Measurement of Gibbs energy of formation of LaGaO₃ using a composition-graded solid electrolyte

Abstract

A composition-graded solid electrolyte (LaF₃)_y· (CaF₂)_1-y was used for the measurement of the standard Gibbs energy of formation of LaGaO₃ from its component oxides. An equimolar mixture of CaO and CaF₂ was employed as the reference electrode. The composition of the working electrode depended on temperature. A three-phase mixture of LaGaO₃+Ga₂O₃+LaF₃ was used in the temperature range from 910 to 1010 K, while a mixture of LaGaO₃+Ga₂O₃+LaO_1-xF_1+2x was employed from 1010 to 1170 K. Both the reference and working electrodes were placed under pure oxygen gas. Because of the high activity of LaF₃ at the working electrode, there was significant diffusion of LaF3 into CaF₂. The composition-graded electrolyte was designed to minimize the electrode–electrolyte interaction. The concentration of LaF₃ varied across the solid electrolyte; from y = 0 near the reference electrode to a maximum value y = 0.32 at the working electrode. For the correct interpretation of the electromotive force at T > 1010 K, it was necessary to use thermodynamic properties of the lanthanum oxyfluoride solid solution. The standard Gibbs energy of formation of LaGaO3 from its component oxides according to the reaction, 1/2La₂O₃ (A-rare earth) + 1/2Ga₂O₃ (b)→LaGaO₃ (rhombohedral) can be represented by the equation: δG°_f,(ox)/J mol^-1 = –46 230 + 7.75 T/K (±1500).