Dr. Anna Kimmel


National Physical Laboratory
Hampton Road Teddington UK
TW11 0LW
and
Department of Physics and Astronomy
University College London
Gower Street London UK
WC1E 6BT


E-mail: a.kimmel@ucl.ac.uk


RESEARCH INTERESTS


My research interests include modelling materials properties using Density Functional Methods, classical potentials and multiscale methods;
Development of ab initio random structure searching technique for multiphase description of complex multiphase, ceramic compounds and design and prediction of new materials;
Studying atomic-scale mechanisms of defect stability, defect induced chemical reactions;
Interfacial phenomena: interfacial chemistry and defects formation, charge depletion and transfer, 2DEG at interfaces;
Development of force fields for large scale modelling of domain dynamics in ferroelectric hetero-structures, parameterisation of screening, strain, domain morphologies at functional interfaces.




Multiphase description of complex multiphase, ceramic compounds

Complex perovskite solid solution materials are of great interest for actual or potential uses, based on their exceptional piezoelectric properties. Among them lead zirconate titanate compound Pb(Zr1−xTix)O3 (PZT) exhibits an exceptionally high piezoelectric response thus has been intensively used in various industrial applications. Despite the long history of experimental and theoretical studies the structure of this material is still under intensive discussion. We have applied structure searching coupled with Density Functional Theory methods to provide a multiphase description of this material. We demonstrate that the permutational freedom of B-site cations leads to the stabilisation of a variety of local phases reflecting the relatively flat energy landscape of PZT. Using a set of predicted local phases we reproduce the experimental pair distribution function (PDF) profile with a high accuracy. We introduce a complex multiphase picture of the structure of PZT and show that additional monoclinic and rhombohedral phases account for better description of the experimental PDF profile. We propose that such a multiphase picture reflects the entropy reached in the sample during preparation process. Our approach could provide an insight into the structural definition of complex multiphase, functional ceramics (as electro-caloric, piezoelectric, magneto-electric compounds); materials with dopants that display fractional site-occupancy; compounds that exhibit degradation of properties or fatigue due to the formation of ad-phase, or phase separation.

A. Bogdanov, A. Mysovsky, C. Pickard, A. V. Kimmel PCCP 18, 28316 (2016).
A. Bogdanov, A. Mysovsky, A. V. Kimmel Ferroelectrics (2016).


Parametrisation of force field for large scale modelling of domain dynamics in ferroelectric hetero-structures

Understanding of the properties of solid solution compounds at an atomistic level is particularly important for the design of high performance piezoelectric materials. Density functional theory (DFT) calculations being extensively applied to various ferroelectric materials provides fundamental insight into the origin of ferroelectricity, domain walls behaviour, grain boundaries and interfacial phenomena, however, modelling solid solution systems (in particular, PZT) with DFT is challenging: the disorder of B-cations creates a statistical gap between small DFT accessible systems and the system size necessary to fully capture the effects of the disorder. To bridge this statistical gap we use of large scale atomistic simulations with interatomic potentials based on the shell model, which are computationally efficient method for studying properties of ferroelectric systems.
We have developed a shell model force-field that reproduces the details of phase diagram of the PZT solid solution compound including the low and high temperature phases of PbZrO3 and PbTiO3. The developed force-field supports the temperature induced phase transitions from cubic to low symmetry phases over the whole composition range and additionally reproduces the composition driven phase transitions.

O. Gindele, D. Duffy, A. V. Kimmel “Shell model for PbZrO3, PbTiO3 and Pb(ZrxTi1-x)O3 ferroelectric compounds”, J. Phys. Chem. Lett. 4, 333 (2015).


Large scale modelling of domain morphology and dynamics in ferroelectric materials

The insight into the atomistic mechanisms of formation of dense domain morphologies is fundamentally and industrially important for the next generation of emergent technologies that aim to develop concepts to integrate and exploit exotic and novel phenomena in dimensionally constrained materials. Ferroelectric films exhibit a delicate balance of the screening of depolarising field, orientation of polarisation and strain. The misfit strain from growth substrates has been shown of promoting exotic polar orientations that are not observed in bulk.
We found that epitaxial strain induces the formation of a variety of closure- and in-plane domain morphologies. Remarkably, we identified a new nano-bubble domain morphology stable at high-temperature regime. We demonstrate that the formation mechanism of the novel nano-bubbled domain morphology is related to the wandering of flux closure domain walls related to the ferroelectric-antiferrodistortive coupling on the film surface.

J.B.J. Chapman, J. M. Gregg, A. Schilling, A.V. Kimmel, D.M. Duffy ”Novel Ferroelectric Nanobubble Domains in Strained Tetragonal Prototypical Perovskite Films” (2016) submitted.


Two-Dimensional Electron Gas Near SiO2/BaTiO3 Interfaces

Oxide interfaces are known to exhibit properties that are not found in the constituent materials when they are considered separately. At the nanoscale, the properties associated with interfaces often define the properties of entire nanostructures. The materials with new properties and functions can be fabricated by assembling complex hetero-structures, such as epitaxial thin films and core-shell (CS) nanoparticles. CS nanoparticles of BaTiO3 (BTO), in the form of nano-sized (300-500 nm) grains coated with ~5nm thick silica layer, demonstrated a dielectric constant over 105 and quite constant in a wide temperature range. These properties make the BTO-SiO2 nanoparticles promising for the applications that require high dielectric permittivity, such as solid-state energy storage and random access memory devices.
An atomistic model of SiO2/BaTiO3 interface was constructed using ab initio molecular dynamics. Analysis of its atomistic structure and electronic properties reveals the closure of the band gap at the stoichiometric SiO2/BaTiO3 interface that is significantly smaller than that of the bulk BaTiO3 and SiO2. The interface contains high concentration of oxygen vacancies in the outermost TiO2 plane of the BaTiO3. The formation of Si-O-Ti bonds between silica and BTO results in breaking Si-O-Si and Ti-O-Ti bonds and subsequent rearrangement of the atoms. This structure gives rise to the interface polar region with positive and negative charges localized in the BaTiO3 and SiO2 parts of the interface, respectively. The high dielectric response, observed experimentally in the SiO2-coated nanoparticles of BaTiO3, was proposed to be due to the electron gas formed in oxygen-deficient BaTiO3 and localized in the vicinity of the polar interface.

A. V. Kimmel, P. V. Sushko “Mechanism of formation of chemical bonds and defects at SiO2/BaTiO3 interfaces”, J. Phys. Cond. Matt. 24(47) 475006 (2015).
A.V. Kimmel, J. Íñiguez, M.G. Cain, P.V. Sushko "Neutral and charged oxygen vacancies induce two-dimensional electron gas near SiO2/BaTiO3 interfaces" J. Phys. Chem. Lett. 4, 333 (2013).


The effect of defects on properties of ferroelectric perovskites

In ferroic materials, unwanted changes of the ferroelectric, dielectric and piezoelectric properties have been attributed to the presence of defects. Oxygen vacancies, being the most abundant defects in oxide materials, are known to induce local perturbation of the lattice, affect dielectric properties, conductivity, polarization, and overall lifetime of ferroelectric devices. A mechanism by which an oxygen vacancy in a tetragonal ferroelectric material can be stabilized by local rotation of polar axis was found. It was suggested that this mechanism is dominant in materials with slow oxygen vacancy diffusion and low formation energy of 90 domain walls.

A.V. Kimmel, P. Weaver, M.G. Cain, P.V. Sushko "Defect-mediated lattice relaxation and domain stability in ferroelectric oxides" Phys. Rev. Lett. 117601, 109 (2012).


Role of charged defects and electronic excitations in stability of molecular crystals

Many organic molecules in condensed phase form molecular crystals (MC), which differ considerably by their optical, electronic and mechanic properties from conventional solids such as covalent or ionic crystals. In MC the bonded intra-molecular interactions are considerably stronger than the weak non-bonded inter-molecular interactions, therefore the presence of structural and/or electronic defects significantly affects the stability of these materials. Due to considerably large primitive unit cells of these materials it is a challenge to perform ab initio modelling of structural, electronic defects, and electronic excitations. The hybrid Embedded Cluster scheme was developed to model polarons in several types of MC. A comprehensive study of initial stages of decomposition of molecular crystals has been performed. An important role of charge trapping and electronic excitations in decomposition process was demonstrated.

A.V. Kimmel, P.V. Sushko, M.M. Kuklja "The structure and decomposition chemistry of cis- trans-isomers in a crystalline DADNE" Journal of Energetic Materials 28 Suppl. 1, 128-139 (2010).
A.V. Kimmel, D. Munoz Ramo, P.V. Sushko, A.L. Shluger, M.M. Kuklja "Modeling proton transfer and polarons in a molecular crystal diamino-dinitroethylene" Phys. Rev. B, 80, 134108 (2009).
A.V. Kimmel, P.V. Sushko, M.M. Kuklja, A.L. Shluger, "Role of hydrogen migration in stability prediction of molecular crystals diamino-dinitroethylene and triamino-trinitrobenzene" J. Phys. Chem. A, 112 (19), 4496 (2008).
A.V. Kimmel, P.V. Sushko, M.M. Kuklja, A.L. Shluger "An effect of charged and excited states on the decomposition of 1,1-diamino-2,2-dinitroethylene molecules" J. Chem. Phys. 126 (23), 234711 (2007).


Electronic defects in silica glass

Studies of the radiation-induced defects present high interests in terms of formation processes, stability, diffusion, and evolution(or decomposition) mechanisms. Irradiation of the silica samples induces the formation of polarons, which further can be stabilised by being trapped by the impurities, intrinsic defects, or imperfections of the oxide structure. One of the extensively studied intrinsic defects is the oxygen deficient centre, V. It has been found to exist in various charge states (from -2 to +2). The interplay of lattice relaxation and electrostatic interactions was shown to stabilize the V2+ via creating specific topology defects.
Inhomogenities of amorphous matrix lead to creation of a specific class of electronic defects, such as self-trapped holes and electrons. The variation of structural parameters of disordered matter generates a favourable trapping potential profile for stabilisation of those defects. A predictive method for characterisation potential energy landscape of amorphous silica has been developed. The method presents technological importance since and allows a description, guidance, spectroscopic identification of vulnerable systems sites, which are most likely to form defects under irradiation with ultra-violet light or due to the charge injection.

A.V. Kimmel, A.L. Shluger "Back-projected E' centres in amorphous silica" J. Non-Cryst. Solids, 355(18-21), 1113 (2009).
K. McKenna, P.V. Sushko, A.V. Kimmel, D. Munoz Ramo and A.L. Shluger "Modelling of electron and hole trapping in oxides" Modelling and Simulation in Materials Science and Engineering, 17, 084004 (2009).
A.V. Kimmel, P.V. Sushko, A.L. Shluger, G.L. Bersuker "Positive and Negative Oxygen Vacancies in Amorphous Silica" ECS Transactions 19(2), 3 (2009).
A.V. Kimmel, P.V. Sushko, A.L. Shluger "Structure and Spectroscopic Properties of Trapped Holes in Silica" J. Non.-Cryst. Sol. 353 (5-7), 599 (2007).
A.M. Stoneham, J.L. Gavartin, A.L. Shluger, A.V. Kimmel, D. Munoz Ramo, H. M. Ronnow, G. Aeppli and C. Renner "Trapping, self-trapping and the polaron family" J. Phys.: Condens. Matter. 19(25), 255208 (2007).



INVITED TALKS

Fundamental Physics of Ferroelectrics, Jan 2015
Knoxville, USA
Defect properties in functional perovskites and at oxide interfaces

International conference on Advanced Nanomaterials, June 2014
Aveiro, Portugal
Functional interfaces: the role of chemical and electrostatic environment

CONFERENCE TALKS

13th Russia/CIS/Baltic/Japan Symposium on Ferroelectricity (RCBJSF13)
June 2016, Shimane, Japan
Multiphase modelling of PbZrTiO3 at morphotropic phase boundary

Study of Matter at Extreme Conditions (SMEC2015)
March 2015, Miami, USA
Ab initio structure search for new incommensurate phase of KNbO3

E-MRS-2014 spring meeting
May 2014, Lille, France
Defect induced 2DEG at oxide interfaces
Properties of metal perovskite interfaces
Ab initio structure search for functional perovskites

PIEZO2013
March 2013, Les Arch, France
Defect-mediated phase stability in ferroelectric oxides
Oxygen vacancies induced two-dimensional electron gas near SiO2BaTiO3 interfaces

PIEZO2011
February 2011, Sestriere, Italy
Oxygen vacancies in BaTiO3 and KNbO3: lattice response

11th Europhysical Conference on Defects in Insulating Materials (EURODIM2010)
July 2010, Pecs, Hungary
Atomistic initiation mechanisms in molecular crystals
Structure and spectroscopic properties of negatively charged defects in silica
The effect of neutral oxygen vacancies in ferroelectric BaTiO3

PUBLICATIONS

O. Gindele, C. Vecchini, P. Thompson, M. Cain, D. Duffy, A. V. Kimmel
Low Temperature Ferroelectric Behavior in Pb(Zr1−xTix)O3 (2016) submitted.
J.B.J. Chapman, J. M. Gregg, A. Schilling, A.V. Kimmel, D.M. Duffy
Novel Ferroelectric Nanobubble Domains in Strained Tetragonal Prototypical Perovskite Films (2016) submitted.

A. Bogdanov, A. Mysovsky, C. Pickard, A. V. Kimmel
Modelling the structure of Zr-rich Pb(Zr1-xTix)O3, x=0.4 with a multiphase approach PCCP 18, 28316 (2016).
A. Bogdanov, A. Mysovsky, A. V. Kimmel
Multiphase modelling of Pb(Zr1-xTix)O3 structure Ferroelectrics (2016), accepted.
A. V. Kimmel, P. V. Sushko
Mechanism of formation of chemical bonds and defects at SiO2/BaTiO3 interfaces, J. Phys. Cond. Matt. 24(47) 475006 (2015).
O. Gindele, D. Duffy, A. V. Kimmel
Shell model for PbZrO3, PbTiO3 and Pb(ZrxTi1-x)O3 ferroelectric compounds, J. Phys. Chem. Lett. 4, 333 (2015). A.V. Kimmel, J. Íñiguez, M.G. Cain, P.V. Sushko
Neutral and charged oxygen vacancies induce two-dimensional electron gas near SiO2/BaTiO3 interfaces, J. Phys. Chem. Lett. 4, 333 (2013).
A.V. Kimmel, P. Weaver, M.G. Cain, P.V. Sushko
Defect-mediated lattice relaxation and domain stability in ferroelectric oxides, Phys. Rev. Lett. 117601, 109 (2012).
A.V. Kimmel, P.V. Sushko, M.M. Kuklja
The structure and decomposition chemistry of cis- trans-isomers in a crystalline DADNE, Journal of Energetic Materials 28 Suppl. 1, 128-139 (2010).
A.V. Kimmel, D. Munoz Ramo, P.V. Sushko, A.L. Shluger, M.M. Kuklja
Modeling proton transfer and polarons in a molecular crystal diamino-dinitroethylene, Phys. Rev. B, 80, 134108 (2009).
A.V. Kimmel, A.L. Shluger
Back-projected E' centres in amorphous silica, J. Non-Cryst. Solids, 355(18-21), 1113 (2009).
K. McKenna, P.V. Sushko, A.V. Kimmel, D. Munoz Ramo, A.L. Shluger
Modelling of electron and hole trapping in oxides, Modelling and Simulation in Materials Science and Engineering, 17, 084004 (2009).
A.V. Kimmel, P.V. Sushko, A.L. Shluger, G.L. Bersuker
Positive and Negative Oxygen Vacancies in Amorphous Silica, ECS Transactions 19(2), 3 (2009).
A.V. Kimmel, P.V. Sushko, M.M. Kuklja, A.L. Shluger
Role of hydrogen migration in stability prediction of molecular crystals diamino-dinitroethylene and triamino-trinitrobenzene, J. Phys. Chem. A, 112 (19), 4496 (2008).
A.V. Kimmel, P.V. Sushko, M.M. Kuklja, A.L. Shluger
An effect of charged and excited states on the decomposition of 1,1-diamino-2,2-dinitroethylene molecules, J. Chem. Phys. 126 (23), 234711 (2007).
A.V. Kimmel, P.V. Sushko, A.L. Shluger
Structure and Spectroscopic Properties of Trapped Holes in Silica, J. Non.-Cryst. Sol. 353 (5-7), 599 (2007).
A.M. Stoneham, J.L. Gavartin, A.L. Shluger, A.V. Kimmel, D. Munoz Ramo, H. M. Ronnow, G. Aeppli and C. Renner
Trapping, self-trapping and the polaron family, J. Phys.: Condens. Matter. 19(25), 255208 (2007).
A. Kimmel, I. Gusenkov
Temperature evolution of the structure of liquid 3d transition metals: MD study, Physica B 382, 257 (2006).
A. V. Kimmel, V. Ladyanov, I. Gusenkov
Molecular dynamics investigations of the liquid nickel's structural properties, Proceeding of the International Summer School of Physics Enrico Fermi, p.521-529. Varenna, Italy (2004).