Enthalpies of formation and insights into defect association in ceria singly and doubly doped with neodymia and samaria

It has been suggested that co-doping ceria with two trivalent ions of different sizes to minimize lattice strain produces materials with better ionic conductivity. To investigate the thermodynamic basis of such behavior, enthalpies of formation at room temperature of samarium-doped ceria (Ce1 − xSmxO2 − 0.5x with 0 < x < 0.3), neodymium-doped ceria (Ce1 − xNdxO2 − 0.5x with 0 < x < 0.3), and neodymia–samaria co-doped ceria (Ce1 − xNdx/2Smx/2O2 − 0.5x with 0 < x < 0.3) have been measured by high temperature oxide melt solution calorimetry. The energetics of the solid solutions were analyzed in terms of cation size mismatch and defect association. At concentrations below x = 0.05, endothermic (destabilization) heat of formation is attributed to the dominance of size mismatch. Considerable energetic stabilization at x > 0.05 for singly doped ceria systems can be attributed to defect associates of trivalent cations coupled with charge-balancing oxygen vacancies. For co-doped Ce1 − xNdx/2Smx/2O2 − 0.5x, there is less destabilization at low x compared to singly doped CeO2 but less stabilization at high x and a shift in the composition of maximum (most endothermic) formation enthalpy toward higher dopant concentration. Enthalpies of defect association of Ce1 − xNdx/2Smx/2O2 − 0.5x are less exothermic than those of singly doped materials.