Ab initio study of the effect of charge localisation on the properties of defects in magnesium oxide and zirconolite

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

Modelling the effects of electronic excitations in ionic-covalent materials

High energy radiation events in ionic and covalent materials can lead to highly excited electronic configurations which, over time, relax to the ground state, either radiatively by emitting photons, or non-radiatively. Non-radiative relaxation involves the transfer of energy to the lattice and this can result in lattice heating, defect formation or even phase changes. The effects of the relaxation mechanisms on the atomic configuration are challenging to model accurately by standard methods. The situation is further complicated by interactions between electronic excitations and pre-existing defects, possibly created by other radiation events. In this paper we describe a range of mechanisms by which the electronic energy is transferred to the lattice and the resulting effects on the atomic configuration, along with the different techniques that are used to model these effects

Trapping of He in intrinsic defects in zirconolite

Zirconolite (CaZrTi2O7) is a proposed ceramic for the disposal of plutonium. Density functional theory with the dispersion correction (DFT-D3) has been used to study the behaviour of the He defect in zirconolite. The lowest energy He interstitial site is located in the channels and found to have a migration barrier of 1.46 eV. There was a significant charge state dependence on the binding energies of a He atom to the vacancies, with the neutral 5-fold coordinated Ti having the strongest binding followed by the Ca vacancies. Multiple He interstitials were studied to examine if He bubbles were likely to form in bulk zirconolite. It was found that it was unfavourable for He to cluster at the concentrations studied. © 2013 Elsevier B.V. All rights reserved