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 (Ce _1-xSm _xO _2-0.5x with 0 < x < 0.3), neodymium-doped ceria (Ce _1-xNd _xO _2-0.5x with 0 < x < 0.3), and neodymia-samaria co-doped ceria (Ce _1-xNd _x/2Sm _x/2O _2-0.5x 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 Ce _1-xNd _x/2Sm _x/2O _2-0.5x, there is less destabilization at low x compared to singly doped CeO ₂ 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 Ce _1-xNd _x/2Sm _x/2O _2-0.5x are less exothermic than those of singly doped materials.