A series of compounds with the general formula La1-xSrxFeO3-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 drops solution calorimetry in molten 2PbO·B2O3 at 702 °C was performed to determine their enthalpies of formation from oxides at room temperature. the enthalpy of oxidation involved in the reaction 2FeXFe + V O•• + 0.5O 2(g) = 2Fe Fe + O OX is independent of oxygen nonstoichiometry in each La1-xSrxFeO3-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 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. (c) 2005 American Chemical Society.


Materials Science and Engineering

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19 Apr 2005