Enthalpies of formation of LaMO 3 perovskites (M = Cr, Fe, Co, and Ni)
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Xiao-Dong Zhou and Harlan U. Anderson Electronic Materials Applied Research Center, University of Missouri–Rolla, Rolla, Missouri 65401 (Received 17 June 2004; accepted 12 October 2004)
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 LaMO3 (M ⳱ Cr, Fe, Co, and Ni). The enthalpies of formation of LaCrO3 and LaFeO3 from oxides (La2O3 and Cr2O3 or Fe2O3) are –70.06 ± 2.79 kJ/mol and –64.58 ± 2.32 kJ/mol, respectively. The enthalpies of formation of LaCoO3 and LaNiO3 from oxides (La2O3 and CoO or NiO) and O2 are −107.64 ± 1.77 kJ/mol and –57.31 ± 2.55 kJ/mol, respectively. All these data are evaluated and found to be consistent with literature values obtained using other methods. The relative stability among these four perovskites decreases in the order of Cr, Fe, Co, Ni.
I. INTRODUCTION
Solid oxide fuel cells (SOFCs) are a promising power generation device because of their high energyconversion efficiency, good reliability, and low pollution.1 A single cell essentially consists of two porous electrodes (the anode and cathode) separated by a dense, oxygen-ion conducting electrolyte. To reach a workable power output, cells are stacked via another dense component, the interconnect, which serves as an electrical contact. Perovskite-type oxides (ABO3) find a wide range of applications as different component materials in SOFCs by engineering doping schemes on A and/or B sites. Doping results in defects (e.g., oxygen vacancies, electron holes, or a combination of both) that enable tuning of electrical properties at high temperature. An ideal solid oxide electrolyte should possess high and exclusive ionic conductivity and be stable in both oxidizing and reducing atmospheres. Alkaline earth doped perovskite-type LaAlO3,2 LaGaO3,3 LaScO3,4 and LaInO35 have been extensively studied as alternatives to replace the commercialized yttria-stabilized zirconia (YSZ) with a fluorite-type structure. Charge imbalance after doping is virtually all compensated by oxygen vacancies. The stringent requirements for an interconnect are very similar to the electrolyte except the need is for high
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0018 J. Mater. Res., Vol. 20, No. 1, Jan 2005
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and pure electronic conductivity. LaCrO3 is one of the few electronically conductive yet refractory ceramic materials.1 It is a p-type semiconductor due to holes in the 3d band of the chromium ions. Calcium, strontium, and magnesium doped LaCrO3 are most commonly used for the interconnect because of their much improved electronic conductivity.6 Doping introduces electronic holes on chromium sites to form Cr4+ and enhances the electronic conduction through a small polaron hopping mechanism between Cr4+ and Cr3+.7 A good cathode material needs to be an excellent mixed ionic and electronic conductor (MIEC), which m
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