Thermodynamic properties and structural stability of thorium dioxide

TL;DR

This study uses density functional theory to analyze the thermodynamic properties, structural stability, defect formation, and helium diffusion in thorium dioxide, providing insights into its behavior at various temperatures and Fermi energy conditions.

Contribution

It offers a comprehensive theoretical investigation of ThO₂'s thermodynamic properties and defect behaviors, including the impact of Fermi energy shifts on structural stability.

Findings

Thermal expansion coefficient and heat capacities agree with experiments.

Interstitial Thₙ^{4+} defects are most probable in intrinsic ThO₂.

Helium diffusion occurs mainly through thorium vacancies with a low energy barrier.

Abstract

Using density functional theory (DFT) calculations, we have systematically investigated the thermodynamic properties and structural stabilities of thorium dioxide (ThO$_2$). Based on the calculated phonon dispersion curves, we calculate the thermal expansion coefficient, bulk modulus, and heat capacities at different temperatures for ThO$_2$ under the quasi-harmonic approximation. All the results are in good agreement with corresponding experiments proving the validity of our methods. Our theoretical studies can help people more clearly understand the thermodynamic behaviors of ThO$_2$ at different temperatures. In addition, we have also studied possible defect formations and diffusion behaviors of helium in ThO$_2$, to discuss its structural stability. It is found that in intrinsic ThO$_2$ without any Fermi energy shifts, the interstitial Th$_i^{4+}$ defect other than oxygen or thorium vacancies, interstitial oxygen, and any kinds of Frenkel pairs, is most probable to form with an energy release of 1.74 eV. However, after upshifting the Fermi energy, the formation of the other defects also becomes possible. For helium diffusion, we find that only through the thorium vacancy can it happen with the small energy barrier of 0.52 eV. Otherwise, helium atoms can hardly incorporate or diffuse in ThO$_2$. Our results indicate that people should prevent upshifts of the Fermi energy of ThO$_2$ to avoid the formation of thorium vacancies and so as to prevent helium caused damages.