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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

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. (C) 2021 Author(s).

DOI： 10.1063/5.0075854

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/jacs.1c04260

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

<title>Abstract</title>Most solid-state materials are composed of p-block anions, only in recent years the introduction of hydride anions (1s²) in oxides (e.g., SrVO₂H, BaTi(O,H)₃) has allowed the discovery of various interesting properties. Here we exploit the large polarizability of hydride anions (H^–) together with chalcogenide (Ch^2–) anions to construct a family of antiperovskites with soft anionic sublattices. The M₃HCh antiperovskites (M = Li, Na) adopt the ideal cubic structure except orthorhombic Na₃HS, despite the large variation in sizes of M and Ch. This unconventional robustness of cubic phase mainly originates from the large size-flexibility of the H^– anion. Theoretical and experimental studies reveal low migration barriers for Li⁺/Na⁺ transport and high ionic conductivity, possibly promoted by a soft phonon mode associated with the rotational motion of HM₆ octahedra in their cubic forms. Aliovalent substitution to create vacancies has further enhanced ionic conductivities of this series of antiperovskites, resulting in Na_2.9H(Se_0.9I_0.1) achieving a high conductivity of ~1 × 10^–4 S/cm (100 °C).

DOI： 10.1038/s41467-020-20370-2

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Other Link： http://www.nature.com/articles/s41467-020-20370-2

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

DOI： 10.1038/s41467-020-15619-9

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

© 2019 Acta Materialia Inc. 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

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.jpcc.5c01341

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsaem.4c03142

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Publishing type：Research paper (scientific journal)  

DOI： 10.1111/jace.20438

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/jacs.4c10520

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.inorgchem.4c01193

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DOI： 10.1021/acs.chemmater.4c00392

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.commatsci.2023.112722

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Ceramic Society of Japan  

DOI： 10.2109/jcersj2.23066

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

Unlike perovskite oxides, antiperovskites M₃HCh and M₃FCh (M = Li, Na; Ch = S, Se, Te) mostly retain their ideal cubic structure owing to anionic size flexibility and low-energy phonon modes that promote their ionic conductivity.

DOI： 10.1039/d3dt01039b

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Understanding the mechanisms that connect heat and electron transport with crystal structures and defect chemistry is fundamental to develop materials with thermoelectric properties. In this work, we synthesized a series of self‐doped compounds Cu_2+xMn_1−xGeS₄ through Cu for Mn substitution. Using a combination of powder X‐ray diffraction, high resolution transmission electron microscopy and precession‐assisted electron diffraction tomography, we evidence that the materials are composed of interconnected enargite‐ and stannite‐type structures, via the formation of nanodomains with a high density of coherent interfaces. By combining experiments with ab initio electron and phonon calculations, we discuss the structure–thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. We demonstrate that excess Cu⁺ substituted for Mn²⁺ dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT.

DOI： 10.1002/anie.202210600

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/anie.202210600

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.chemmater.2c00958

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

We report numerical analyses to examine the impact of edge dislocation onto phonon thermal conduction in its vicinity. Two types of edge dislocations, one with open core and another with densely-packed core, show different magnitude of suppression of thermal conductivity across dislocation lines, more than crystallographic anisotropy. It is also found that small density of dislocations effectively scatters phonons lowering thermal conductivity while higher density of dislocations is less effective to suppress thermal conduction. Detailed analyses on bond strains and atomic vibrational states indicated that, while linear dependence of atomic thermal conductivity is found when bond strain is small as in elastic strain field, non-linear/anharmonic dependence of atomic thermal conductivity emerges for the highly strained and under-coordinated atoms at the cores. A combination of these leads to remarkable suppression in thermal conductivity at non-arrayed dislocations even if atoms are positioned away from dislocation core.

DOI： 10.1016/j.scriptamat.2021.113991

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

<p>The Jahn–Teller-like effect on the tetrahedral geometry of d⁶ iron(<sc>ii</sc>) is caused by its unequally occupied e orbitals with non-bonding nature.</p>

DOI： 10.1039/D0DT04155F

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/ange.202008187

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/anie.202008187

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

© 2019 Thermodynamic phase stabilities of α, β, γ and δ polymorphs of RE2Si2O7 (RE = Yb, Gd), promising candidates for environmental barrier coating systems of gas turbine engines in next generations, were investigated using harmonic and quasi-harmonic lattice dynamics in conjunction with ab initio calculations. By quantifying their Gibbs free energies with dynamics of atoms taken into account, we showed the phase stability of β-Yb2Si2O7 up to melting temperature under an ideal condition, and also under thermal stresses, ensuring the reliability of β-Yb2Si2O7 in environmental barrier coating systems. It was also found that the difference of anharmonic phonons in these polymorphs, which was measured as mode Grüneisen parameters, significantly changed their phonon free energies and invoked the formation of δ-Gd2Si2O7 at high temperature. These results enable a quantitative comprehension of the phase transformation of RE2Si2O7 and future discussions of other factors which may affect the phase stability such as lattice defects.

DOI： 10.1016/j.jeurceramsoc.2019.10.060

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Publishing type：Research paper (international conference proceedings)  

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

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Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Optimizing multiple materials properties which are simultaneously in competition with each other is one of the chief challenges in thermoelectric materials research. Introducing greater anharmonicity to vibrational modes is one strategy for suppressing phonon thermal transport in crystalline oxides without detrimentally affecting electronic conductivity, so that the overall thermoelectric efficiency can be improved. Based on perturbed molecular dynamics and associated numerical analyses, we show that CoO2 layers in layered cobaltite thermoelectrics NaxCoO2 and Ca3Co4O9 are responsible for most of the in-plane heat transport in these materials, and that the non-conducting intermediate layers in the two materials exhibit different kinds of anharmonicity. More importantly, thermal conduction is shown to be altered by modifying the structure of the intermediate layers. The simulation methods developed to quantify the effect of anharmonic atomic vibrations on thermal conductivity provide a new tool for the rational design of thermoelectric materials, and the insights gained should hasten the attainment of higher conversion efficiencies so that thermoelectrics can be put to widespread practical use.

DOI： 10.1038/s41598-018-29259-z

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PubMed

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Other Link： http://www.nature.com/articles/s41598-018-29259-z

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsanm.8b00278

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The phonon thermal conductivities of misfit-layered Ca3Co4O9, Sr3Co4O9, and Ba3Co4O9 were calculated using the perturbed molecular dynamics method to clarify the impact of lattice misfit on the phonon thermal conduction in misfit-layered cobaltites. Substitution of Sr and Ba for Ca substantially modified the magnitude of the lattice misfit between the CoO2 and rock salt (RS) layers, because of the different ionic radii, increasing overall phonon thermal conductivity. Further analyses with intentionally changed atomic masses of Ca, Sr, or Ba revealed that smaller ionic radius at the Ca site in the RS layer, instead of heavier atomic mass, is a critical factor suppressing the overall thermal conductivity of Ca3Co4O9, since it determines not only the magnitude of lattice misfit but also the dynamic interference between the two layers, which governs the phonon thermal conduction in the CoO2 and RS layers. This concept was demonstrated for Sr-doped Ca3Co4O9 as an example of atomistic manipulation for better thermoelectric properties. Phonon thermal conductivities not only in the RS layer but also in the CoO2 layer were reduced by the substitution of Sr for Ca. These results provide another strategy to improve the thermal conductivity of this class of misfit cobaltites, that is, to control the thermal conductivity of the CoO2 layer responsible for electronic and thermal conductivity by atomistic manipulation in the RS layer adjacent to the CoO2 layer.

DOI： 10.1007/s11664-015-3938-7

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The phonon thermal conductivity of misfit-layered Ca3Co4O9 has been calculated by perturbed molecular dynamics using a classical force field. Detailed numerical analyses reveal that, in spite of its smaller cross-sectional area, the CoO2 layer transports more heat than the thicker rock salt (RS) layer, although its local thermal conduction is more suppressed than in another layered cobaltite, Na (x) CoO2. The origins of these differences have been elucidated through careful examination of the atomic arrangements in each layer. Since thermal conduction in the RS layer can be reduced without deteriorating electronic properties for which the CoO2 layer is responsible, it is suggested that the RS layer should be modified to further suppress the overall in-plane thermal conductivity. Computational experiments with increasing number of Ca-O planes in the RS layer showed the opposite trend to what can be predicted based on the misfit between two dissimilar layers. Further analyses to reveal the origin of these unexpected results provide yet another strategy to further decrease the thermal conductivity, namely to control the dynamic interference between atoms across the interface between two layers.

DOI： 10.1007/s11664-013-2902-7

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Event date： 2022.9

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Event date： 2021.12

Presentation type：Oral presentation (invited, special)  

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Event date： 2021.9

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Event date： 2024.12

Presentation type：Oral presentation (general)  

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Event date： 2024.9

Presentation type：Poster presentation  

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Event date： 2024.9

Presentation type：Poster presentation  

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Event date： 2023.6

Presentation type：Oral presentation (general)  

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Event date： 2022.3

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Event date： 2021.11

Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Oral presentation (invited, special)  

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Language：Japanese   Presentation type：Oral presentation (invited, special)  

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

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J-GLOBAL

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J-GLOBAL

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Authorship：Lead author,　Corresponding author   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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