Quantum mechanical studies of the early actinide compounds

Abstract

This study involves the investigation of the early actinide systems using ab initio techniques based on density functional theory (DFT). It was motivated by: (i) the incomplete description of these systems using conventional DFT because they are strongly correlated, (ii) the usefulness of these systems in nuclear energy generation, (iii) the complexity that arises in experimentally studying these systems due to their inherent radioactive nature and (iv) their limited availability. The results obtained from this study are divided into two broad sections. The first comprises chapters 3 and 4 while the second comprises chapters 5 and 6. Thorium based compounds are studied in chapters 3 and 4. In the first section, the Hubbard U parameter is not necessary to accurately describe the electronic, elastic and mechanical properties of these systems. In the second, the inclusion of the Hubbard U parameter is shown to be paramount for the accurate description of most compounds considered. Chapter 3 presents the electronic, structural and bonding character of thorium based nitrides. We obtained the result that Th2N2NH, which is crystallographically equivalent to metallic Th2N3, is insulating. Chapter 4 demonstrates that the formation of a meta-stable thorium-titanium based alloy is plausible and also further information on bonding, electronic and elastic properties of the determined meta-stable alloy is provided. This has provided important new knowledge about these bulk systems. In Chapter 5 the DFT + U based study on Pa and its oxides is presented. The electronic, structural and bonding character of these systems was studied. We found that PaO2 is a Mott-Hubbard insulator with an indirect band gap of 3.48 eV within the generalized gradient approximation GGA + U. Chapter 6 discusses various actinide nitrides. We explored the electronic properties, elastic properties, lattice dynamics and the energetics of the various compounds using GGA + U. Also, we investigated the e ect of the Hubbard U parameter and magnetic configuration on these systems. The use of the DFT + U based method provides a rapid way to study strongly correlated systems, while other methods such as Hybrid functional,GW, DMFT+DFT, etc. are highly intensive, computationally speaking. Finally, the results obtained with the inclusion of this ad hoc parameter give a very good description of these systems.