担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The lattice dynamics of CsSnX3 (X = Cl, Br, and I) and CsPbI3, which are low-thermal-conductivity materials, are investigated using first-principles phonon calculations. Because of the strong lattice anharmonicity and the accompanying instability of high-temperature cubic phases, the self-consistent phonon theory, which can incorporate the effect of lattice anharmonicity at a mean-field level, is applied in this study. The calculated lattice thermal conductivity reproduced a low thermal conductivity, as shown experimentally, owing to the short phonon lifetime due to the incoherent scattering contribution of Cs atoms. The halogen ion dependence on thermal conductivity reveals that CsSnCl3 exhibits an anomalous lattice thermal conductivity that is as low as that of CsSnBr3. This indicates that the lattice dynamics cannot be explained merely in terms of the atomic mass of the compounds. The low thermal conductivity of CsSnCl3 is caused by the exceptionally short phonon lifetime; further, a bonding analysis suggests that covalent bonding contributes significantly to the unusual anharmonicity of CsSnCl3.

DOI： 10.1021/acs.jpcc.0c08324

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2020 Elsevier B.V. For thermoelectric materials, the figure of merit depends on the nanostructure, morphology, element doping, and the structure of the interface with other materials. Control of this interface is important in relation to a wide range of thermoelectric material designs such as in organic–inorganic hybrid compounds. Therefore, we evaluated the ease of forming various Bi2Te3 (0 0 1) termination surfaces using formation energy and calculation of phonon dispersion after structure optimization; TeI-1 and TeII terminations exhibited static and dynamic stabilities within various termination structures. In density-of-state and charge-distribution calculations, the TeII termination form exhibited distinctive electronic states around the Fermi energy that originated from the exposed Te atoms; therefore, it is suggested that efficient carrier transfer shall occur at the interface between a TeII surface and another material.

DOI： 10.1016/j.apsusc.2020.146454

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019 Elsevier B.V. To study the formation mechanism of the long-period stacking ordered (LPSO) structures, the reaction pathways of solid–solid transformations from a hexagonal close-packed (HCP) structure to LPSO structures in Mg-Y-Zn alloys were calculated using the generalized solid-state nudged elastic band method. The energy increases along the transition from HCP to 18R, and the peak positions represent the activation energy for the transition. Y substitution hardly changes the activation energy but makes the 18R-type LPSO structure more stable than HCP. In contrast, Zn or Y + Zn substitution results in higher activation energy and makes the 18R-type LPSO structure less stable than HCP. The calculated results for 14H and 24R LPSO structures also show similar activation energy and LPSO stability to the HCP-18R transition. Therefore, Y substitution plays an important role in stabilizing the stacking faults in LPSO systems. For the microscopic mechanism, the volume dependence of the total energy in pure FCC and HCP Y were examined, and the result suggests that FCC-Y is stable than HCP-Y under pressure. Therefore, the effect of substitution of Y in HCP Mg can be explained by the characteristics of Y under the chemical pressure exerted by the small size of Mg lattice.

DOI： 10.1016/j.commatsci.2019.109210

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019, The Minerals, Metals & Materials Society and ASM International. This study uses first-principles calculations to investigate the effect of hydrogen on the chemical driving force of the transformation of iron from the face centered cubic (FCC) to hexagonal close packed (HCP) phase. The minimum energy path from FCC to HCP phases shows that FCC becomes stable with increasing hydrogen content. Furthermore, the energy difference between the FCC and HCP phases is observed to be smaller in Fe2H than in Fe throughout the temperature region. These results clearly explain the observed anomalous suppression of the martensitic transformation in the hydrogen-charged steel.

DOI： 10.1007/s11661-019-05237-6

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2019 American Chemical Society. The reaction selectivity of an electrode catalyst can be modulated by regulating its crystal structure, and the modified electrode may show different CO 2 reduction selectivity from that of its constituent metal. In this study, we investigated the mechanisms of the electrochemical CO 2 reduction on an electrodeposited Cu 3 Sn alloy by experimental and theoretical analyses. The electrodeposited Cu 3 Sn alloy electrode showed selectivity for CO production at all the applied potentials, and HCOOH production increased with an increase in the applied potential. In particular, hydrocarbon generation was well suppressed on Cu 3 Sn(002). To understand this selectivity change in electrochemical CO 2 reduction, we conducted density functional theory calculations for the reaction on the Cu 3 Sn(002) surface. According to the theoretical analysis, the Cu sites in Cu 3 Sn(002) contributed more to the stabilization of H, COOH, and CO∗ as compared with the Sn sites. Furthermore, the results indicated that Cu 3 Sn(002) decreased the surface coverage of reaction intermediates such as H, COOH, and CO. We believe that these effects promoted CO∗ desorption while suppressing H 2 generation, CO∗ protonation, and C-C bond formation. The results also suggested that the surface Sn concentration significantly affected the reaction selectivity for HCOOH production from CO 2 .

DOI： 10.1021/acs.jpcc.8b11431

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記述言語：英語   掲載種別：研究論文（学術雑誌）

©2018 The Japan Institute of Metals and Materials. Cast Mg 85 Y 9 Zn 6 has an 18R-type LPSO structure. However, Mg 85 Y 9 Zn 6 recovered after being subjected to a loading pressure of 7 GPa at 973 K shows a fine dual-phase structure composed of a face-centered cubic (fcc) structure showing a superlattice (D0 3 ), as well as a hexagonal close-packed structure (hcp:2H). The D0 3 /hcp structure transformed to 18R-type LPSO during heating at ambient pressure. In this research, the transformation process from the D0 3 /hcp structure to 18R-type LPSO structure was discussed by means of in situ XRD and first-principles calculation. At first, lattice volume of 2H increased with an increase in the temperature, because of the Zn and Y emitted from the D0 3 phase into the 2H lattice. After the volume expansion of 2H lattice, the structure collapsed due to insert of random stacking faults (SFs). Then, a formation of 18R-type LPSO structure occurred. Based on a first-principles calculation for pure Mg, volume expansion of the 2H lattice causes the transformation to an 18R structure. Furthermore, the results of free energy calculations for the hcp and fcc structures in the MgYZn ternary system show that the segregation of Y and Zn atoms on SFs occurs by the Suzuki effect. These segregated Y and Zn atoms in SF layers, which have a local fcc structure, create a synergy between the stacking and chemical modulations. Present result insists that the volume increase of 2H lattice takes place first, and then the transformation from the hcp structure to 18R stacking occurs. [doi:10.2320/matertrans.M2018213]

DOI： 10.2320/matertrans.M2018213

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記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2018 Author(s). By combining theoretical predictions and in-situ X-ray diffraction under high pressure, we found a novel stable crystal structure of Li3PS4 under high pressures. At ambient pressure, Li3PS4 shows successive structural transitions from γ-type to β-type and from β-type to α type with increasing temperature, as is well established. In this study, an evolutionary algorithm successfully predicted the γ-type crystal structure at ambient pressure and further predicted a possible stable δ-type crystal structures under high pressure. The stability of the obtained structures is examined in terms of both static and dynamic stability by first-principles calculations. In situ X-ray diffraction using a synchrotron radiation revealed that the high-pressure phase is the predicted δ-Li3PS4 phase.

DOI： 10.1063/1.5011401

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2018 The Japan Institute of Metals and Materials. To investigate the stability of various Mg-based Laves phases, the formation enthalpy and phonon dispersion were obtained by firstprinciples calculation. The calculated formation enthalpy and phonon dispersion indicate that MgX2 (X = Al, Co, Ni, Cu, Zn) and Mg2X (X = Ca, Sr, Y, Ba, La) are stable both statically and dynamically. These results are consistent with the experimental results except for MgAl2 and Mg2La. These compounds are considered to be in a metastable state in each binary system. We also used the cluster expansion method to examine the possibility of adding a third element to MgZn2. Our theoretical investigations suggest attractive interaction between Zn and a third element such as Ag, Ca, and Zr in the MgZn2 lattice. However, Ca and Zr replace a small amount of Zn in MgZn2 owing to the instability of MgCa2 and MgZr2, in agreement with the experimental result. Furthermore, it is suggested that Zr becomes stable at the Mg site in the MgZn2 lattice owing to the stability of ZrZn2.

DOI： 10.2320/matertrans.M2018079

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

© 2017 Universiti Putra Malaysia Press. In this study, we investigated the anisotropic Li diffusion in Li10GeP2S12 using a molecular dynamics method. Although the calculated ion conductivity is slightly lower in comparison to the calculated and experimental ion conductivity of previous studies, our results support anisotropic Li diffusion. These findings show that the anisotropic Li diffusion estimated from the activation energy is an important property of Li10 GeP2 S12, and a potential factor for good ion conductivity. Further, a high diffusion coefficient was observed in Li10P3S12, which is a good Li-ion conductor.

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