Atsushi Togo 교수

The tetragonal to orthorhombic ferroelastic phase transition between rutile- and ${\text{CaCl}}_{2}$-type ${\text{SiO}}_{2}$ at high pressures is studied using first-principles calculations and the Landau free-energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with ${\text{B}}_{1g}$ and ${\text{A}}_{g}$ symmetries, shear moduli, transverse-acoustic mode, rotation angle of the ${\text{SiO}}_{6}$ octahedra, spontaneous symmetry-breaking an

Harmonic, quasi-harmonic, and anharmonic phonon properties of crystals are getting to be better predicted using firstprinciples phonon calculations by virtue of the progress of the calculation methods and increasing computer power. In this review, basic formulae of phonon properties are reviewed with the phonon calculation examples performed using the phonon calculation codes, phonopy and phono3py, combined with the first-principles calculations. The computational workflow to utilize the first-p

A collaboration of researchers from Japan and France present a comprehensive study of phonon lifetimes and thermal conductivity for 33 zincblende- and wurtzite compounds using linearized phonon Boltzmann equation and first-principles anharmonic phonon calculations. The software that the authors created for this study will be released as an open source package and should be of help in the search of new materials for thermoelectric applications.

Scientific simulation codes are public property sustained by the community. Modern technology allows anyone to join scientific software projects, from anywhere, remotely via the internet. The phonopy and phono3py codes are widely used open-source phonon calculation codes. This review describes a collection of computational methods and techniques implemented in these codes and shows their implementation strategies as a whole, aiming to be useful for the community. Some of the techniques presented

Thermal properties of ternary carbides with composition ${\text{Ti}}_{3}{\text{SiC}}_{2}$, ${\text{Ti}}_{3}{\text{AlC}}_{2}$, and ${\text{Ti}}_{3}{\text{GeC}}_{2}$ were studied using the first-principles phonon calculations. The thermal expansions, the heat capacities at constant pressure, and the isothermal bulk moduli at finite temperatures were obtained under the quasiharmonic approximation. Comparisons were made with the available experimental data and excellent agreements were obtained. Pho

The formation energies and electronic structure of native defects in tin monoxide are investigated by first-principles calculations. Equilibrium defect concentrations, which are obtained using the calculated formation energies and charge neutrality, indicate that the tin vacancy is the dominant defect under oxygen-rich conditions. It forms shallow acceptor states, suggesting that the $p$-type conductivity of tin monoxide originates from the tin vacancy. The equilibrium concentration of the oxyge

A computer algorithm to search the symmetries of crystal structures, as implemented in the spglib code, is described. An iterative algorithm is employed to robustly identify space group types, tolerating a certain amount of distortion in the crystal structures. The source code is distributed under the 3-Clause BSD License, a permissive open-source software license. This paper focuses on the algorithm for identifying the space group symmetry of crystal structures.

Crystal structures of metals are often treated as dense packing of atomic spheres. Face-centered cubic and hexagonal close-packed structures are favored in many metals. Long-period-stacking structures such as 9$R$ are sometimes formed. However, nonclose packed structures such as body centered cubic and $\ensuremath{\omega}$ are formed depending upon chemistry and process conditions. Even in metallic elements, it is a priori unknown how such close/nonclose packed structures are formed and what ar

A computer algorithm to search symmetries of crystal structures as implemented in the \texttt{spglib} code is described. An iterative algorithm is employed to robustly identify space group types tolerating a certain amount of distortion in the crystal structures. The source code is distributed under the 3-Clause BSD License, a permissive open-source software license. This paper focuses on the algorithm for identifying the space group symmetry of the crystal structures.

The pressure-induced transitions from molecular to nonmolecular $\mathrm{C}{\mathrm{O}}_{2}$ crystals are systematically investigated by using first-principles lattice dynamics calculations. Geometrically, likely transition pathways are derived from the dynamical instability of the molecular crystals under high pressures. Layered $\mathrm{C}{\mathrm{O}}_{2}$ crystals composed of a two-dimensional network of corner-sharing $\mathrm{C}{\mathrm{O}}_{4}$ tetrahedra are proposed as metastable product

The phase boundary between beta-Si(3)N(4) and gamma-Si(3)N(4) is investigated at high pressure and high temperature using first-principles lattice dynamics calculations within the quasi-harmonic approximation. We find a positive slope of the phase boundary, hence, at higher temperatures it requires higher pressures to synthesize the high-pressure polymorph of silicon nitride. It turns out that the thermal expansion of the spinel-type gamma-phase is larger than that of the phenacite-type beta-pha

The application of first-principles calculations for predicting lattice thermal conductivity (LTC) in crystalline materials, in conjunction with the linearized phonon Boltzmann equation, has gained increasing popularity. In this calculation, the determination of force constants through first-principles calculations is critical for accurate LTC predictions. For material exploration, performing first-principles LTC calculations in a high-throughput manner is now expected, although it requires sign

Phonon plays essential roles in dynamical behaviors and thermal properties, which are central topics in fundamental issues of materials science. The importance of first principles phonon calculations cannot be overly emphasized. Phonopy is an open source code for such calculations launched by the present authors, which has been world-widely used. Here we demonstrate phonon properties with fundamental equations and show examples how the phonon calculations are applied in materials science.

Longitudinal-optical (LO) mode phonon branches of KCl and NaCl were measured using inelastic x-ray scattering (IXS) at 300 K and calculated by the first-principles phonon calculation with the stochastic self-consistent harmonic approximation. Spectral shapes of the IXS measurements and calculated spectral functions agreed well. We analyzed the calculated spectral functions that provide higher resolutions of the spectra than the IXS measurements. Due to strong anharmonicity, the spectral function