First-principles study of ground state properties and high pressure behavior of ThO2

TL;DR

This study uses first-principles density-functional theory to analyze the ground state properties, electronic structure, and high-pressure behavior of ThO2, revealing phase stability, bonding nature, and phase transitions up to 240 GPa.

Contribution

It provides a comprehensive first-principles analysis of ThO2's structural, electronic, and mechanical properties under high pressure, including phase transition predictions.

Findings

Both fluorite and cotunnite ThO2 are mechanically stable.

The phase transition from fluorite to cotunnite occurs at 26.5 GPa.

An isostructural transition is predicted between 80 and 130 GPa.

Abstract

The mechanical properties, electronic structure and phonon dispersion of ground state ThO$_{2}$ as well as the structure behavior up to 240 GPa are studied by using first-principles density-functional theory. Our calculated elastic constants indicate that both the ground state fluorite structure and high pressure cotunnite structure of ThO$_{2}$ are mechanically stable. The bulk modulus, shear modulus, and Young's modulus of cotunnite ThO$_{2}$ are all smaller by approximately 25% compared with those of fluorite ThO$_{2}$. The Poisson's ratios of both structures are approximately equal to 0.3 and the hardness of fluorite ThO$_{2}$ is 27.33 GPa. The electronic structure and bonding nature of fluorite ThO$_{2}$ are fully analyzed, which show that the Th-O bond displays a mixed ionic/covalent character. The valence of Th and O ions in fluorite ThO$_{2}$ can be represented as Th$^{3.834+}$ and O$^{0.452-}$. The phase transition from the fluorite to cotunnite structure is calculated to be at the pressure of 26.5 GPa, consistent with recent experimental measurement by Idiri \emph{et al}. \cite{Idiri}. For the cotunnite phase it is further predicted that an isostructural transition takes place in the pressure region of 80 to 130 GPa.