PhD Theses - Samuel Murphy
Atomistic Simulation of Defects and Diffusion in Oxides
Abstract
Magnesium aluminate spinel, MgAl2O4, is an excellent model tertiary oxide which has been the subject to a significant number of experimental and theoretical studies since its structure was first reported by Bragg in 1915 [29]. One of the defining characteristics of this material is its ability to tolerate a significant degree of inversion between the cation sublattices: that is, magnesium ions, which Bragg allocated to the tetrahedral sublattice can become exchanged with aluminium ions that he assigned to the octahedral sublattice. This apparently simple modification has profound affects on the physical properties of this material. The presence of antisite defects can affect both the concentrations of other intrinsic point defects as well as cation migration energies. Antisite defects can also accommodate MgO or Al2O3 excess in nonstoichiometric spinel. There are three principle factors that can affect transport of cation species in spinel, these are: (i) the concentration and hence availability of the mediating point defect, (ii) the activation energy for an isolated ‘hop’ and (iii) the diffusion prefactor, D0. This prefactor contains information such as the number of equivalent pathways, jump distances and the attempt frequency. In this thesis atomistic simulation is used to determine the structures and relative concentrations of the isolated intrinsic point defects as well as the diffusion migration energies energies and prefactors. This allows a thorough understanding of the kinetic and thermodynamic processes occurring in a spinel (assuming defect concentrations are very low, ie. at the dilute limit). In order to extend this study to cover real materials, the accelerated dynamical technique, TAD, was applied to point defects that are clustered to oppositely charged point defects, such as those responsible for nonstoichiometry. The results demonstrate the immense complexity that can arise from the formation of defect clusters but allow some overarching conclusions to be ii bstract drawn concerning the affect of defect clustering on ion diffusion. Atomistic simulation was also used to examine isovalent cation substitution into BO oxides. This allowed the construction of a general relationship between the difference in size of the host and substitutional cations.