Composition-graded solid electrolyte for determination of the Gibbs energy of formation of lanthanum zirconate

A composition-graded solid electrolyte has been used to determine the standard Gibbs free energy of formation of lanthanum zirconate (La2Zr2O7) from the component oxides lanthana (La2O3) (A-rare earth) and zirconia (ZrO2) (monoclinic) in the temperature range of 870-1240 K. The cell used for measurement can be represented as Pt, O-2, CaO + CaF(2)parallel to CaF2 \(x=0) (LaF3)(x)(CaF2)(1-x) parallel to(x=0.32) LaF3 + La2Zr2O7 + ZrO2, O-2, Pt A composition-graded electrolyte has been introduced to compensate the solubility effects of the electrode material (lanthanum fluoride, LaF3) in the solid electrolyte (calcium fluoride, CaF2), The ability of the graded electrolyte to generate a Nernstian response is demonstrated, using electrodes with known fluorine chemical potentials. For the reaction La2O3 (A-rare earth) and (ZrO2) (monoclinic) La2Zr2O7 (pyrochlore), the Gibbs free energy change (Delta G(f,ox)degrees) is given by the formula -133800 - 10.32T (+/-4500) (in units of J/mol), The enthalpy and entropy of formation of La2Zr2O7 obtained in this study are in good agreement with calorimetric data. The "third-law" enthalpy of formation of La2Zr2O7, from the component oxides at 298.15 K, is -133.8 +/- 5 kJ/mol.