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

Abstract

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, O₂, CaO + CaF₂∥ CaF₂| (LaF₃)_x(CaF₂)_1-x∥LaF₃+ La₂Zr₂O₇+ ZrO₂, O₂, Pt x=0 x=0.32 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 gen-erate a Nernstian response is demonstrated, using electrodes with known fluorine chemical potentials. For the reaction La₂O₃ (A-rare earth) + 2ZrO2 (monoclinic)→La₂Zr₂O₇ (pyrochlore), the Gibbs free energy change (ΔG°_f,ox) 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.