Enthalpy of formation of yttria-doped ceria

Solid solutions (1 - x)CeO2 - xYO(1.5) (0 <= x <= 0.36) were prepared by coprecipitation and sol-gel methods. Their enthalpy of formation relative to the end-members, fluorite-type cubic CeO2 and C-type YO1.5 was determined by oxide melt solution calorimetry. The enthalpy of drop solution shows a roughly linear trend with composition. Extrapolation to x = 1 gives the transition enthalpy of C-type to cubic fluorite YO1.5 as 22.2 +/- 6.7 kJ/mol. This linear behavior is in contrast to the strong curvature seen in the ZrO2 - YO01.5 and HfO2 - YO1.5 systems. The slightly positive enthalpy of formation of CeO2 - YO1.5 is strikingly different from the strongly negative enthalpies of formation of ZrO2 - YO1.5 and HfO2 - YO1.5. The thermodynamics of CeO2 - YO1.5 is analyzed in terms of defect association and oxygen vacancy distribution. Specifically, the association of oxygen vacancies with the tetravalent cations in the zirconia and hafnia systems, in contrast to the preference of vacancies for nearest neighbor yttrium sites in the ceria systems, may explain the different energetics.