Calorimetric measurements of energetics of defect interactions in fluorite oxides

Direct measurement by oxide melt solution calorimetry of energetics of mixing in rare earth and yttrium doped zirconia, hafnia, and ceria systems provides support for spectroscopic and computational studies of the location and clustering of vacancies in these systems. Strongly negative heats of mixing are seen when the vacancy is transferred from being nearest neighbor to Y or RE in the sesquioxide to being nearest neighbor to Zr or Hf in the cubic solid solution. In the absence of such redistribution, small positive enthalpies of mixing are seen in CeO₂-YO_1.5 and CeO₂-REO _1.5 systems. Strongly positive enthalpies of mixing are seen in CeO₂-ZrO₂, which has a large difference in cation sizes and no vacancy formation. The system Ce_0.8Y_0.2O _1.9-Zr_0.8Y_0.2O_1.9 shows small positive heats of formation with less destabilization in the Ce-rich region, suggestive of "scavenging" of oxygen vacancies by Zr. The calorimetric data obtained in these studies offer direct comparison with the results of computations on defect clusters and their binding energies.