Energetics of rare-earth-doped hafnia

The enthalpies of formation of rare-earth (RE)-doped Hf_1-xRE_xO_2-x/2 solid solutions (RE = Sm, Gd, Dy, Yb; x = 0.25 to 0.62) with respect to the oxide end members, monoclinic HfO₂ and C-type REO₁.₅, were determined using oxide melt solution calorimetry. The enthalpies of formation fit a function quadratic in composition. The strongly negative interaction parameters in all solid solutions confirm a strong tendency for short-range order. Though strongly negative for all systems, the interaction parameters become less negative with increasing ionic potential (decreasing RE radius). Crystallization energetics were investigated for amorphous coprecipitation products with x = 0.4. The energy difference between the crystalline (fluorite and pyrochlore) and amorphous phases decreases with decreasing dopant radius. This suggests that systems doped with small RE ions have more similar local structures in the fluorite and amorphous phases. These observations may result in a smaller kinetic barrier to recrystallization and account for the greater radiation resistance of materials with smaller RE cations.