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 similar to 33-35 mol% La2O3 at 1500 degrees 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 degrees C, Delta H-f,H-ox,(0) , for stoichiometric lanthanum zirconate pyrochlore is -107.3 +/- 5.1 kJ/mol, and the standard enthalpy of formation from elements, Delta H-f,H-el,(0) 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 degrees C and similar to 350 kJ/mol, respectively.