Thermochemical study of trivalent-doped ceria systems:: CeO2-MO1.5 (M = La, Gd, and Y)

The formation enthalpies from the oxide end-members (Delta H-f,H-ox) of the CeO2-MO1.5 (M = La, Gd, and Y) systems were determined by high temperature oxide melt drop solution calorimetry. In each system, Delta H-f,H-ox is slightly positive over the investigated composition range with a maximum at a certain doping level. Above that concentration, Delta H-f,H-ox decreases rapidly and stays almost constant. Such behavior is strikingly different from the strongly negative Delta H-f,H-ox of the ZrO2-YO1.5 and HfO2-YO1.5 systems. The absence of substantial energetic stabilization in the CeO2-MO1.5 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-MO1.5 are proposed to be neutral trimers with oxygen vacancies nearest neighbor to the dopant cations. It is also suggested that the maximum Delta H-f,H-ox (destabilization) of CeO2-MO1.5 is determined by the local site distortion rather than the global lattice deformation. The relatively stable region after the maximum Delta H-f,H-ox may be attributed to the somewhat stabilizing long-range defect-defect interactions, which become effective above a certain doping level.