Towards an accurate and transferable charge transfer model in polarisable interatomic potentials
File(s)
Author(s)
Nemytov, Vadim
Type
Thesis
Abstract
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.
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.
Version
Open Access
Date Issued
2018-12
Date Awarded
2019-03
Copyright Statement
Creative Commons Attribution Non-Commercial No Derivatives licence
Advisor
Tangney, Paul
Haynes, Peter
Sponsor
Materials Design s.a.r.l.
Engineering and Physical Sciences Research Council
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)