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

The mechanical properties, electronic structure and phonon dispersion of ground state ThO₂ as well as the structure behavior up to 240 GPa are studied using first-principles density-functional theory. Our calculated elastic constants indicate that both the ground-state fluorite structure and high pressure cotunnite structure of ThO₂ are mechanically stable. The bulk modulus, shear modulus, and Young's modulus of cotunnite ThO₂ are all smaller by approximately 25% compared with those of fluorite ThO₂. The Poisson's ratios of both structures are approximately equal to 0.3 and the hardness of fluorite ThO₂ is 22.4 GPa. The electronic structure and bonding nature of fluorite ThO₂ are fully analyzed, and show that the Th-O bond displays a mixed ionic/covalent character. The phase transition from the fluorite to cotunnite structure is calculated to occur at the pressure of 26.5 GPa, consistent with recent experimental measurement by Idiri et al. [1]. For the cotunnite phase it is further predicted that an isostructural transition takes place in the pressure region of 80-130 GPa. Crown