研究者詳細 - 横井　達矢

Atomic-Scale Insights into Yttrium-Induced Grain Boundary Structure Modification in Al2O3

Yan, JY; Yokoi, T; Nakano, Y; Kondo, S; Feng, B; Shibata, N; Matsunaga, K; Ikuhara, Y

ADVANCED SCIENCE 頁： e15350 2025年12月

Application of neural-network potential based on trainable descriptor to crystallographically complex lattice defects in silicon 査読有り Open Access

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

Physical Review Materials 9 巻 ( 11 ) 2025年11月

担当区分：責任著者掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review Materials

A trainable descriptor based on artificial neural networks (ANN), developed in our recent study [M. Uchida et al., Phys. Rev. Mater.8, 103805 (2025) 10.1103/PhysRevMaterials.8.103805], is integrated into a neural-network potential (NNP) and applied to various lattice defects in molecular simulations of silicon. The ANN descriptor achieves higher predictive accuracy for point defects, surfaces, and symmetric tilt grain boundaries (STGBs) than analytic-function-based descriptors. As a case study of crystallographically complex defects, the ANN descriptor is applied to asymmetric tilt grain boundaries (ATGBs), where favorable atomic structures are governed by the competition between multiple structural units. For the Σ9(111)||(115) ATGB, the predicted atomic structure shows a grain boundary energy close to the DFT value and agrees with previous electron microscopy observations. Similarly, Σ3, Σ5, and Σ9 ATGBs are also systematically examined with the combination of the ANN descriptor and DFT calculations. ATGBs in the Σ5 system can be described by structural units of two STGBs with the same Σ value, indicating that Σ5 ATGBs ideally facet into Σ5 STGB structures. In contrast, this trend does not hold for the Σ3 and Σ9 ATGBs due to the crystallographic incompatibility of the constituent STGBs. As a result, Σ3 and Σ9 ATGBs involve structural units that cannot be predicted by the crystallographic misorientation of the two gains. Electronic-structure analysis reveals that specific atoms at ATGBs create defect levels within the band gap, depending on the inclination angle of ATGBs. These results highlight that the high-accuracy ANN descriptor provides a deeper understanding of atomic and electronic structures and energetics of complex lattice defects.

DOI： 10.1103/3gx6-zp38

Phase Transition of MnN2 Associated with N2 Dimer Alignment under High Pressure and High Temperature 査読有り

Direct observation of substitutional and interstitial dopant diffusion in oxide grain boundary. 査読有り Open Access

Futazuka T, Ishikawa R, Yokoi T, Matsunaga K, Shibata N, Ikuhara Y

Nature communications 16 巻 ( 1 ) 頁： 9043 2025年10月

Origin of reduced thermal conduction by native point defects in PbTe: Perturbed molecular dynamics with neural network potential 査読有り Open Access

Tatsuya Yokoi, Susumu Fujii, Yu Ogura, Katsuyuki Matsunaga

Physical Review B 112 巻頁： 024111 2025年7月

Asano, S; Niwa, K; Yokoi, T; Sasaki, T; Kawaguchi-Imada, S; Matsunaga, K; Hasegawa, M

INORGANIC CHEMISTRY 64 巻 ( 22 ) 頁： 10700 - 10705 2025年5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：Inorganic Chemistry

Manganese pernitride (MnN<inf>2</inf>) was synthesized at a pressure of approximately 60 GPa and a temperature of approximately 2000 K for the first time. A phase transition between rhombohedral and triclinic symmetry associated with the alignment of the N<inf>2</inf> dimer was also clearly observed at high temperatures under high pressure or along the decompression process at room temperature. The present experimental results with DFT calculations offer new insights and diversity into the crystal chemistry of the transition metal pernitrides concerning temperature and pressure.

DOI： 10.1021/acs.inorgchem.5c01118

PubMed

Transferability of machine-learning interatomic potential to α-Fe nanocrystalline deformation 査読有り

Ito, K; Yokoi, T; Hyodo, K; Mori, H

INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES 291 巻 2025年4月

掲載種別：研究論文（学術雑誌）出版者・発行元：International Journal of Mechanical Sciences

To improve the mechanical properties of polycrystalline metallic materials, understanding the elementary processes involved in their deformation at the atomic level is crucial. In this study, firstly, we evaluate the transferability of the recently proposed α-Fe machine-learning interatomic potential (MLIP), constructed from mechanically generated training data based on crystal space groups, to the tensile deformation process of nanopolycrystals. The transferability was evaluated by comparing the physical properties and lattice defect formation energies, which are important in the deformation behavior of nanopolycrystals, with those obtained from density functional theory (DFT) and by comprehensively calculating extrapolation grades based on active learning methods for the local atomic environment in the nanopolycrystal during tensile deformation. These evaluations demonstrate the superior transferability of the MLIP to the tensile deformation of the nanopolycrystals. Furthermore, large-scale molecular dynamics calculations were performed using the MLIP and the most commonly used embedded atom method (EAM) potential to investigate the effect of grain size on the deformation behavior of α-Fe polycrystals and the effect of interatomic potentials on them. The uniaxial tensile deformation behavior of the nanopolycrystals obtained from EAM was qualitatively consistent with that obtained from MLIP. This result supports the results of many studies conducted using EAM and is an important conclusion considering the high computational cost of the MLIP. Furthermore, the construction method for the MLIP used in this study is applicable to other metals. Therefore, this study considerably contributes to the understanding and material design of various metallic materials through the construction of highly accurate MLIPs.

DOI： 10.1016/j.ijmecsci.2025.110132

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

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

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

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 Open Access

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

PubMed

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 Open Access

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 Open Access

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

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.

DOI： 10.2109/jcersj2.23060

CiNii Research

Grain boundary segregation of Y and Hf dopants in α-Al2O3: A Monte Carlo simulation with artificial-neural-network potential and density-functional-theory calculation Open Access

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

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(4510)/[0001] grain boundary (GB) in α-Al2O3. 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.

DOI： 10.2109/jcersj2.23044

CiNii Research

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

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

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.

DOI： 10.2109/jcersj2.23055