Thermodynamic Study of Barium Cerate (BaCeO3) by Knudsen Effusion Mass Spectrometry

The vaporization/decomposition of single-phase BaCeO(3)samples was studied by the Knudsen Effusion Mass Spectrometry technique in the overall temperature range 1647-1923 K. The partial pressures of the gaseous species BaO(g), CeO2(g) and CeO(g) were monitored as a function of temperature. The formation of reduced form of cerium oxide as a product of the high temperature decomposition of BaCeO(3)under the given experimental conditions was suggested by the low value of the measured CeO2(g)/CeO(g) pressure ratio and confirmed by the XRD spectra of the vaporization residues. The equilibrium data were tentatively processed according to various heterogeneous equilibria that may be established in the cell: (i) BaCeO3(s) = BaO(g) + CeO2(s); (ii) BaCeO3(s) + CeO(g) = BaO(g) + Ce2O3(s); (iii) BaCeO3(s) = BaO(g) + CeO2(g). The corresponding enthalpy changes were calculated by the third-law method and the enthalpy of formation of BaCeO(3)derived thereafter. The thermodynamic analysis confirms that the decomposition process forming CeO2(s) is very unlikely to occur under the high temperature, high vacuum conditions used in the present study. The analysis of process BaCeO3(s) = BaO(g) + CeO2(g), not involving solid products, was selected as the most reliable and leads to a value for the enthalpy of formation of barium cerate (Delta H-f(298)degrees(BaCeO3) = -1688.7 +/- 6.2 kJ/mol) in agreement with the available calorimetric determinations.