Use of Composition-Graded Bi-Electrolyte Cells for Thermodynamic Studies on Lanthanum Aluminates

Abstract

Innovative bi–electrolyte solid-state cells incorporating single crystal CaF2 and composition-graded solid electrolyte (LaF3)y·(CaF2)1-y (y = 0 to 0.32) were used for measurement of the standard Gibbs energy of formation of hexagonal La0.885Al11.782O19 and cubic LaAlO3 from component binary oxides La2O3 and α-Al2O3 in the temperature range from 875 to 1175 K. The cells were designed based on experimentally verified relevant phase relations in the systems La2O3–Al2O3–LaF3 and CaF2–LaF3. The results can be summarized as: 5.891 α-Al2O3 + 0.4425 La2O3 (A-rare earth) → La0.885Al11.782O19 (hex), ΔGof(ox)( ± 2005)/J mol− 1 = −80982 + 7.313(T/K); 1/2 La2O3 (A-rare earth) + 1/2 α-Al2O3 → LaAlO3 (cubic), ΔGof(ox)( ± 2100)/J mol− 1 = −59810 + 4.51(T/K). Electron probe microanalysis was used to ascertain the non-stoichiometric range of the hexaaluminate phase. The results are critically analyzed in the light of earlier electrochemical measurements. Several imperfections in the electrochemical cells used by former investigators are identified. Data obtained in the study for LaAlO3 are consistent with calorimetric enthalpy of formation and entropy derived from heat capacity data. Estimated are the standard entropy and the standard enthalpy of formation from elements of hexagonal La0.885Al11.782O19 and rhombohedral LaAlO3 at 298.15 K.