Thermochemical study of trivalent-doped ceria systems: CeO₂–MO_1.5(M = La, Gd, and Y)

The formation enthalpies from the oxide end-members (ΔH_f,ox) of the CeO₂–MO_1.5(M = La, Gd, and Y) systems were determined by high temperature oxide melt drop solution calorimetry. In each system, ΔH_f,oxis slightly positive over the investigated composition range with a maximum at a certain doping level. Above that concentration, ΔH_f,oxdecreases rapidly and stays almost constant. Such behavior is strikingly different from the strongly negative ΔH_f,oxof the ZrO₂–YO_1.5and HfO₂–YO_1.5systems. The absence of substantial energetic stabilization in the CeO₂–MO_1.5systems may be attributed to the large size of Ce⁴⁺, which has no preference for 7-coordination like the smaller Zr⁴⁺or Hf⁴⁺ions. The primary defect associates in CeO₂–MO_1.5are proposed to be neutral trimers with oxygen vacancies nearest neighbor to the dopant cations. It is also suggested that the maximum ΔH_f,ox(destabilization) of CeO₂–MO_1.5is determined by the local site distortion rather than the global lattice deformation. The relatively stable region after the maximum ΔH_f,oxmay be attributed to the somewhat stabilizing long-range defect–defect interactions, which become effective above a certain doping level.