Sushko, Petr Valentinovich; (2000) Development and application of embedded cluster methodologies for defects in ionic materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The main objectives of the present Thesis were to develop a method and computer code for calculating the properties of point defects in insulators, and to study defects and the mechanisms of defect processes in the bulk and at surfaces of ionic crystals. We develop an embedded cluster method and a computer code, which allows us to treat a defect in a crystal quantum mechanically in the effective potential of the rest of the polarisable crystal. This method has several advantages with respect to existing methods: i) it can be applied to bulk and surface defects; ii) it allows application of several quantum-mechanical methods, including the Hartree-Fock method and different configuration interaction techniques, and methods based on the Density Functional Theory; iii) it allows one to calculate the spectroscopic properties of point defects, such as optical absorption, vibrational spectra and hyperfine interactions, taking into account the defect interaction with the rest of the crystal; iv) the atomic structure of the defect can be optimised self-consistently using an effective procedure. The new method and the computer code have been tested on a number of well-established systems. The method has been applied to study the electronic structure and properties of several defects, and to model the mechanisms of various surface processes, i) We have calculated the position of the top of the (001) MgO surface valence band with respect to the vacuum level and the energy levels of neutral and charged oxygen vacancies with respect to the top of the valence band and the vacuum level, ii) The study of the electronic structure of excitons at low coordinated oxygen sites of the MgO surface has demonstrated a significant dependence of the excitation energies on oxygen coordination. iii) A variety of geometric configurations of the [FeCln(CN)6-n]³⁻ and [FeCln(CN)6-n]⁴⁻ impurities in the bulk of NaCl were studied and the most stable configurations identified. iv) It was demonstrated that the accurate account of lattice polarisation is crucial for quantitative agreement of the optical absorption and luminescence energies with the experimentally observed properties of the Ce³⁺ centres in LiBaF₃. v) Modelling of the interaction of a silicon tip with the NaCl (001) surface has demonstrated that the tip-surface interaction can lead to transfer of surface ions to the tip and allowed us to establish the charge state of the transferred Cl ions. vi) The analysis of the mechanism of laser-induced desorption of positive Mg ions from the (001) MgO surface allowed us to elucidate the atomistic stages of the desorption process. The results of this Thesis have demonstrated the applicability of the method developed to a wide range of defect properties in ionic materials.

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