THERMODYNAMIC PROPERTIES OF TERNARY BARIUM OXIDES

The vapor pressure over the two-phase mixture of CeO2 + BaCeO3 has been measured by mass spectrometry in the temperature range 1774-1947 K. BaO(g) only was identified as being the main gas species. From the vapor pressure of BaO(g) over CeO2 + BaCeO3, the standard molar enthalpy of formation of BaCeO3 has been determined by second and third law treatments and is compared with those values calculated by the use of Aronson's semiempirical equation in this study and obtained previously by solution calorimetry. The standard molar Gibbs energies of formation of BaCeO3, BaZrO3 and Ba2ZrO4 have also been determined as a function of temperature from the respective vapor pressures of BaO(g) over CeO2 + BaCeO3 in this study and those over ZrO2 + BaZrO3 and BaZrO3 + Ba2ZrO4 previously determined by the present author and co-workers. The experimental values of the standard molar Gibbs energy of formation and the standard molar enthalpy of formation of other ternary barium oxides such as BaMoO3, BaMoO4, BaUO3, BaUO4, BaTiO3, Ba2TiO4, BaThO3 and BaPuO3 are summarized and the latter is compared with calculated values from Aronson's equation, The relative stability of barium perovskite oxides is briefly discussed from the Goldschmidt tolerance factors and the molar enthalpies of formation of the perovskite oxides from the constituent binary oxides.