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

摘要

Innovative bi-electrolyte solid-state cells incorporating single crystal CaF_2 and composition-graded solid electrolyte (LaF_3)_y·(CaF_2)_(1-y) (y = 0 to 0.32) were used for measurement of the standard Gibbs energy of formation of hexagonal La_(0.885)Al_(11.782)O_(19) and cubic LaAlO_3 from component binary oxides La_2O_3 and α-Al_2O_3 in the temperature range from 875 to 1175 K. The cells were designed based on experimentally verified relevant phase relations in the systems La_2O_3-Al_2O_3-LaF_3 and CaF_2-LaF_3. The results can be summarized as: 5.891 α-Al_2O_3 + 0.4425 La_2O_3 (A-rare earth) →-La_(0.885)Al_(11.782)O_(19) (hex), ΔG_(f(ox))~0)(±2005)/Jmol~(-1) = -80982 + 7.313(T/K); 1/2 La_2O_3 (A-rare earth) + 1/2 α-Al_2O_3 → LaAlO_3 (cubic), ΔG_(f(ox))~0(±2100)/Jmol~(-1) =-59810 + 4.5KT/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 LaAlO_3 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 La_(0.885)A_(11.782)O_(19) and rhombohedral LaAlO_3 at298.15K.