研究者詳細 - 横井　達矢

論文 - 横井　達矢

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Grain-boundary thermodynamics with artificial-neural-network potential: Its ability to predict the atomic structures, energetics, and lattice vibrational properties for Al

Yokoi Tatsuya, Matsuura Maya, Oshima Yu, Matsunaga Katsuyuki

PHYSICAL REVIEW MATERIALS 7 巻 ( 5 ) 2023年5月

出版者・発行元：Physical Review Materials

An artificial neural network (ANN) potential for Al, trained with density-functional-theory (DFT) data, is constructed to accurately predict lattice vibrational properties and thermodynamics of grain boundaries (GBs) in Al. The ANN potential is demonstrated to accurately predict not only atomic structures and energetics of the GBs at 0 K but also partial phonon densities of states and vibrational entropies, even for GBs absent in the training data sets. In addition, their total potential energies and atomic forces by DFT at elevated temperatures up to 800 K can also be well reproduced by molecular dynamics with the ANN potential. In contrast, a modified embedded atom method (MEAM) potential shows larger errors in phonon frequencies and atomic forces for atoms at GBs, as well as in the bulk, than the ANN potential. The MEAM potential is thus likely to be inadequate to quantitatively predict thermodynamic properties of GBs, particularly at high temperature. The present ANN potential is also applied to systematically examine thermodynamic stability of asymmetric tilt GBs. It is predicted that for the ς9 system, the GB free-energy profile as a function of inclination angle exhibits a cusp at elevated temperatures, due to its larger vibrational entropies of asymmetric tilt GBs than those of ς9 symmetric tilt GBs.

DOI： 10.1103/PhysRevMaterials.7.053803

Web of Science

Scopus

Atomic and electronic structure of grain boundaries in a-Al2O3: A combination of machine learning, first-principles calculation and electron microscopy

Yokoi T., Hamajima A., Wei J., Feng B., Oshima Y., Matsunaga K., Shibata N., Ikuhara Y.

SCRIPTA MATERIALIA 229 巻 2023年5月

出版者・発行元：Scripta Materialia

To accurately determine the atomic and electronic structures of symmetric tilt grain boundaries (GBs) in α-Al2O3, this work employed an artificial-neural-network (ANN) interatomic potential, density-functional-theory (DFT) calculation and scanning transmission electron microscopy (STEM) observation. An ANN-based simulated annealing method was demonstrated to efficiently screen candidate low-energy structures with reasonably high accuracy. For Σ7 and Σ31GBs with the [0001] tilt axis, which were absent in the training datasets for the ANN potential, their lowest-energy structures predicted from ANN and DFT calculations were in quantitative agreement with STEM images in terms of both Al- and O-column positions. The exact GB structures have enabled us to analyze quantitatively the relationship between their atomic and electronic structure. This work will be an important model case where a combination of machine-learning, theoretical calculation and experiment has successfully solved the problem of determining complicated GB structures and their electronic structures in α-Al2O3.

DOI： 10.1016/j.scriptamat.2023.115368

Web of Science

Scopus

Atomic structures of grain boundaries for Si and Ge: A simulated annealing method with artificial-neural-network interatomic potentials (vol 173, 111114, 2023) Open Access

Yokoi Tatsuya, Kato Hirotaka, Oshima Yu, Matsunaga Katsuyuki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 176 巻 2023年5月

Ca-vacancy effect on the stability of substitutional divalent cations in calcium-deficient hydroxyapatite

Saito Tatsushi, Ishikawa Yuto, Noda Yusuke, Yokoi Tatsuya, Oshima Yu, Nakamura Atsutomo, Matsunaga Katsuyuki

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 106 巻 ( 2 ) 頁： 1587 - 1596 2023年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the American Ceramic Society

First-principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M2+ (M = Mg, Zn, Sr) in Ca-deficient hydroxyapatite (dHAp). M2+ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M2+ are lower in dHAp than in stoichiometric HAp (sHAp) at all M2+ concentrations. This indicates that these M2+ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca–O bond lengths over a wide area so that Ca–O polyhedrons with various sizes are produced. As a result, M2+ ions can predominantly occupy Ca sites at which M2+ fits better, depending on the ionic radii of M2+. For Zn2+ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp.

DOI： 10.1111/jace.18853

Web of Science

Scopus

Atomic structures of grain boundaries for Si and Ge: A simulated annealing method with artificial-neural-network interatomic potentials

Yokoi Tatsuya, Kato Hirotaka, Oshima Yu, Matsunaga Katsuyuki

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 173 巻 2023年2月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Physics and Chemistry of Solids

To accurately predict low-energy structures for symmetric tilt grain boundaries (GBs) in Si and Ge, artificial-neural-network (ANN) interatomic potentials are constructed and are combined with a simulated annealing (SA) method based on molecular dynamics simulations. The ANN-driven SA method is demonstrated to predict GB structures that are in good agreement with previous electron microscopy observations, without prior knowledge about their atomic configurations. Their GB energies also reasonably agree with density-functional-theory (DFT) calculations. By contrast, a conventional empirical potential fails to predict those GB structures. For misorientation angles 2θ≥93.37°, the lowest-energy structures are found to contain atomic configurations that cannot be reproduced by one repeat unit of the perfect crystal along the tilt axis. Such GB structures cannot be obtained using the γ-surface method, although it is most commonly used for exploring low-energy GB structures. These results highlight the importance of using simulation cells with multiple repeat units along the tilt axis and of performing the SA method with high-accuracy interatomic potentials transferable to GBs.

DOI： 10.1016/j.jpcs.2022.111114

Web of Science

Scopus

DFT calculations of carrier-trapping effects on atomic structures of 30° partial dislocation cores in zincblende II-VI group zinc compounds

Hoshino S., Oshima Y., Yokoi T., Nakamura A., Matsunaga K.

Physical Review Materials 7 巻 ( 1 ) 2023年1月

掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review Materials

It was experimentally reported that II-VI group inorganic semiconductor crystals become harder and brittle by external light illumination. In order to reveal essential factors of the light illumination effect, systematic GGA+U calculations are performed for 30° partial dislocations in ZnSe and ZnTe, and the obtained results are discussed together with those for ZnS reported previously. It is found in these three crystal systems that the cores of pristine partial dislocations have unreconstructed atomic structures whereas their cores undergo atomic reconstruction energetically more favorably by trapping excess carriers. Such carrier trapping and atomic reconstruction of the dislocation cores are ascribed to the presence of excess electrostatic potentials at the cores due to ionic bonding characters of the host crystals. The dislocation core reconstructions decrease potential energies of the partial dislocations and can in turn increase Peierls-potential barriers for dislocation glide, corresponding to the observed hardening and brittleness by light illumination. The energy gains due to the dislocation-core reconstructions also depend on energy positions of the defect-induced levels that appear within the band gaps.

DOI： 10.1103/physrevmaterials.7.013603

Scopus

その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.7.013603/fulltext

Atomic structures and stability of finite-size extended interstitial defects in silicon: Large-scale molecular simulations with a neural-network potential Open Access

Ohbitsu M., Yokoi T., Noda Y., Kamiyama E., Ushiro T., Nagakura H., Sueoka K., Matsunaga K.

Scripta Materialia 214 巻 2022年6月

記述言語：日本語出版者・発行元：Scripta Materialia

The energetics of various extended interstitial (I) defects in crystalline Si is examined by constructing an artificial-neural-network (ANN) potential trained with density-functional-theory (DFT) data, enabling us to perform accurate large-scale simulations and to obtain well-converged formation energies (Ef). By varying the number of interstitials n to around 1,000, Ef is calculated for the compact cluster, I12-like, (001)-plane, (311)-rod-like and Frank-loop defects. For n ≤ 36, the compact cluster or (311)-rod-like defect is found to be most stable, depending on n. This trend strongly depends on simulation cell sizes, suggesting the importance of sufficiently large cells. For 36 < n ≲ 860, the (311)-rod-like defect is most stable whereas the Frank-loop defect becomes most stable for larger n. The ANN potential is demonstrated to outperform empirical potentials in prediction of Ef and defect structures. Furthermore, ANN values of Ef are fitted to analytic functions with the aim of refining macroscopic simulations for device manufacturing processes.

DOI： 10.1016/j.scriptamat.2022.114650

Scopus

First-Principles Analysis of the Oxide-Ion Conduction Mechanism in Si-Deficient Lanthanum Silicate Apatite

Ogura Yusuke, Yokoi Tatsuya, Fujii Kotaro, Yashima Masatomo, Matsunaga Katsuyuki

JOURNAL OF PHYSICAL CHEMISTRY C 126 巻 ( 13 ) 頁： 5805 - 5812 2022年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Physical Chemistry C

Si-deficient lanthanum silicate apatites (La9.33+0.67xSi6-0.5xO26, LSOs) have attracted attention for their high oxide-ion conductivities. However, the carrier species and the conduction pathways responsible for the oxide-ion conduction are still unclear. In this study, the conduction mechanism of oxide ions in Si-deficient LSO was studied by first-principles calculations. It was found that Si-deficient LSO can include a small amount of excess oxide ions, which are located at the interstitial site along the c axis. The excess oxide ions in Si-deficient LSO can rapidly diffuse along the c axis by the push-pull mechanism with a potential barrier of 0.35 eV, which involves cooperative movements of two neighboring oxide ions, O4 ions, and the excess oxide ions. The potential barrier height was in good agreement with the experimentally reported activation energy. It is shown that a small amount of excess oxygen contributes critically to oxide-ion conduction in Si-deficient LSO, as was also discussed in stoichiometric La9.33Si6O26. These findings indicate the existence of a mechanism whereby a small amount of excess carriers in the ion-conduction pathways can significantly change the energy profiles for ionic conduction and enhance ion conductivity.

DOI： 10.1021/acs.jpcc.1c10486

Web of Science

Scopus

Accurate prediction of grain boundary structures and energetics in CdTe: a machine-learning potential approach

Yokoi Tatsuya, Adachi Kosuke, Iwase Sayuri, Matsunaga Katsuyuki

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 24 巻 ( 3 ) 頁： 1620 - 1629 2022年1月

記述言語：日本語出版者・発行元：Physical Chemistry Chemical Physics

To accurately predict grain boundary (GB) atomic structures and their energetics in CdTe, the present study constructs an artificial-neural-network (ANN) interatomic potential. To cover a wide range of atomic environments, large amounts of density functional theory (DFT) data are used as a training dataset including point defects, surfaces and GBs. Structural relaxation combined with the trained ANN potential is applied to symmetric tilt and twist GBs, many of which are not included in the training dataset. The relative stability of the relaxed structures and their GB energies are then evaluated with the DFT level. The ANN potential is found to accurately predict low-energy structures and their energetics with reasonable accuracy with respect to DFT results, while conventional empirical potentials critically fail to find low-energy structures. The present study also provides a way to further improve the transferability of the ANN potential to more complicated GBs, using only low-Σ GBs as training datasets. Such improvement will offer a way to accurately predict atomic structures of general GBs within practical computational cost. This journal is

DOI： 10.1039/d1cp04329c

Web of Science

Scopus

PubMed

Effect of La vacancies on the oxide-ion conduction in lanthanum silicate apatites

Ogura Yusuke, Yokoi Tatsuya, Fujii Kotaro, Yashima Masatomo, Matsunaga Katsuyuki

SOLID STATE IONICS 373 巻 2021年12月

Grain-size dependence and anisotropy of nanoscale thermal transport in MgO Open Access

Fujii Susumu, Funai Kohei, Yokoi Tatsuya, Yoshiya Masato

APPLIED PHYSICS LETTERS 119 巻 ( 23 ) 2021年12月

記述言語：日本語出版者・発行元：Applied Physics Letters

Controlling thermal conductivity in nanocrystalline materials is of great interest in various fields such as thermoelectrics. However, its reduction mechanism has not been fully given due to the difficulty to assess local thermal conduction at grain boundaries (GBs) and grain interiors. Here, we calculated spatially decomposed thermal conductivities across and along MgO symmetric GBs using perturbed molecular dynamics, varying the GB separation from 2.1 to 20.0 nm. This reveals the different length scale of GB scattering for two directions: over hundreds of nanometers across GBs while within a few nanometers along GBs. Numerical analyses based on the spatially decomposed thermal conductivities demonstrate that the former is dominant upon suppressing thermal conductivity in polycrystalline materials, whereas the latter has a non-negligible impact in nanocrystalline materials because of a large reduction of intragrain thermal conductivity along GBs. These insights provide the exact mechanisms of heat transport in nanocrystalline materials toward more precise control of thermal conductivity.

DOI： 10.1063/5.0075854

Open Access

Web of Science

Scopus

Preferential growth mode of large-sized vacancy clusters in silicon: A neural-network potential and first-principles study 招待有り査読有り

T. Ushiro, T. Yokoi, Y. Noda, E. Kamiyama, M. Ohbitsu, H. Nagakura, K. Sueoka, K. Matsunaga

Journal of Physical Chemistry C 125 巻 ( 48 ) 頁： 26869 - 26882 2021年11月

Differential clustering of self-interstitials during Si crystal growth Open Access

Kamiyama Eiji, Yokoi Tatsuya, Noda Yusuke, Sueoka Koji

JOURNAL OF CRYSTAL GROWTH 574 巻 2021年11月

First-principles based theoretical calculations of atomic structures of hydroxyapatite surfaces and their charge states in contact with aqueous solutions Open Access

Saito T., Yokoi T., Nakamura A., Matsunaga K.

RSC ADVANCES 11 巻 ( 54 ) 頁： 34004 - 34014 2021年10月

Segregation mechanism of arsenic dopants at grain boundaries in silicon Open Access

Yutaka Ohno, Tatsuya Yokoi, Yasuo Shimizu, Jie Ren, Koji Inoue, Yasuyoshi Nagai, Kentaro Kutsukake, Kozo Fujiwara, Atsutomo Nakamura, Katsuyuki Matsunaga, Hideto Yoshida

Science and Technology of Advanced Materials: Methods 2021年8月

Direct imaging of atomistic grain boundary migration

Wei Jiake, Feng Bin, Ishikawa Ryo, Yokoi Tatsuya, Matsunaga Katsuyuki, Shibata Naoya, Ikuhara Yuichi

NATURE MATERIALS 20 巻 ( 7 ) 頁： 951 - + 2021年7月

An origin of excess vibrational entropies at grain boundaries in Al, Si and MgO: a first-principles analysis with lattice dynamics

Yokoi T., Ikawa K., Nakamura A., Matsunaga K.

PHYSICAL CHEMISTRY CHEMICAL PHYSICS 23 巻 ( 16 ) 頁： 10118 - 10129 2021年4月

Photoindentation: A New Route to Understanding Dislocation Behavior in Light

Nakamura Atsutomo, Fang Xufei, Matsubara Ayaka, Tochigi Eita, Oshima Yu, Saito Tatsushi, Yokoi Tatsuya, Ikuhara Yuichi, Matsunaga Katsuyuki

NANO LETTERS 21 巻 ( 5 ) 頁： 1962 - 1967 2021年3月

Crystal and Electronic Structures of MoSi2-Type CrGe2 Synthesized under High Pressure

Sasaki Takuya, Kanie Kota, Yokoi Tatsuya, Niwa Ken, Gaida Nico Alexander, Matsunaga Katsuyuki, Hasegawa Masashi

INORGANIC CHEMISTRY 60 巻 ( 3 ) 頁： 1767 - 1772 2021年2月

Stable and metastable structures and their energetics of asymmetric tilt grain boundaries in MgO: a simulated annealing approach

Yokoi T., Kondo Y., Ikawa K., Nakamura A., Matsunaga K.

JOURNAL OF MATERIALS SCIENCE 56 巻 ( 4 ) 頁： 3183 - 3196 2021年2月

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