Measurement of Gibbs energy of formation of LaGaO 3 using a composition-graded solid electrolyte
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Niladri Dasgupta Ceramic Technology Institute, Bharat Heavy Electricals Limited, Bangalore 560 012, India
Helfried Na¨fe and Fritz Aldinger Max-Planck-Institut fu¨r Metallforschung, Heisenbergstrasse 5, D-70569 Stuttgart, Germany (Received 24 July 2000; accepted 16 September 2000)
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 + LaO1−x F1+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 () → LaGaO3 (rhombohedral) can be represented by the equation: ⌬Gof,(ox)/J mol−1 ⳱ −46 230 + 7.75 T/K (±1500).
I. INTRODUCTION
Doped LaGaO3 is an excellent oxide ion conductor with potential application in solid-oxide fuel cells.1,2 Doping of Sr on the La site and Mg on the Ga site is an effective method for increasing oxygen vacancies and enhancing ionic conductivity of LaGaO3. Thermodynamic data for pure and doped LaGaO3 are useful for evaluating the stability of the electrolyte under reducing conditions and its compatibility with electrode materials. Phase diagram for the system La2O3–Ga2O3 shows the presence of two congruent melting interoxide compounds; LaGaO3 melting at 1988 (±20) K, and La4Ga2O9 melting at 1977 (±20) K.3 Azad et al.4 attempted to measure the standard Gibbs energy of formation of LaGaO3 from its constituent sesquioxides using a cell based on CaF2 as the solid elec-
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J. Mater. Res., Vol. 15, No. 12, Dec 2000 Downloaded: 18 Mar 2015
trolyte, La2O3 + LaF3 as the reference electrode, and LaF3 + LaGaO3 + Ga2O3 as the working electrode. Both electrodes were placed under flowing oxygen gas. The formation of lanthanum oxyfluoride from a mixture of La2O3 and LaF3 is well established in the literature.5–7 Therefore, the reference electrode used by Azad et al.4 is unstable in the temperature range of their measurements. Because Azad et al.4 ignored the formation of the oxyfluoride
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