Thermochemical study of trivalent-doped ceria systems: CeO₂-M O_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,ox is slightly positive over the investigated composition range with a maximum at a certain doping level. Above that concentration, ΔH_f,ox decreaseses rapidly and stays almost constant. Such behavior is strikingly different from the strongly negative ΔH_f,ox of the ZrO₂-YO_1.5 and HfO₂-YO_1.5 systems. The absence of substantial energetic stabilization in the CeO₂-MO_1.5 systems 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_1.5 are 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-M O_1.5 is determined by the local site distortion rather than the global lattice deformation. The relatively stable region after the maximum ΔHf_,ox may be attributed to the somewhat stabilizing long-range defect-defect interactions, which become effective above a certain doping level.