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

Abstract

The formation enthalpies from the oxide end-members (ΔHf,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, ΔHf,oxis slightly positive over the investigated composition range with a maximum at a certain doping level. Above that concentration, ΔHf,oxdecreases rapidly and stays almost constant. Such behavior is strikingly different from the strongly negative ΔHf,oxof the ZrO2–YO1.5and HfO2–YO1.5systems. The absence of substantial energetic stabilization in the CeO2–MO1.5systems 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–MO1.5are proposed to be neutral trimers with oxygen vacancies nearest neighbor to the dopant cations. It is also suggested that the maximum ΔHf,ox(destabilization) of CeO2–MO1.5is determined by the local site distortion rather than the global lattice deformation. The relatively stable region after the maximum ΔHf,oxmay be attributed to the somewhat stabilizing long-range defect–defect interactions, which become effective above a certain doping level.