Enthalpy of formation of cubic yttria-stabilized hafnia

Abstract

The enthalpy of formation of cubic yttria-stabilized hafnia from monoclinic hafnia and C-type yttria was measured by oxide melt solution calorimetry. The enthalpies of formation fit a function independent of temperature and quadratic in composition. The enthalpies of transition from m-HfO 2 and C-type YO 1.5 , to the cubic fluorite phase are 32.5 ± 1.7 kJ/mol and 38.0 ± 13.4 kJ/mol, respectively. The interaction parameter in the fluorite phase is strongly negative, −155.2 ± 10.2 kJ/mol, suggesting even stronger short range order than in ZrO 2 -YO 1.5 . Regular solution theory or any other model assuming random mixing on the cation and /or anion sublattice is not physically reasonable. A more complex solution model should be developed to be consistent with the new calorimetric data and observed phase relations.

Formation enthalpy of ThSiO4 and enthalpy of the thorite → huttonite phase transition

“…Enthalpy of formation from binary oxides of zircon-type orthosilicates MSiO 4 as function of the effective ionic radius of the tetravalent metal in eightfold coordination (Shannon, 1976). Data for cubic HfO 2 and ZrO 2 are taken from Lee and Navrotsky (2004) and Molodetsky et al (1998). Error bars for U are absent, but data should be considered semiquantitative since this is obtained rather indirectly (Langmuir, 1978) (half-filled square).…”

Computational study of the energetics and defect clustering tendencies for Y- and La-doped UO2

“…We first investigate the energetic stability of U 1Àx M x O 2À0:5x (M = Y, La) through consideration of the formation enthalpy DH f with respect to constituent oxides, as defined in Eq. (2). Results for the lowest energy structures of Y-and La-substituted UO 2 are shown in Fig.…”

“…Comparisons between such DFT calculations and experiments can be justified if the latter display appreciable short-range order. While substantial ordering effects have been reported in experimental studies on similar fluoritestructured solid solutions [4,[1][2][3], it is difficult to quantify whether the effects are sufficiently pronounced for direct comparison with DFT calculations on fully ordered compounds. Nevertheless, it is assumed that the trends with respect to trivalent cation species should be comparable regardless of the extent of order/disorder, as was found to be the case in previous studies for La-and Y-substituted ThO 2 .…”

“…Under reducing conditions this doping leads to the formation of charge-compensating oxygen vacancy defects. Recent calorimetry studies on trivalent-cation-doped ZrO 2 [1], HfO 2 [2], CeO 2 [3] and ThO 2 [4] provide evidence for strong defect association in such systems. The experimental findings have been shown to be consistent with recent computational work on some of these same systems, which demonstrate energetic binding/clustering tendencies between cations and oxygen vacancies, to a degree that has been found to correlate strongly with ionic radii [5][6][7][8][9][10][11][12].…”

Hydrothermal crystal growth of yttrium and rare earth stabilized hafnia

“…Stabilization of cubic hafnia with low dopant concentrations is regularly achieved by a variety of methods including coprecipitation [19], flux growth [14], and the skull melting technique [20]. However, enthalpy of formation studies has determined the most favorable formation of the cubic hafnia species occurs when using 30-40% stabilizer concentrations [21]. The low viscosity of hydrothermal fluids, in comparison to flux and skull melting synthesis, would allow ample circulation of the stabilizer dopant promoting the formation of the most energetically favorable composition.…”