研究者詳細 - 横井　達矢

論文 - 横井　達矢

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Spectrophotometric analyses on electronic structures of glide dislocation cores in ZnS

Ogura, Y; Mishina, K; Hoshino, S; Higuchi, K; Yokoi, T; Nakamura, A; Matsunaga, K

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 2024年12月

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

It has been reported that dislocations in ZnS can trap photoexcited carriers and then undergo core reconstruction. However, electronic states of the trapped carriers at the reconstructed dislocations were not detected experimentally. In this study, unique ultraviolet (UV)–visible–infrared (IR) transmission measurements by using a spectrophotometer with UV pump beam were performed for deformed ZnS single crystals having glide dislocations. Optical transmission values of the deformed specimens decreased by the pump beam illumination apparently. Additionally, the optical transmission reductions increased more with rising plastic strains, namely, rising dislocation density. Since the UV pump beam can produce photoexcited carriers of electrons and holes, it is expected that the glide dislocations introduced in the deformed specimens undergo core reconstruction due to carrier trapping. Therefore, it is thought that the observed reduction in optical transmission of the deformed specimens is ascribed to photoabsorption due to in-gap states localized at the reconstructed partial dislocation cores. Moreover, time-dependent transmission measurements revealed that the core reconstruction proceed rapidly while the opposite processes are quite slow, which may be closely related to atomic structure changes of the glide dislocation cores due to the additional pump beam illumination.

DOI： 10.1111/jace.20332

Machine learning interatomic potential with DFT accuracy for general grain boundaries: Analysis of grain boundary energy and atomic structure in α-Fe polycrystals 査読有り

Kazuma Ito, Tatsuya Yokoi, Katsutoshi Hyodo, Hideki Mori

npj Computational Materials 10 巻頁： 255 2024年11月

Artificial-neural-network descriptor enhancing accuracy of machine-learning interatomic potential and its application to lattice defects in Si

Uchida, M; Yokoi, T; Ogura, Y; Matsunaga, K

PHYSICAL REVIEW MATERIALS 8 巻 ( 10 ) 2024年10月

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

Trainable descriptors based on artificial neural networks (ANNs) are presented with the goal of accurately predicting the structure-property relationship in the presence of various lattice defects for Si. In this architecture, interatomic distances and bond angles are directly fed to the ANNs instead of empirical analytic functions. The ANNs are hierarchically connected with another ANN that outputs the atomic energy involving many-body terms of the potential energy. All ANNs are then simultaneously trained using density-functional-theory data, which enables updating the entire functional forms numerically. For training and test datasets containing point defects, surfaces, and grain boundaries in Si, the ANN descriptor exhibits systematically smaller errors in total potential energy than those based on the symmetry function and the Chebyshev polynomials, which are commonly used in ANN potentials. The outputs of the trained descriptor each have flexible functional forms that are difficult to construct using a single separation-of-variable function in terms of the interatomic distance and the bond angle. This indicates that the ANN descriptor can acquire a better representation of diverse atomic environments via the training, which leads to the decreased error. The present study also discusses an efficient way of training the ANN descriptor in a stepwise manner while maintaining its flexibility. Furthermore, the computational time is found to be comparable to that of the symmetry function, which will pave the way for large-scale molecular simulations involving multiple lattice defects with high accuracy.

DOI： 10.1103/PhysRevMaterials.8.103805

Multicrystalline informatics: a methodology to advance materials science by unraveling complex phenomena.

Usami N, Kutsukake K, Kojima T, Kudo H, Yokoi T, Ohno Y

Science and technology of advanced materials 25 巻 ( 1 ) 頁： 2396272 2024年9月

Atomic-structure changes of 30° partial-dislocation cores due to excess carriers in GaP

Hoshino S., Oi S., Ogura Y., Yokoi T., Li Y., Nakamura A., Matsunaga K.

Physical Review Materials 8 巻 ( 9 ) 2024年9月

出版者・発行元：Physical Review Materials

It was experimentally reported that light illumination leads to reduced deformation stresses in some III-V compound semiconductors such as GaP. This phenomenon is known as the negative photoplastic effect, which is expected to originate from interactions between photoexcited carriers and glide dislocations. To clarify its physical origin at the atomic and electronic levels, density-functional-theory calculations were performed for Shockley 30° partial dislocations in GaP. In the absence of excess carriers, both Ga and P cores of the partial dislocations were found to have reconstructed structures that are energetically most stable. This can be understood by the fact that dangling-bond-like states at undercoordinated atoms of the dislocation cores are removed by core reconstruction. In the presence of excess carriers that would be formed by light illumination, the reconstructed Ga and P cores were able to trap excess holes and electrons, respectively, and were subsequently transformed to unreconstructed structures. It was also found that the unreconstructed structures due to excess carriers tend to have smaller potential barrier heights for dislocation glide, as compared to the pristine reconstructed structures without any excess carriers. This is in good agreement with the increased dislocation mobility in GaP under external light illumination that has been experimentally reported.

DOI： 10.1103/PhysRevMaterials.8.093605

Revealing Atomic Structure of Hybrid Octacalcium-Phosphate Derivative

Susaki, N; Saito, T; Yokoi, T; Ogura, Y; Matsunaga, K

INORGANIC CHEMISTRY 63 巻 ( 34 ) 頁： 15924 - 15930 2024年8月

記述言語：英語出版者・発行元：Inorganic Chemistry

Octacalcium phosphate (OCP), which is one of the bioactive calcium phosphates, can incorporate various organic molecules in its crystal lattice, forming the organic-inorganic hybrid derivatives. However, detailed atomic arrangements of OCP hybridized with organic molecules such as dicarboxylate are still unknown, although many years have passed since the first discovery of the materials systems. In the present study, some black-box optimization methods combined with first-principles calculations were used to theoretically identify the most stable atomic structure of the OCP with the incorporation of malonate ions as a typical case study. The results showed that the calculated interplanar spacing on the (100) plane of the most stable structure agrees well with experimental data, by taking account of implicit solvent of aqueous solution. An underlying mechanism that realizes the bridging feature of the incorporated malonate ions between the apatitic layers is also discussed. The present methodology can pave the way to accurately explore reliable atomic structures of such complicated organic-inorganic hybrid biomaterials with high structural degrees of freedom.

DOI： 10.1021/acs.inorgchem.4c02259

The effect of room-temperature plastic deformation in darkness on the photoluminescence properties of ZnS

Ogura, Y; Nakamura, A; Kameyama, T; Kurokawa, Y; Tochigi, E; Shibata, N; Torimoto, T; Hoshino, S; Yokoi, T; Matsunaga, K

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 107 巻 ( 4 ) 頁： 2040 - 2047 2024年4月

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

Inorganic semiconductors have been considered to be brittle at room temperature generally, but recently zinc sulfide (ZnS), a II–VI compound semiconductor, was found to exhibit extraordinarily large plasticity even at room temperature if it was deformed in darkness. Room-temperature plastic deformation can achieve high density of glide dislocations in the crystal without significantly changing the point defect structure, and these dislocations themselves have a potential to provide unique functional properties different from those of the bulk. In this study, therefore, undoped ZnS crystals were plastically deformed at room temperature in darkness to generate only a large number of dislocations, and the changes in luminescence properties resulting from this process were investigated for the first time. As a result, we found that ZnS deformed in darkness exhibits characteristic photoluminescence and persistent luminescence emissions with a visible green color. SEM-CL analyses also identified the emissions from the dislocations. Theoretical calculations indicated that individual dislocations in ZnS have the ability to trap photo-excited carriers. Such changes in luminescence properties due to room temperature plastic deformation in darkness can be considered to originate from dislocations rather than point defects. In other words, the dislocations themselves serve as the significant recombination centers, realizing visible light emission.

DOI： 10.1111/jace.19564

Multicrystalline Informatics Applied to Multicrystalline Silicon for Unraveling the Microscopic Root Cause of Dislocation Generation

Yamakoshi, K; Ohno, Y; Kutsukake, K; Kojima, T; Yokoi, T; Yoshida, H; Tanaka, H; Liu, X; Kudo, H; Usami, N

ADVANCED MATERIALS 36 巻 ( 8 ) 頁： e2308599 2023年12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：Advanced Materials

A comprehensive analysis of optical and photoluminescence images obtained from practical multicrystalline silicon wafers is conducted, utilizing various machine learning models for dislocation cluster region extraction, grain segmentation, and crystal orientation prediction. As a result, a realistic 3D model that includes the generation point of dislocation clusters is built. Finite element stress analysis on the 3D model coupled with crystal growth simulation reveals inhomogeneous and complex stress distribution and that dislocation clusters are frequently formed along the slip plane with the highest shear stress among twelve equivalents, concentrated along bending grain boundaries (GBs). Multiscale analysis of the extracted GBs near the generation point of dislocation clusters combined with ab initio calculations has shown that the dislocation generation due to the concentration of shear stress is caused by the nanofacet formation associated with GB bending. This mechanism cannot be captured by the Haasen-Alexander-Sumino model. Thus, this research method reveals the existence of a dislocation generation mechanism unique to the multicrystalline structure. Multicrystalline informatics linking experimental, theoretical, computational, and data science on multicrystalline materials at multiple scales is expected to contribute to the advancement of materials science by unraveling complex phenomena in various multicrystalline materials.

DOI： 10.1002/adma.202308599

Anharmonicity in grain boundary energy for Al: thermodynamic integration with artificial-neural-network potential 査読有り国際誌

Scripta Materialia 236 巻頁： 115685 2023年11月

Photoplastic effect in MgO single crystals

Ogura Yu, Tsuchiya Yuki, Hoshino Sena, Yokoi Tatsuya, Matsunaga Katsuyuki

Journal of the Ceramic Society of Japan 131 巻 ( 10 ) 頁： 628 - 631 2023年10月

記述言語：英語出版者・発行元：The Ceramic Society of Japan

<p>The photoplastic effect, phenomena that flow stress and/or hardness change by light illuminations, has been known mainly in II–VI compound semiconductors. In this study, it was revealed for the first time that magnesium oxide (MgO) single crystals exhibit increase in flow stress by light illumination, namely, the positive photoplastic effect. Scanning transmission electron microscope (STEM) observations demonstrated that the plastic deformation of MgO is realized by generation of glide dislocations on the slip system of {110}〈110〉. Therefore, the observed positive photoplastic effect of MgO is likely due to interactions of the glide dislocations and photo-excited carriers. According to the theoretical calculations, individual dislocations in MgO had a specific band structure that differs from the bulk. This also indicates possible interactions between dislocations and carriers. It is expected that light illumination can reduce dislocation mobility in MgO, leading to the increase in flow stress under light illuminations.</p>

DOI： 10.2109/jcersj2.23060

Grain boundary segregation of Y and Hf dopants in α-Al<sub>2</sub>O<sub>3</sub>: A Monte Carlo simulation with artificial-neural-network potential and density-functional-theory calculation

Yokoi Tatsuya, Hamajima Akihiro, Ogura Yu, Matsunaga Katsuyuki

Journal of the Ceramic Society of Japan 131 巻 ( 10 ) 頁： 751 - 761 2023年10月

記述言語：英語出版者・発行元：The Ceramic Society of Japan

<p>Artificial-neural-network (ANN) interatomic potentials for Al–Y–O and Al–Hf–O systems are constructed using density-function-theory (DFT) data and combined with Monte Carlo (MC) simulations in order to predict Y and Hf segregation behavior at the ∑7(4<span style="text-decoration: overline;">5</span>10)/[0001] grain boundary (GB) in α-Al<sub>2</sub>O<sub>3</sub>. The ANN potentials are demonstrated to accurately predict preferential substitutional sites of not only an isolated but also multiple dopant ions. This enables us to circumvent DFT calculations for MC trial moves, thereby greatly reducing computational cost. There is a tendency that both Y and Hf ions substitute for 6-fold Al ions with elongated Al–O bonds at the GB and have coordination numbers greater than 6 after structural relaxation. This may suggest that even at the GB, Y and Hf ions prefer atomic environments in Y- and Hf-containing oxides with 7- and 8-fold coordination. Furthermore, effects of dopant species and concentrations on band-gap reduction at the GB are elucidated by analyzing partial density of states for the dopant-segregated GBs. The ANN-MC method with DFT analysis will pave the way for systematically determining atomic and electronic structures of GBs involving dopants, as demonstrated in this work.</p>

DOI： 10.2109/jcersj2.23044

Electronic and atomic structures of Shockley-partial dislocations in CdX (X = S, Se and Te)

Hoshino Sena, Yokoi Tatsuya, Ogura Yu, Matsunaga Katsuyuki

Journal of the Ceramic Society of Japan 131 巻 ( 10 ) 頁： 613 - 620 2023年10月

記述言語：英語出版者・発行元：The Ceramic Society of Japan

<p>II–VI semiconductors, including Cd compounds, become brittle under light illumination. This phenomenon is known as the photoplastic effect (PPE) and is thought to arise from interactions between glide dislocations and photoexcited carriers. The present study investigated atomic structures of 30° Shockley-partial dislocations with and without excess carriers in CdX (X = S, Se and Te), by density-functional-theory (DFT) calculations. It was found that both Cd and anion cores favor unreconstructed atomic structures when excess carriers are absent. In the presence of excess carriers, on the other hand, reconstructed atomic structures were more stable at the anion cores while the unreconstructed ones were still energetically more favorable at the Cd cores. It is thus expected that only the anion cores change their atomic structures by light illumination, which can retard glide-dislocation motion by forming like-atom bonds. Analyses of local densities of states (LDOSs) revealed that the reconstructed Cd and anion cores form shallow and deep defect states within the band gaps, respectively. This determines the possible atomic reconstructions at the dislocation cores in the presence of excess carriers excited by external light.</p>

DOI： 10.2109/jcersj2.23055

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

First-principles analysis of oxide-ion conduction mechanism in neodymium silicate

Ogura Yusuke, Yokoi Tatsuya, Toyoura Kazuaki, Matsunaga Katsuyuki

SOLID STATE IONICS 355 巻 2020年11月

Formation energies and site preference of substitutional divalent cations in carbonated apatite

Saito Tatsushi, Yokoi Tatsuya, Nakamura Atsutomo, Matsunaga Katsuyuki

JOURNAL OF THE AMERICAN CERAMIC SOCIETY 103 巻 ( 9 ) 頁： 5354 - 5364 2020年9月

Room-temperature creep deformation of cubic ZnS crystals under controlled light conditions

Oshima Yu, Nakamura Atsutomo, Lagerlof K. Peter D., Yokoi Tatsuya, Matsunaga Katsuyuki

ACTA MATERIALIA 195 巻頁： 690 - 697 2020年8月

Carrier-trapping induced reconstruction of partial-dislocation cores responsible for light-illumination controlled plasticity in an inorganic semiconductor

Matsunaga Katsuyuki, Hoshino Sena, Ukita Masaya, Oshima Yu, Yokoi Tatsuya, Nakamura Atsutomo

ACTA MATERIALIA 195 巻頁： 645 - 653 2020年8月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Acta Materialia

Inorganic semiconductors show interesting mechanical behavior in response to their surrounding lighting environments. In fact, we recently found out that zinc sulfide (ZnS) shows the light-illumination controlled plasticity at room temperature. However, its mechanism is poorly understood. Here we report systematic density functional theory results of electronic and atomic structures of glide partial dislocations in ZnS. We have revealed that partial dislocations in ZnS have excess electrostatic fields localized around their cores and can trap electrons or holes, depending on excess ionic species at the cores. Such carrier-dislocation interactions can induce energetically more stable bond reconstruction at their cores, as compared with the case before carrier trapping. Reconstructed bonds at the dislocation cores should be broken upon dislocation glide, and thus can restrict the dislocation motion significantly. Such reduced dislocation motion due to core reconstruction should give rise to increased plastic deformation stresses, namely, hardening of the materials under light illumination. The present results provide a critical understanding of the experimentally observed light-illumination controlled plasticity in ZnS at the electronic level, which is opposed to previously reported plasticity and dislocation motion in other elemental and III-V semiconductors.

DOI： 10.1016/j.actamat.2020.06.010

Quantitative prediction of grain boundary thermal conductivities from local atomic environments

Fujii Susumu, Yokoi Tatsuya, Fisher Craig A. J., Moriwake Hiroki, Yoshiya Masato

NATURE COMMUNICATIONS 11 巻 ( 1 ) 2020年4月

Direct Measurement of Electronic Band Structures at Oxide Grain Boundaries

Wei Jiake, Ogawa Takafumi, Feng Bin, Yokoi Tatsuya, Ishikawa Ryo, Kuwabara Akihide, Matsunaga Katsuyuki, Shibata Naoya, Ikuhara Yuichi

NANO LETTERS 20 巻 ( 4 ) 頁： 2530 - 2536 2020年4月

Role of phonons on phase stabilization of RE2Si2O7 over wide temperature range (RE = Yb, Gd)

Fujii Susumu, Ioki Arata, Yokoi Tatsuya, Yoshiya Masato

JOURNAL OF THE EUROPEAN CERAMIC SOCIETY 40 巻 ( 3 ) 頁： 780 - 788 2020年3月

Dependence of excess vibrational entropies on grain boundary structures in MgO: A first-principles lattice dynamics

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

PHYSICAL REVIEW MATERIALS 4 巻 ( 2 ) 2020年2月

Neural-network interatomic potential for grain boundary structures and their energetics in silicon

Yokoi T., Noda Y., Nakamura A., Matsunaga K.

PHYSICAL REVIEW MATERIALS 4 巻 ( 1 ) 2020年1月

Atomistic mechanisms of thermal transport across symmetric tilt grain boundaries in MgO

Fujii Susumu, Yokoi Tatsuya, Yoshiya Masato

ACTA MATERIALIA 171 巻頁： 154 - 162 2019年6月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Acta Materialia

Grain boundaries often have a decisive effect on the macroscopic properties of polycrystalline materials, but the wide variety of their atomic structures, interfacial lengths, and compositions makes them difficult to characterize all-encompassingly. An indispensable example in which an understanding of the relationship between grain boundary structures and properties would greatly facilitate development of superior materials is thermal transport, especially with respect to microstructure evolution, thermoelectrics and thermal barrier coatings. To contribute to a more comprehensive understanding, we performed a systematic study of lattice thermal conduction across a wide range of symmetric tilt grain boundaries in MgO using perturbed molecular dynamics. It was found that thermal conductivities vary significantly with grain boundary structure but are strongly correlated with excess volume, which is a measure of the number density of atoms in the vicinity of the grain boundary planes. Real-space analysis revealed that dislocation densities determine the phonon transport paths and thermal conductivity in low-angle boundaries whereas it is the amount of open volume rather than the shape of structural units in high-angle boundaries that determine thermal conductivity. We also found that low-angle boundaries mainly reduce phonon transports at low frequencies whereas high-angle boundaries also reduce it at intermediate and high frequencies effectively, regardless of the shape of structural units. These insights are expected to be applicable to other close-packed oxide systems, and should aid the design of next-generation thermal materials through tailoring of grain boundaries.

DOI： 10.1016/j.actamat.2019.04.009

Theoretical Calculations of Characters and Stability of Glide Dislocations in Zinc Sulfide

Ukita Masaya, Nagahara Ryota, Oshima Yu, Nakamura Atsutomo, Yokoi Tatsuya, Matsunaga Katsuyuki

MATERIALS TRANSACTIONS 60 巻 ( 1 ) 頁： 99 - 104 2019年

Oxide-ion conduction via interstitials in scheelite-type LaNbO4: a first-principles study

Toyoura Kazuaki, Sakakibara Yusuke, Yokoi Tatsuya, Nakamura Atsutomo, Matsunaga Katsuyuki

JOURNAL OF MATERIALS CHEMISTRY A 6 巻 ( 25 ) 頁： 12004-12011 2018年7月

Interaction of sodium atoms with stacking faults in silicon with different Fermi levels

Ohno Yutaka, Morito Haruhiko, Kutsukake Kentaro, Yonenaga Ichiro, Yokoi Tatsuya, Nakamura Atsutomo, Matsunaga Katsuyuki

APPLIED PHYSICS EXPRESS 11 巻 ( 6 ) 2018年6月

Atomistic simulations of grain boundary transformation under high pressures in MgO 査読有り

Yokoi T., Yoshiya M.

PHYSICA B-CONDENSED MATTER 532 巻頁： 2 - 8 2018年3月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Physica B: Condensed Matter

This study focuses on transformation of grain boundary (GB) structures under high pressures up to 60 GPa by using a simulated annealing technique with molecular dynamics and lattice statics calculations for various symmetric tilt GBs (STGBs) of MgO. It is found that except for the Σ3(111)/[11¯0] that is a rather stable GB, all the STGBs studied transform into a metastable structure more than once at threshold pressures. In addition, the GBs with an open-core structure and small tilt angle are found to be more “flexible” to transform into different structures than the GBs with a dense structure. For polycrystalline MgO, therefore, GBs may also exhibit GB transformation under high pressures and flexible GBs may govern overall transformation and deformation. These findings also suggest that polycrystals sintered at high pressures consist of more pressure-resistant GBs than those at normal pressures.

DOI： 10.1016/j.physb.2017.03.014

Structure of the Basal Edge Dislocation in ZnO

Nakamura Atsutomo, Tochigi Eita, Nagahara Ryota, Furushima Yuho, Oshima Yu, Ikuhara Yuichi, Yokoi Tatsuya, Matsunaga Katsuyuki

CRYSTALS 8 巻 ( 3 ) 2018年3月

Electronic and atomic structures of edge and screw dislocations in rock salt structured ionic crystals

Ukita Masaya, Nakamura Atsutomo, Yokoi Tatsuya, Matsunaga Katsuyuki

PHILOSOPHICAL MAGAZINE 98 巻 ( 24 ) 頁： 2189-2204 2018年

Atomically ordered solute segregation behavior in an oxide grain boundary 査読有り

B. Feng, T. Yokoi, A. Kumamoto, M. Yoshiya, Y. Ikuhara, N. Shibata

Nature Communications 2016年3月

Atomistic analyses of Competition between Site-Selective Segregation and Association of Point Defects at Grain Boundary in Y₂O₃-doped ZrO₂ 査読有り

T. Yokoi, M. Yoshiya, H. Yasuda

Material Transaction 2015年8月

On modeling of grain boundary segregation in aliovalent cation doped ZrO₂: Critical factors in site-selective point defect occupancy 査読有り

T. Yokoi, M. Yoshiya, H. Yasuda

Scripta Materialia 2015年3月

Nonrnadom Point Defect Configurations and Driving Force Transitions for Grain Boundary Segregation in Trivalent Cation Doped ZrO₂ 査読有り

T. Yokoi, M. Yoshiya, H. Yasuda

Langmuir 2014年11月

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