Modelling of Trapped Hole Polarons in Oxides

This thesis presents Density Functional Theory (DFT) modelling results for the V- centre in MgO, and self-trapped holes in the bulk and at the surface of monoclinic (m-)ZrO2. These systems exhibit polaronic self-trapping of a hole, a situation with which conventional DFT struggles due to the self-interaction error (SIE). The V- centre in MgO comprises a hole polaron trapped at a Mg vacancy, and has been the subject of many experimental and theoretical studies. Hybrid DFT was applied within an embedded cluster scheme to investigate how the optical and paramagnetic properties of the centre are affected by the proportion of Hartree-Fock exchange present in the functional, and also to elucidate the nature of the transitions involved in the optical spectrum, which were found to be more complicated than previously thought. Semi-local DFT under periodic boundary conditions, augmented with the cancellation of non-linearity (CON) method to correct for the SIE, was used to determine whether holes self-trap in the bulk of m-ZrO2. Self-trapped holes were found to be stable on the sub-lattice of 3-coordinated (3-C) oxygens, with a trapping energy of 0.13 eV. These hole polarons are highly mobile, but di_use primarily in 2-C planes. Having identified the (-111), (111) and (001) as the three most prevalent facets of a model nanoparticle, the same method as described in the previous paragraph was applied to these surfaces. It was found that self-trapped holes are stable at all under-coordinated oxygen sites, but that they trap particularly strongly at 2-coordinated sites on the (-111) terrace, with a trapping energy of 1.18 eV, while still being relatively mobile. Two model monolayer steps were then constructed by intersecting the (-111) and (111) terraces, and found that trapping was less favourable near the step, in contrast with simpler oxides such as MgO and CaO