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Title: Towards an accurate and transferable charge transfer model in polarisable interatomic potentials
Author: Nemytov, Vadim
ISNI:       0000 0004 7659 1660
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Understanding the properties of defects, interfaces and other heterogeneities in metal oxides is crucial for enabling new applications. Some properties can be explored in a computer simulation using classical interatomic potentials (IPs) - analytic functions of nuclear positions that provide interatomic forces. In this thesis I have focused on developing, testing and analysing an ionic IP which combines variable-charge (QEq) and polarisable-ion (PI) models, allowing ions' charges and induced dipole moments to vary in response to their changing environments. We aim to improve the limited transferability to heterogeneous environments of the current state-of-the-art, PI-based, IPs. For several bulk and heterogeneous systems, past applications of QEq-based IPs showed no improvement over fixed-charge IPs - at variance with common expectations. Two reasons for this have been suggested in our work - in some systems ionic charges are essentially-constant, in other systems the bonding doesn't adhere well to the limit of the QEq model. Using density functional theory and the QEq-PI IP we calculated and analysed the charge distributions and interatomic forces in BaTiO3 at finite temperature, in a pristine crystal and near neutral O (VO) and Ti (VTi) vacancies. Ionic charges were found to be essentially-constant in the defect-free BaTiO3, but to deviate from their bulk values near the vacancies. The improvement of QEq-PI IP over fixed-charge IP was greater for the system with VO than with VTi, which was explained by the bonding near VO adhering better to the limit of this model. For systems with VO the QEq-PI IP demonstrated a marked improvement in accuracy and transferability over the fixed-charge PI IP; this is the first time, to our knowledge, that a clear improvement over PI IPs has been demonstrated.
Supervisor: Tangney, Paul ; Haynes, Peter Sponsor: Materials Design s.a.r.l. ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral