The enthalpies of formation from binary oxide components at 25 °C of Ba(Zr_1−xY_x)O_3−δ, x = 0.1 to 0.5 solid solutions are measured by high temperature oxide melt solution calorimetry in a molten solvent, 3Na₂O·4MoO₃ at 702 °C. The enthalpy of formation is exothermic for all the compositions and becomes less negative when increasing yttrium content from undoped (−115.12 ± 3.69 kJ mol⁻¹) to x = 0.5 (−77.09 ± 4.31 kJ mol⁻¹). The endothermic contribution to the enthalpy of formation with doping content can be attributed to lattice distortions related to the large ionic radius difference of yttrium and zirconium and vacancy formation. For 0.3 ≤ x ≤ 0.5, the enthalpy of formation appears to level off, consistent with an exothermic contribution from defect clustering. Raman spectra indicate changes in short range structural features as a function of dopant content and, suggests that from x = 0.3 to 0.5 the defects begins to cluster significantly in the solid solution, which corroborates with the thermodynamic data and the drop-off in proton conductivity from x > 0.3.

The energetics of the order-disorder phase transformation in the binary oxide system, Eu₂O₃–ZrO₂, is studied by powder X-ray diffraction and high temperature drop solution calorimetry. The nanocrystalline defect fluorite phase of Eu₂Zr₂O₇ is synthesized on crystallization of an amorphous precursor from aqueous precipitation. The defect fluorite transforms to an ordered pyrochlore above 1200 °C. Aerodynamic levitation combined with laser heating is used to prepare coarse defect fluorite, which is otherwise impossible by conventional synthesis techniques. Formation enthalpies from oxides are −62.4 ± 2.6 and −24.6 ± 3.7 kJ mol⁻¹ for the pyrochlore and defect fluorite phase, respectively. The transformation enthalpy from pyrochlore to defect flourite in the coarse sample is 37.8 ± 3.1 kJ mol⁻¹ at 25 °C. The enthalpy of water vapor adsorption on the surface of the nanocrystalline defect fluorite Eu₂Zr₂O₇ is −75 ± 2.5 kJ mol⁻¹ H₂O for coverage of 9.5 ± 0.8 H₂O/nm². The calculated surface enthalpies for the anhydrous and hydrous surfaces of defect fluorite Eu₂Zr₂O₇ are 1.47 ± 0.13 and 1.01 ± 0.15 J m⁻², respectively.

The formation enthalpies of perovskite-structured lithium lanthanum titanate (LLTO) Li_3xLa_0.67−xTiO₃ (compositions x = 0.04 to 0.15) are exothermic and subject to two regimes with structural transformation.

Pyrochlore, an ordered derivative of the defect fluorite structure, shows complex disordering behavior as a function of composition, temperature, pressure, and radiation damage. We propose a thermodynamic model to calculate the disordering enthalpies for several RE₂Zr₂O₇ (RE = Sm, Eu, Gd) pyrochlores from experimental site distribution data obtained by in situ high temperature synchrotron X-ray diffraction. Site occupancies show a gradual increase in disorder on both cation and anion sublattices with increasing temperature and even greater disorder is achieved close to the phase transition to defect fluorite. The enthalpy associated with cation disorder depends on the radius of the rare earth ion, while the enthalpy of oxygen disordering is relatively constant for different compositions. The experimental data support trends predicted by ab initio calculations, but the obtained enthalpies of disordering are less endothermic than the predicted values. Thermal expansion coefficients are in the range (8.6–10.8) × 10⁻⁶ K⁻¹. These new experimental determinations of defect formation energies are important for understanding the stability of pyrochlore oxides and their disordering mechanisms, which are essential in the context of their potential applications in nuclear waste management and other technologies.