Formation Enthalpies of Tetravalent Lanthanide Perovskites by High Temperature Oxide Melt Solution Calorimetry
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Formation enthalpies of Tetravalent Lanthanide Perovskites by High Temperature Oxide Melt Solution Calorimetry S. V. Ushakov, J. Cheng, A. Navrotsky, J. R. Wu1, S. M. Haile1 Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA 1 Materials Science Department, California Institute of Technology, Pasadena, CA 91125, USA ABSTRACT High-temperature oxide melt solution calorimetry was used to measure formation enthalpies for several compositions of perovskites of nominal stoichiometry BaPrO3 and BaCeO3. Samples were synthesized from chemical solution methods followed by calcination at 1100-1300 °C. PrO2 was synthesized by oxidation of Pr6O11 in an oxygen flow at 280 °C. The samples were characterized by microprobe, thermogravimetric and differential thermal analyses. Cell parameters were refined by the Rietveld method. Barium excess in the samples with respect to ideal stoichiometry was detected. Drop solution enthalpies were measured in a Calvet type twin microcalorimeter, using 3Na2O·4MoO3 solvent at 702 °C. Preliminary values of the formation enthalpy of BaPrO3 and BaCeO3 from oxides were –70 ±10 kJ/mol and –51 ±10 kJ/mol, respectively. They fall on the normal trend of energetics versus Goldschmidt tolerance factor and do not show any special stabilization of BaPrO3 relative to other MLnO3 perovskites. INTRODUCTION Tetravalent lanthanide perovskites, MLnO3, where M = Ba, Sr and Ln = Ce, Pr, Tb represent an unusual case of lanthanide stabilization in octahedral coordination. These materials are of interest due to their ionic and proton conducting properties. While enthalpies of formation of MLnO3 from oxides follow the usual trend for perovskites, namely decreasing stability with increasing structural distortion [1], BaPrO3 was reported to be anomalously stable [2]. The thermochemistry of barium cerate was studied much more extensively, but reported data on formation enthalpy scatter from positive values [3] to –53 kJ/mol [4]. The aim of present work is to obtain formation enthalpies from oxides for BaPrO3 and BaCeO3 using high-temperature oxide melt solution calorimetry [5]. EXPERIMENTAL Two batches of BaCeO3 and BaPrO3 were synthesized, characterized, and used for hightemperature oxide solution calorimetry measurements. In order to close the cycle for BaPrO3 in an unambiguous fashion, in addition to measurements on perovskites, we synthesized, characterized and performed calorimetric measurements on PrO2. Synthesis BaCeO3 and BaPrO3 were synthesized by a modified Pechini (MP) process [6]. The precursors were Ba(NO3)2, Ce(NO3)3*6H2O and Pr(NO3)3 xH2O (x was determined by thermogravimetric analysis to be 5.6). EDTA (ethylene diamine tetraacetic acid) and EG (ethylene glycol), were used as polymerization/complexation agents, with the molar ratios EDTA/Metal=2.0, and EDTA/EG=1/3. Powders were calcined for 10 hours at 1000 – 1200 °C. Direct Pr6O11 oxidation into PrO2 was earlier reported at 500 °C under high oxygen pressure [7], but recen
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