Energetics of rare-earth-doped hafnia

Abstract

The enthalpies of formation of rare-earth (RE)-doped Hf 1−x RE x O 2−x/2 solid solutions (RE ‫ס‬ Sm, Gd, Dy, Yb; x ‫ס‬ 0.25 to 0.62) with respect to the oxide end members, monoclinic HfO 2 and C-type REO 1.5 , 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 ‫ס‬ 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.

Systematics of Phase Transition and Mixing Energetics in Rare Earth, Yttrium, and Scandium Stabilized Zirconia and Hafnia

“…Zr 1− x RE x O 2− x /2 samples with 20–65 mol% REO 1.5 (RE=Sm, Gd, Dy, Yb, Sc) were synthesized by a coprecipitation method 12,13 . 0.3 M solutions of zirconium oxychloride (ZrCl 2 O·8H 2 O, Alfa‐Aesar, 99.9%) and 0.3 M solutions of rare earth and scandium chloride solutions (SmCl 3 · x H 2 O, Alfa‐Aesar, 99.99%; GdCl 3 ·6H 2 O, Alfa‐Aesar, 99.99%; DyCl 3 · x H 2 O, Alfa‐Aesar, 99.99%; YbCl 3 · x H 2 O, Alfa‐Aesar, 99.99%, ScCl 3 ·6H 2 O, Alfa‐Aesar, 99.99%) were mixed in the appropriate ratio to produce the desired composition.…”

“…Eight to 12 data points were collected for each ZrO 2 –REO 1.5 sample. The calorimetric methodology was applied previously for a study of the energetics of HfO 2 –REO 1.5 solid solutions 12 …”

“…Although the cubic fluorite phase is considered to be disordered on the cation and oxygen sublattices, several structural, and thermodynamic studies verified strong local ordering in these systems 10,11 . It was shown that the disordered fluorite phase is energetically and structurally closely related to the ordered pyrochlore and δ‐phase 10,12 . The energetics of yttria stabilized zirconia, as well as yttria and rare earth stabilized hafnia have been previously investigated using oxide melt solution calorimetry 10,12,13 .…”

“…The energetics of the ZrO 2 –REO 1.5 and ZrO 2 –ScO 1.5 systems are discussed in relation to the dopant and its concentration. Calorimetric data from this study and studies on HfO 2 –REO 1.5 , 12 HfO 2 –YO 1.5 and ZrO 2 –YO 1.5 10,13 are combined to derive the interaction parameters of these solid solutions and to obtain the enthalpies of transition of their oxide end members to the cubic fluorite structure in an internally consistent manner.…”

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

“…Thus, the estimated transition enthalpies are very approximate. In a recent high-temperature solution calorimetry study of the HfO 2 -GdO 1.5 system, Simoncic et al 22 reported that the formation enthalpy from monoclinic HfO 2 and C-type GdO 1.5 showed a strongly curved trend with negative heat of mixing. The quadratic fit of ⌬H DS as a function of the GdO 1.5 concentration gave an interaction parameter (as in a regular solution) of −237 ± 10 kJ/mol and an estimated transition enthalpy of GdO 1.5 from C-type to fluorite phase as 62 ± 3 kJ/mol, which is quite different from the value estimated in this study.…”