrated (short-range ordering) forx> 0.5. Extrapolation to the end-member (x = 1) gives the enthalpy of formation of perovskite-typeSrFeO2.5 and the enthalpy of the hypothetical brownmillerite−perovskite phase transition at roomtemperature is estimated as 5.5 ± 4.0 kJ/mol. This small value implies extensive short-range order in theperovskite phases with high x. The enthalpies of formation from oxides in the LaFeO3−SrFeO3 solidsolution are virtually independent of Sr composition when x ≤ 0.67 and are a few kJ/mol more exothermicwhen x ≥ 0.7. This is interpreted by phase evolutions from low symmetries to cubic.
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