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 (La2Zr2 O7) from the component oxides lanthana (La2O3) (A-rare earth) and zirc onia (ZrO2) (monoclinic) in the temperature range of 870-1240 K. The c ell used for measurement can be represented as Pt, O-2, CaO + CaF(2)pa rallel 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 int roduced 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 chemic al potentials. For the reaction La2O3 (A-rare earth) and (ZrO2) (monoc linic) La2Zr2O7 (pyrochlore), the Gibbs free energy change (Delta G(f, ox)degrees) is given by the formula -133800 - 10.32T (+/-4500) (in uni ts of J/mol), The enthalpy and entropy of formation of La2Zr2O7 obtain ed in this study are in good agreement with calorimetric data. The ''t hird-law'' enthalpy of formation of La2Zr2O7, from the component oxide s at 298.15 K, is -133.8 +/- 5 kJ/mol.