Probing disorder in pyrochlore oxides using in situ synchrotron diffraction from levitated solids–A thermodynamic perspective

Abstract

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.

Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

“…High temperature in situ diffraction and differential scanning calorimetry studies can be done in the future to nd the temperature dependence of the equilibrium degree of disorder and provide a more realistic thermodynamic model, as has been done for other pyrochlore compounds. 93 However, since at present we have data only for one series of quenched materials, we cannot say anything denitive about the equilibrium degree of ordering and resulting congurational entropy. Further local structure and calorimetry studies of more uorite-related systems would provide insights about the relation between local structure and thermodynamics.…”

Research for the Safe Management of Nuclear Waste at Forschungszentrum Jülich: Materials Chemistry and Solid Solution Aspects

“…The adequate and accurate computational methodology (see Section 2) substantially improved the DFT prediction of the temperature of order-disorder transitions and allowed for derivation of accurate maps of cation antisite and anion Frenkel pair defect formation energies [27,103] (Figure 9), which were recently shown to be the best match to the experimental values. [104] As shown in Figure 9, the obtained maps reveal a clear correlation between the oxygen Frenkel pair defect formation energies and the stability field of pyrochlore. Li et al [27] showed that the compounds that form defect fluorite have small or negative Frenkel pair defect formation energy.…”

“…However, a pure simulation driven research that aims at understanding the performance of pyrochlore as a nuclear waste form on the atomic scale has also been applied. The adequate and accurate computational methodology (see Section ) substantially improved the DFT prediction of the temperature of order–disorder transitions and allowed for derivation of accurate maps of cation antisite and anion Frenkel pair defect formation energies (Figure ), which were recently shown to be the best match to the experimental values . As shown in Figure , the obtained maps reveal a clear correlation between the oxygen Frenkel pair defect formation energies and the stability field of pyrochlore.…”

A Comparison of Order-Disorder in Several Families of Cubic Oxides

“…In situ synchrotron diffraction coupled with aerodynamic levitation was performed on Eu 2 Zr 2 O 7 to study the structural progression of disorder upon increasing temperature ( Maram et al, 2015 ). This was followed by in situ synchrotron diffraction on various zirconium and hafnium-based pyrochlores, RE 2 Zr 2 O 7 (RE = Sm, Eu, and Gd) and RE 2 Hf 2 O 7 (RE = La, Nd, and Sm), and modeling of the thermodynamics of disordering using configurational entropy calculations based on cation and anion site occupancies obtained from Rietveld refinement ( Maram et al, 2018 ). This study extended early work on spinel disordering to fluorite-related systems ( Navrotsky and Kleppa, 1967 ).…”