Ab initio study of transition metal carbides and actinide compounds

Abstract: Two classes of materials are investigated using ab intio methods based on density functional theory. The structural properties, electronic structure and thermodynamic properties of binary and ternary transition metal carbides are discussed in details. In addition, two actinide compounds will be presented. A new actinide monoxide, ThO, is predicted to be stable under pressure, and the weakly correlated UN is investigated as regards to its magnetic properties and electronic structure.The atomic and electronic structures of various types of single defects in TiC such as vacancies, interstitial defects, and antisite defects are investigated systematically. Both the C-poor and C-rich off-stoichiometric Ti1-cCc composition (0.49≤c≤0.51) have been studied. For the electronic structure, the difference of density of states (dDOS) is introduced to characterize the changes produced by the defects. Concerning the atomic structures, both interstitial defects and antisites are shown to induce the formation of C dumbbells or Ti dumbbells. To date, the Ti self-diffusion mechanism in TiC has not been fully understood, and particularly the Ti diffusion is much less studied in comparison with the C diffusion. Therefore, the self-diffusion of Ti in sub-stoichiometric TiC is studied, and the formation energies, migration barriers for Ti interstitials, dumbbells and dumbbell-vacancy clusters are reported. Some of the calculated activation energies are close to the experimental values, and the migration of Ti dumbbell terminated by C vacancies gives the lowest activation energy, which is in good agreement with the experimental data. These studies must be continued to obtain a full description (including phonon contributions, prefactors, etc.) of all the feasible diffusion mechanisms in TiC.The focus is then shifted from the light transition metal carbides to the heavy transition metal carbides. Various structures of Ru2C under ambient conditions are explored by using an unbiased swarm structure searching algorithm. The structures with R3m (one formula unit) and R-3m symmetry (two formula units) have been found to be lower in energy than the P-3m1 structure, and also to be dynamically stable at ambient conditions. The R-3m structure is characterized by emergence of the Ru-Ru metallic bonding, which has a crucial role in diminishing the hardness of this material.The study of correlation and relativistic effects in Ta2AlC is also presented. We have shown that going from a scalar relativity to a fully relativistic description does not have a significant effect on the computed electronic and mechanical properties of Ta2AlC. In addition, the calculations show that the structural and mechanical properties of Ta2AlC are strongly dependent on other details of theoretical treatment, such as the value of the Hubbard U parameter. The comparison between our results and experimental data point to that Ta2AlC is a weakly correlated system, which originates from that the 5d band is relatively wide in comparison with that of the 3d band.The existence of a rock salt Thorium monoxide (ThO) under high pressure is theoretically predicted. A chemical reaction between Th and ThO2 can produce a novel compound thorium monoxide under sufficient external pressure. To determine the pressure range where this reaction can be observed, we have identified two extreme boundaries by means of different theoretical approaches. The first one is given by a fully relativity DFT code in local density approximation (LDA). The second one is given by a scalar relativistic DFT code in generalized gradient approximation (GGA). It is found that ThO is energetically favored between 14 and 26 GPa. The f orbitals are filled at the expense of s and d electrons states of Th metal, under the action of pressure. The d-p hybridization leads to the stability of metallic ThO. Dynamical stability is also investigated by computing the phonon dispersions for the considered structures at high pressure.The electronic structure and magnetic properties of a promising nuclear fuel material, uranium mononitride (UN), are studied by means of density functional theory (DFT) and several extensions, such as dynamical mean-field theory (DMFT), disordered local moment (DLM) approach, and the GW method. The role of the relativistic corrections is analyzed for different levels of approximation. The importance of correlation effects is assessed through a detailed comparison between calculated electronic structure and measured photoemission spectrum, which helps to clarify the dual itinerant/localized nature of the 5f states of U in UN. Important effects are also observed for the 2p states of nitrogen, which are positioned at much lower energies that are difficult to be well treated in the conventional electronic structure calculations.