First-principles investigation of electronic, vibrational, elastic, and structural properties of ThN and UN up to 100 GPa

We have investigated the electronic properties, phonon dispersion relations, elastic constants, structural phase transitions, and pressure-volume equations-of-state of thorium (Th) and uranium (U) mononitrides (ThN and UN) under pressure (0{-}100 GPa) using pseudopotential density functional theoretical methods. The generalized gradient approximation (GGA) is found to describe the ground-state and high-pressure experimental data much better than the local density approximation (LDA) for both compounds. ThN shows acoustic mode phonon softening along the {Γ}{-}X direction of the Brillouin zone in the NaCl phase under pressure, followed by a transition to a CsCl structure at 72.5 GPa. Detailed electronic structure analysis revealed an electronic topological transition under pressure that could be responsible for acoustic mode phonon softening and structural phase transition. Unlike ThN, UN shows a structural phase transition from an NaCl to R-3m structure at a much smaller pressure (18 GPa), and the calculated C₄₄ shear elastic constant decreases with pressure and becomes negative at 15 GPa. A Peierls-like distortion, due to f states, is found responsible for elastic instability and structural phase transition. Our results are in reasonably good agreement with available experimental data. We have also tested the effect of on-site Coulomb interactions on a few ground-state properties and on the phase-transition behavior of UN.