Enthalpies of formation of LaBO 3 perovskites (B = Al, Ga, Sc, and In)
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Enthalpies of formation from constituent oxides and elements at 298 K were determined by high-temperature oxide melt solution calorimetry for a group of technologically important perovskites LaBO3 (B ⳱ La, Ga, Sc, and In). Enthalpies of formation from oxides of LaAlO3 and LaGaO3 are −69.61 ± 3.23 kJ/mol and −52.39 ± 1.99 kJ/mol, respectively. The data were consistent with literature values obtained using other methods. The enthalpies of formation of LaScO3 and LaInO3 from oxides were reported for the first time as −38.64 ± 2.30 kJ/mol and −23.99 ± 2.31 kJ/mol, respectively. As seen for other perovskites, as the tolerance factor deviates more from unity (in the order Al, Ga, Sc, In), the enthalpy of formation from oxides becomes less exothermic, indicating a less stable structure with respect to the constituent oxides.
I. INTRODUCTION
LaBO3 phases (B ⳱ Al, Ga, Sc, and In) with the perovskite structure represent a group of excellent parent materials for electrolyte and cathode candidates in solid oxide fuel cells (SOFCs). Sr- and Mg-doped LaGaO3 has been studied as one of the most promising electrolytes to replace the currently used yttria-stabilized zirconia (YSZ) because of its high and exclusive ionic conductivity at intermediate temperatures (973–1073 K) and excellent stability in both reducing and oxidizing atmospheres.1,2 LaAlO3 doped with alkaline earth cations on the La site (Ca or Sr) and/or on the Al site (Mg) is an oxygen ion conductor at low oxygen partial pressures and a mixed ionic and p-type electronic conductor at high oxygen partial pressures.3–5 The electrical behavior of alkaline-earth-doped LaScO3 is similar to that of the corresponding aluminates,6 but the ionic conductivity consists of oxygen ion and proton conduction.7–9 Sr-doped LaInO3 shows dominant p-type, ionic, and n-type conduction when at high, intermediate, and low oxygen partial pressures, respectively.10,11 This variation suggests a conceptually new SOFC, namely, a single-layer SOFC (SLFC), which uses only one material to construct the electrolyte, cathode, and anode simultaneously.10 Thermodynamic data are of critical importance to evaluate and predict the stability and compatibility of components at fuel cell operating temperatures. Unfortunately, relatively little attention has been given to the a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 10, Oct 2003
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thermodynamic properties of these perovskites. Hightemperature oxide melt solution calorimetry provides a unique tool for studying the energetics of refractory inorganic materials, especially those insoluble in aqueous acids.12,13 In the current work, we report the enthalpies of formation of LaBO3 perovskites (B ⳱ Al, Ga, Sc, and In) and compare the new thermodynamic data with those available in the literature. We discuss the variation of energetics of LaBO3 with structural parameters.
II. EXPERIMENTAL A. Sample preparation and characterization
All samples were p
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