Thermochemistry of lanthanum zirconate pyrochlore

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 La_1.98Zr_2.01O₇ to La_2.07Zr_1.95O₇. The enthalpy of formation from oxides at 25 °C, ΔH⁰_f,ox, for stoichiometric lanthanum zirconate pyrochlore is −107.3 ± 5.1 kJ/mol, and the standard enthalpy of formation from elements, ΔH⁰_f,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.