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Cui CX, Jiang JW. Molecular dynamics simulation for phase transition of CsPbI3 perovskite with the Buckingham potential. J Chem Phys 2024; 161:104501. [PMID: 39248238 DOI: 10.1063/5.0221731] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2024] [Accepted: 08/22/2024] [Indexed: 09/10/2024] Open
Abstract
The CsPbI3 perovskite is a promising candidate for photovoltaic applications, for which several critical phase transitions govern both its efficiency and stability. Large-scale molecular dynamics simulations are valuable in understanding the microscopic mechanisms of these transitions, in which the accuracy of the simulation heavily depends on the empirical potential. This study parameterizes two efficient and stable empirical potentials for the CsPbI3 perovskite. In these two empirical potentials, the short-ranged repulsive interaction is described by the Lennard-Jones model or the Buckingham model, while the long-ranged Coulomb interaction is summed by the damped shifted force method. Our molecular dynamics simulations show that these two empirical potentials accurately capture the γ ↔ β ↔ α and δ → α phase transitions for the CsPbI3 perovskite. Furthermore, they are up to two orders of magnitude more efficient than previous empirical models, owing to the high efficiency of the damped shifted force truncation treatment for the Coulomb interaction.
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Affiliation(s)
- Chuan-Xin Cui
- Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai Institute of Applied Mathematics and Mechanics, Shanghai Frontier Science Center of Mechanoinformatics, School of Mechanics and Engineering Science, Shanghai University, Shanghai 200072, People's Republic of China
| | - Jin-Wu Jiang
- Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai Institute of Applied Mathematics and Mechanics, Shanghai Frontier Science Center of Mechanoinformatics, School of Mechanics and Engineering Science, Shanghai University, Shanghai 200072, People's Republic of China
- Zhejiang Laboratory, Hangzhou 311100, People's Republic of China
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2
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Lim VY, Righetto M, Yan S, Patel JB, Siday T, Putland B, McCall KM, Sirtl MT, Kominko Y, Peng J, Lin Q, Bein T, Kovalenko M, Snaith HJ, Johnston MB, Herz LM. Contrasting Ultra-Low Frequency Raman and Infrared Modes in Emerging Metal Halides for Photovoltaics. ACS ENERGY LETTERS 2024; 9:4127-4135. [PMID: 39144815 PMCID: PMC11320646 DOI: 10.1021/acsenergylett.4c01473] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/31/2024] [Revised: 07/14/2024] [Accepted: 07/19/2024] [Indexed: 08/16/2024]
Abstract
Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3-x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and "liquid-like" Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose-Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.
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Affiliation(s)
- Vincent
J.-Y. Lim
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Marcello Righetto
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Siyu Yan
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Jay B. Patel
- Department
of Physics, King’s College London, London WC2R 2LS, United Kingdom
| | - Thomas Siday
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Benjamin Putland
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Kyle M. McCall
- Department
of Chemistry and Applied Biosciences, Institute of Inorganic Chemistry, ETH Zürich, Zürich 8093, Switzerland
- Empa-Swiss
Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland
| | - Maximilian T. Sirtl
- Department
of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstr. 11, 81377 Munich, Germany
| | - Yuliia Kominko
- Department
of Chemistry and Applied Biosciences, Institute of Inorganic Chemistry, ETH Zürich, Zürich 8093, Switzerland
- Empa-Swiss
Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland
| | - Jiali Peng
- Key Lab of
Artificial Micro- and Nano-Structures of Ministry of Education of
China, School of Physics and Technology, Wuhan University, Wuhan 430072, Hubei, China
| | - Qianqian Lin
- Key Lab of
Artificial Micro- and Nano-Structures of Ministry of Education of
China, School of Physics and Technology, Wuhan University, Wuhan 430072, Hubei, China
| | - Thomas Bein
- Department
of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstr. 11, 81377 Munich, Germany
| | - Maksym Kovalenko
- Department
of Chemistry and Applied Biosciences, Institute of Inorganic Chemistry, ETH Zürich, Zürich 8093, Switzerland
- Empa-Swiss
Federal Laboratories for Materials Science and Technology, Dübendorf 8600, Switzerland
| | - Henry J. Snaith
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Michael B. Johnston
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
| | - Laura M. Herz
- Department
of Physics, Clarendon Laboratory, University
of Oxford, Parks Road, Oxford OX1
3PU, United Kingdom
- Institute
for Advanced Study, Technical University
of Munich, Lichtenbergstrasse
2a, D-85748 Garching, Germany
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3
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Mattoni A, Argiolas S, Cozzolino G, Dell'Angelo D, Filippetti A, Caddeo C. Many-Body MYP2 Force-Field: Toward the Crystal Growth Modeling of Hybrid Perovskites. J Chem Theory Comput 2024. [PMID: 39066691 DOI: 10.1021/acs.jctc.4c00391] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/30/2024]
Abstract
Hybrid perovskites are well-known for their optoelectronic and photovoltaic properties. Molecular dynamics simulations allow the study of these soft and ionic crystals by including dynamical effects (e.g., molecular rotations, octahedra tilting, ionic diffusion and hysteresis), yet the high computational cost restricts the use of accurate ab initio forces for bulk or small atomic systems. Hence, great interest exists in the development of classical force-fields for hybrid perovskites of low and linear scaling computational cost, via both empirical methods and machine-learning. This work aims at extending the transferability of our MYP0 model, which has been successfully tailored to methylammonium lead iodide (MAPI) and applied to the study of molecular rotations, vibrations, diffusion of defects, and many other properties. The extended model, named MYP2, improves the description of inorganic or hybrid fragments and the processes of crystal formation while preserving a good description of bulk properties. More importantly, it allows for the direct simulation of the crystal growth of cubic MAPI from deposition of PbI and MAI precursors on the surfaces. Our findings pave the way toward classical force-fields able to model the microstructure evolution of hybrid perovskites and the crystalline synthesis from deposition in vacuo.
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Affiliation(s)
- Alessandro Mattoni
- CNR - Istituto Officina dei Materiali (IOM), Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
| | - Simone Argiolas
- CNR - Istituto Officina dei Materiali (IOM), Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
- Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
| | - Giacomo Cozzolino
- CNR - Istituto Officina dei Materiali (IOM), Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
| | - David Dell'Angelo
- CNR - Istituto Officina dei Materiali (IOM), Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
- Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
| | - Alessio Filippetti
- CNR - Istituto Officina dei Materiali (IOM), Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
- Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
| | - Claudia Caddeo
- CNR - Istituto Officina dei Materiali (IOM), Cagliari, Cittadella Universitaria, Monserrato (CA) 09042, Italy
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4
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Caicedo-Dávila S, Cohen A, Motti SG, Isobe M, McCall KM, Grumet M, Kovalenko MV, Yaffe O, Herz LM, Fabini DH, Egger DA. Disentangling the effects of structure and lone-pair electrons in the lattice dynamics of halide perovskites. Nat Commun 2024; 15:4184. [PMID: 38760360 PMCID: PMC11101661 DOI: 10.1038/s41467-024-48581-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2023] [Accepted: 05/06/2024] [Indexed: 05/19/2024] Open
Abstract
Halide perovskites show great optoelectronic performance, but their favorable properties are paired with unusually strong anharmonicity. It was proposed that this combination derives from the ns2 electron configuration of octahedral cations and associated pseudo-Jahn-Teller effect. We show that such cations are not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in these materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and molecular dynamics to contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds that are structurally similar but with the former lacking ns2 cations with the propensity to form electron lone pairs. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint of anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This highlights the role of structure in perovskite lattice dynamics, providing design rules for the emerging class of soft perovskite semiconductors.
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Affiliation(s)
- Sebastián Caicedo-Dávila
- Physics Department, TUM School of Natural Sciences, Technical University of Munich, Garching, Germany
| | - Adi Cohen
- Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot, Israel
| | - Silvia G Motti
- Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, UK
- School of Physics and Astronomy, Faculty of Engineering and Physical Sciences, University of Southampton, Southampton, UK
| | - Masahiko Isobe
- Max Planck Institute for Solid State Research, Stuttgart, Germany
| | - Kyle M McCall
- Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Zürich, Switzerland
- Laboratory for Thin Films and Photovoltaics, EMPA - Swiss National Laboratories for Materials and Technology, Dübendorf, Switzerland
- Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, USA
| | - Manuel Grumet
- Physics Department, TUM School of Natural Sciences, Technical University of Munich, Garching, Germany
| | - Maksym V Kovalenko
- Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Zürich, Switzerland
- Laboratory for Thin Films and Photovoltaics, EMPA - Swiss National Laboratories for Materials and Technology, Dübendorf, Switzerland
| | - Omer Yaffe
- Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot, Israel
| | - Laura M Herz
- Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, UK
- TUM Institute for Advanced Study, Technical University of Munich, Garching, Germany
| | - Douglas H Fabini
- Max Planck Institute for Solid State Research, Stuttgart, Germany.
- Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA.
| | - David A Egger
- Physics Department, TUM School of Natural Sciences, Technical University of Munich, Garching, Germany.
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5
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Leppert L. Excitons in metal-halide perovskites from first-principles many-body perturbation theory. J Chem Phys 2024; 160:050902. [PMID: 38341699 DOI: 10.1063/5.0187213] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2023] [Accepted: 12/19/2023] [Indexed: 02/13/2024] Open
Abstract
Metal-halide perovskites are a structurally, chemically, and electronically diverse class of semiconductors with applications ranging from photovoltaics to radiation detectors and sensors. Understanding neutral electron-hole excitations (excitons) is key for predicting and improving the efficiency of energy-conversion processes in these materials. First-principles calculations have played an important role in this context, allowing for a detailed insight into the formation of excitons in many different types of perovskites. Such calculations have demonstrated that excitons in some perovskites significantly deviate from canonical models due to the chemical and structural heterogeneity of these materials. In this Perspective, I provide an overview of calculations of excitons in metal-halide perovskites using Green's function-based many-body perturbation theory in the GW + Bethe-Salpeter equation approach, the prevalent method for calculating excitons in extended solids. This approach readily considers anisotropic electronic structures and dielectric screening present in many perovskites and important effects, such as spin-orbit coupling. I will show that despite this progress, the complex and diverse electronic structure of these materials and its intricate coupling to pronounced and anharmonic structural dynamics pose challenges that are currently not fully addressed within the GW + Bethe-Salpeter equation approach. I hope that this Perspective serves as an inspiration for further exploring the rich landscape of excitons in metal-halide perovskites and other complex semiconductors and for method development addressing unresolved challenges in the field.
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Affiliation(s)
- Linn Leppert
- MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands
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6
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Gelžinytė E, Öeren M, Segall MD, Csányi G. Transferable Machine Learning Interatomic Potential for Bond Dissociation Energy Prediction of Drug-like Molecules. J Chem Theory Comput 2024; 20:164-177. [PMID: 38108269 PMCID: PMC10782450 DOI: 10.1021/acs.jctc.3c00710] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2023] [Revised: 11/30/2023] [Accepted: 11/30/2023] [Indexed: 12/19/2023]
Abstract
We present a transferable MACE interatomic potential that is applicable to open- and closed-shell drug-like molecules containing hydrogen, carbon, and oxygen atoms. Including an accurate description of radical species extends the scope of possible applications to bond dissociation energy (BDE) prediction, for example, in the context of cytochrome P450 (CYP) metabolism. The transferability of the MACE potential was validated on the COMP6 data set, containing only closed-shell molecules, where it reaches better accuracy than the readily available general ANI-2x potential. MACE achieves similar accuracy on two CYP metabolism-specific data sets, which include open- and closed-shell structures. This model enables us to calculate the aliphatic C-H BDE, which allows us to compare reaction energies of hydrogen abstraction, which is the rate-limiting step of the aliphatic hydroxylation reaction catalyzed by CYPs. On the "CYP 3A4" data set, MACE achieves a BDE RMSE of 1.37 kcal/mol and better prediction of BDE ranks than alternatives: the semiempirical AM1 and GFN2-xTB methods and the ALFABET model that directly predicts bond dissociation enthalpies. Finally, we highlight the smoothness of the MACE potential over paths of sp3C-H bond elongation and show that a minimal extension is enough for the MACE model to start finding reasonable minimum energy paths of methoxy radical-mediated hydrogen abstraction. Altogether, this work lays the ground for further extensions of scope in terms of chemical elements, (CYP-mediated) reaction classes and modeling the full reaction paths, not only BDEs.
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Affiliation(s)
- Elena Gelžinytė
- Engineering
Laboratory, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, U.K.
| | - Mario Öeren
- Optibrium
Limited, Cambridge Innovation Park, Denny End Road, Cambridge CB25 9GL, U.K.
| | - Matthew D. Segall
- Optibrium
Limited, Cambridge Innovation Park, Denny End Road, Cambridge CB25 9GL, U.K.
| | - Gábor Csányi
- Engineering
Laboratory, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, U.K.
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7
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Mączka M, Ptak M, Gągor A, Zaręba JK, Liang X, Balčiu̅nas S, Semenikhin OA, Kucheriv OI, Gural’skiy IA, Shova S, Walsh A, Banys J, Šimėnas M. Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites. CHEMISTRY OF MATERIALS : A PUBLICATION OF THE AMERICAN CHEMICAL SOCIETY 2023; 35:9725-9738. [PMID: 38047186 PMCID: PMC10687860 DOI: 10.1021/acs.chemmater.3c02200] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 08/29/2023] [Revised: 10/31/2023] [Accepted: 10/31/2023] [Indexed: 12/05/2023]
Abstract
Hybrid organic-inorganic lead halide perovskites are promising candidates for next-generation solar cells, light-emitting diodes, photodetectors, and lasers. The structural, dynamic, and phase-transition properties play a key role in the performance of these materials. In this work, we use a multitechnique experimental (thermal, X-ray diffraction, Raman scattering, dielectric, nonlinear optical) and theoretical (machine-learning force field) approach to map the phase diagrams and obtain information on molecular dynamics and mechanism of the structural phase transitions in novel 3D AZRPbX3 perovskites (AZR = aziridinium; X = Cl, Br, I). Our work reveals that all perovskites undergo order-disorder phase transitions at low temperatures, which significantly affect the structural, dielectric, phonon, and nonlinear optical properties of these compounds. The desirable cubic phases of AZRPbX3 remain stable at lower temperatures (132, 145, and 162 K for I, Br, and Cl) compared to the methylammonium and formamidinium analogues. Similar to other 3D-connected hybrid perovskites, the dielectric response reveals a rather high dielectric permittivity, an important feature for defect tolerance. We further show that AZRPbBr3 and AZRPbI3 exhibit strong nonlinear optical absorption. The high two-photon brightness of AZRPbI3 emission stands out among lead perovskites emitting in the near-infrared region.
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Affiliation(s)
- Mirosław Mączka
- Institute
of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland
| | - Maciej Ptak
- Institute
of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland
| | - Anna Gągor
- Institute
of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland
| | - Jan K. Zaręba
- Institute
of Advanced Materials, Faculty of Chemistry, Wrocław University of Science and Technology, 50-370 Wrocław, Poland
| | - Xia Liang
- Department
of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
| | | | - Oleksandr A. Semenikhin
- Department
of Chemistry, Taras Shevchenko National
University of Kyiv, 64 Volodymyrska St., Kyiv 01601, Ukraine
| | - Olesia I. Kucheriv
- Department
of Chemistry, Taras Shevchenko National
University of Kyiv, 64 Volodymyrska St., Kyiv 01601, Ukraine
| | - Il’ya A. Gural’skiy
- Department
of Chemistry, Taras Shevchenko National
University of Kyiv, 64 Volodymyrska St., Kyiv 01601, Ukraine
| | - Sergiu Shova
- Department
of Inorganic Polymers, Petru Poni Institute
of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41-A, Iasi 700487, Romania
| | - Aron Walsh
- Department
of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
- Department
of Physics, Ewha Womans University, Seoul 03760, Korea
| | - Ju̅ras Banys
- Faculty
of Physics, Vilnius University, LT-10257 Vilnius, Lithuania
| | - Mantas Šimėnas
- Faculty
of Physics, Vilnius University, LT-10257 Vilnius, Lithuania
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8
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Liang X, Klarbring J, Baldwin WJ, Li Z, Csányi G, Walsh A. Structural Dynamics Descriptors for Metal Halide Perovskites. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2023; 127:19141-19151. [PMID: 37791100 PMCID: PMC10544022 DOI: 10.1021/acs.jpcc.3c03377] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/19/2023] [Revised: 08/03/2023] [Indexed: 10/05/2023]
Abstract
Metal halide perovskites have shown extraordinary performance in solar energy conversion technologies. They have been classified as "soft semiconductors" due to their flexible corner-sharing octahedral networks and polymorphous nature. Understanding the local and average structures continues to be challenging for both modeling and experiments. Here, we report the quantitative analysis of structural dynamics in time and space from molecular dynamics simulations of perovskite crystals. The compact descriptors provided cover a wide variety of structural properties, including octahedral tilting and distortion, local lattice parameters, molecular orientations, as well as their spatial correlation. To validate our methods, we have trained a machine learning force field (MLFF) for methylammonium lead bromide (CH3NH3PbBr3) using an on-the-fly training approach with Gaussian process regression. The known stable phases are reproduced, and we find an additional symmetry-breaking effect in the cubic and tetragonal phases close to the phase-transition temperature. To test the implementation for large trajectories, we also apply it to 69,120 atom simulations for CsPbI3 based on an MLFF developed using the atomic cluster expansion formalism. The structural dynamics descriptors and Python toolkit are general to perovskites and readily transferable to more complex compositions.
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Affiliation(s)
- Xia Liang
- Department
of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
| | - Johan Klarbring
- Department
of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
- Department
of Physics, Chemistry and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden
| | - William J. Baldwin
- Department
of Engineering, University of Cambridge, Cambridge CB2 1PZ, U.K.
| | - Zhenzhu Li
- Department
of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
| | - Gábor Csányi
- Department
of Engineering, University of Cambridge, Cambridge CB2 1PZ, U.K.
| | - Aron Walsh
- Department
of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
- Department
of Physics, Ewha Womans University, Seoul 03760, Korea
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