A correlation between the ionic conductivities and the formation enthalpies of trivalent-doped ceria at relatively low temperatures

We report a correlation between oxygen ionic conductivity and the enthalpy of formation of trivalent-doped ceria from the component binary oxides observed at relatively low temperatures (150-275 °C). The bulk conductivities of La-doped ceria samples identical to those previously examined by thermochemical studies were measured as a function of La content for a direct comparison. The conductivity showed a maximum at a La concentration of 5 mol%, implying that the number of freely mobile oxygen vacancies reaches a maximum near that doping level in the temperature range of interest. The formation enthalpies previously reported by Chen and Navrotsky also show a maximum, indicating destabilization near that composition. Additional measurements show that this maximum is very pronounced and sharply peaked near that composition. These enthalpies suggest that the energetically favorable long-range interactions between the charged defects that trap the oxygen vacancies become dominant above 5 mol% doping in the CeO₂-LaO_1.5 solid solution. In addition, the conductivities measured from independently prepared Gd-doped ceria samples show a maximum at around 10 mol% doping below 450 °C as anticipated from a pronounced maximum in the formation enthalpies of the CeO₂-GdO _1.5 solid solution. These empirical findings confirm that the ionic conductivity of trivalent-doped ceria is strongly enough correlated with its formation enthalpy at relatively low temperatures so that information about the critical dopant concentration associated with the conductivity maximum may be gained from the formation enthalpies of the solid solutions, and vice versa. We have no direct information about this correlation at higher temperatures; both thermodynamics and conductivity maximum might change if the defect clusters dissociate to any significant extent.