Mulroue, J; (2013) Ab initio study of the effect of charge localisation on the properties of defects in magnesium oxide and zirconolite. Doctoral thesis , UCL (University College London).

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Abstract

The localisation of excited electrons on defects in ceramic materials has a significant effect on the evolution of damage resulting from irradiation. The localisation of charge on a defect will change the charge state of that defect, which will affect the position of the defect level and change the defect properties. In ceramic materials for encapsulating radioactive waste the alpha decay of the actinide results in the accumulation of helium within the lattice, which will affect the durability of the waste and alter the performance of the waste form. DFT was used to study the structure and mobility of defects in different charge states for two ceramic materials. MgO was used as a model oxide due to the simple crystal structure. It was found that the charge state has a significant effect on the structure and mobility of the oxygen defects. The localisation of a hole onto the O2- interstitial significantly reduces its migration barrier. The effect of charge localisation on a hexa-interstitial cluster was investigated and it was found that the charge state affects the migration barriers, with the singly-charged cluster again having the lowest migration barrier. Zirconolite, a proposed encapsulation matrix for plutonium, was also studied. The monoclinic crystal structure comprises of layers of alternating 5 and 6 coordinated Ti-O polyhedra, separated by layers of alternating Ca and Zr ions. The structures of intrinsic defects, in different charge states, were studied and a significant effect on the defective structure of Zr and Ti vacancies was observed. Ab initio random structure searching was used to identify the lowest energy interstitial site for each species. DFT-D3 was used to study the structure, mobility and binding of a He atom in zirconolite. It was found that the neutral 5-fold coordinated Ti vacancy was the strongest binding site.

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