Wellington, Joseph Paul William; (2017) Applications of the periodic electrostatic embedded cluster method to solid state actinide chemistry. Doctoral thesis , UCL (University College London).

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Abstract

The work described in this thesis uses density functional theory (DFT) with an embedded cluster method, known as the periodic electrostatic embedded cluster method (PEECM) to study solid state actinide systems. The theoretical background of electronic structure calculations is discussed in the first chapter, while the remaining chapters deal with results of the studies. In Chapter 2 the PEECM is used to include long-range electrostatic interactions in calculations of Quantum Theory of Atoms in Molecules (QTAIM) bond critical point and delocalisation index metrics for the actinide-element bonds in Cs2UO2Cl4, U(Se2PPh2)4 and Np(Se2PPh2)4. The effects of the environment are seen to be minor, suggesting they do not account for the differences previously observed between the experimental and theoretical QTAIM data. In Chapter 3 the electronic structure of actinide dioxide systems has been investigated by examining the projected density of states (PDOS). While PBE incorrectly predicts these systems to be metallic, PBE0 finds them to be insulators, with the composition of the valence and conduction levels agreeing well with experiment. In Chapter 4 molecular and dissociative water adsorption on the (111) and (110) surfaces of UO2 and PuO2 has been investigated, with that on the (110) surface being stronger than on the (111). Similar energies are found for molecular and dissociative adsorption on the (111) surfaces, while on the (110) there is a clear preference for dissociative adsorption. Adsorption energies and geometries on the (111) surface of UO2 are in good agreement with recent periodic DFT studies using the GGA+U approach. In Chapter 5 oxygen vacancies are investigated on the actinide oxide surfaces. Oxygen vacancy formation energies are found to be much greater on UO2 than PuO2 surfaces. Oxygen vacancies lead to a preference for dissociative adsorption of water on both the (111) and (110) surfaces, with adsorption energies being much greater on PuO2 than UO2 surfaces.

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