Thermochemistry of lanthanum zirconate pyrochlore
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A thermodynamic study was carried out to resolve discrepancies in the enthalpy of formation and related parameters for lanthanum zirconate pyrochlore. The homogeneity field for single phase pyrochlore formation was determined to be 33–35 mol% La2O3 at 1500 C. High-temperature oxide melt drop solution calorimetry was performed in sodium molybdate and lead borate solvents on three compositions ranging from La1.98Zr2.01O7 to La2.07Zr1.95O7. The enthalpy of formation from oxides at 25 C, DH0f,ox, for stoichiometric lanthanum zirconate pyrochlore is 107.3 5.1 kJ/mol, and the standard enthalpy of formation from elements, DH0f,el, is 4102.2 6.0 kJ/mol. La2Zr2O7 pyrochlore was found by differential thermal analysis to be stable up to its melting point. The melting point and the fusion enthalpy of La2Zr2O7 pyrochlore were measured as 2295 10 C and 350 kJ/mol, respectively. I. INTRODUCTION
Rare earth zirconates are developed as solid oxide fuel cell (SOFC) electrode materials,1 fluorescence screens,2 thermal-barrier coatings,3 high-temperature heating elements, and oxidation catalysts.4 Lanthanum zirconate, La2Zr2O7, in particular, shows excellent protonic conductivity on doping with alkaline earth ions5 and is used as an electrolyte in high-temperature galvanic cells and as a material for a thick-film pyroelectric ceramic sensor.5,6 The A2B2O7 pyrochlore structure (Fd3m) can be described as a defect (A,B)O2-x fluorite structure (Fm3m), in which both cations and oxygen vacancies are ordered. In rare earth zirconate pyrochlores, the rare earth cation retains 8-fold coordination as in the ideal fluorite structure, whereas the coordination of zirconium drops to six. The disordering of cations and oxygen vacancies in this structure leads to either partially disordered pyrochlore or disordered fluorite structures.1,7 Figure 1 shows the fluorite and pyrochlore structure types for lanthanum zirconate. Thermodynamic properties of lanthanum zirconate pyrochlore have been studied using different measurement techniques. The heat capacity of La2Zr2O7 was measured by Bolech et al.8 using adiabatic calorimetry (4–400 K) and drop calorimetry (from 500 to 900 K). Sedmidubsky et al.9 further extended the heat capacity data from 888 to 1567 K using drop calorimetry. Korneev et al.10 measured the enthalpy of formation of La2Zr2O7 from oxides, DH0f,ox, using oxygen bomb calorimetry as 125 20 kJ/mol. The first measurement a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0401
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J. Mater. Res., Vol. 24, No. 11, Nov 2009
on La2Zr2O7 using high-temperature oxide melt solution calorimetry in lead borate was performed by Bolech et al.11 in 1995, and the formation enthalpy from oxides was calculated as 136 6 kJ/mol. A similar formation enthalpy value was later calculated from the Gibbs free energy of formation using galvanic cells with lanthanum b-alumina12 and composition graded electrolytes.13 More recently, the formation enthalpy of La2Zr2O7 was measured by oxide melt so
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