Enthalpy of formation of cubic yttria-stabilized zirconia

Abstract

Oxide melt solution calorimetric measurements were made to determine the enthalpy of formation of cubic-yttria-stabilized zirconia (c-YSZ) with respect to the oxides m-ZrO2 and C-type YO1.5. The enthalpy of formation can be fit either by a quadratic equation or by two straight line segments about the minimum near x = 0.40. The quadratic fit gives a strongly negative interaction parameter, Ω=-93.7 ± 12.0 kJ/mol, but does not imply regular solution behavior because of extensive short-range order. In this fit, the enthalpy of transition of m-ZrO2 to c-ZrO2, 9.7 ± 1.1 kJ/mol, is in reasonable agreement with earlier estimates and that of C-type to cubic fluorite YO1.5, 24.3 ±14.4 kJ/mol, is consistent with an essentially random distribution of oxide ions and anion vacancies in the high-temperature fluorite phase. The two straight-line segments give 6.1 ± 0.6 kJ/mol and 5.5 ± 2.5 kJ/mol for these transitions, respectively. The latter value would imply strong short-range order in cubic fluorite YO1.5. Clearly more complex solution thermodynamic descriptions need to be developed. The enthalpy of transition from the disordered c-YSZ phase to the ordered δ-phase at 25 °C was also measured and was 0.4 ± 1.6 kJ/mol. No energetic difference between the disordered c-YSZ phase and the ordered δ-phase underscores the importance of short-range order in c-YSZ. Enthalpy data were combined with Gibbs free energy data to calculate entropies of mixing. Using the quadratic fit, negative excess entropy of mixing in the cubic solid solution, relative to a system with maximum randomness on cation and anion sublattices, was found and was another indication of extensive short-range order in c-YSZ.

Calorimetric determination of energetics of solid solutions of UO2+x with CaO and Y2O3

“…With such large negative deviations from ideality, it is clear that the regular solution formalism, which assumes random entropy of mixing, is not applicable. This point has been discussed previously in the context of yttria-stabilized zirconia [15] and indeed was already noted in the early work on urania systems [20]. Rather, the oxidation of uranium and formation of complex defect clusters dominate the system and are responsible for the large stabilization.…”

“…Understanding the systematics of defect chemistry in doped fluorite structures and energetics of these materials in a variety of systems forms the microscopic basis for tailoring materials with optimized properties. Recent calorimetric studies in this laboratory of other fluorite-structured systems such as yttria-doped zirconia, hafnia and ceria based systems [13][14][15][16][17] provide such understanding, and the urania-based systems are also of interest in further developing systematics in terms of defect chemistry.…”

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

“…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].…”

ΔThe effect of co-doping on the yttrium local environment and ionic conductivity of yttria-stabilised zirconia

“…Ordering in YSZ is known to increase with increasing yttria content [3,20]. Therefore, using the above logic, ΔS m would be expected to increase with yttria content.…”