Akihide Koura

Local clusters in a distorted rocksalt gete crystal found by X-ray fluorescence holography

A Ge Kα X-ray fluorescence holography (XFH) measurement was performed on a single-crystal GeTe film with a deformed rocksalt (rhombohedral) lattice, for which careful analyses and observations of the reconstructed atomic images were needed. The three-dimensional atomic images obtained around the central Ge atom were compared with the theoretical images obtained from a simulated hologram in the full sphere range and in the limited θ range (0 < θ < 74°), as in the experiment. It was found that reliable atomic images were realized only when neighboring atoms are located on the surface side of the central Ge atom. GeTe3 clusters were found by XFH, which cannot be easily detected by diffraction, and confirmed by ab initio molecular dynamics simulation.

Structural Changes of Short- and Intermediate-Range Order in Liquid Arsenic under Pressure

The pressure dependence of the structural properties of liquid arsenic (l-As) is studied in detail by ab initio molecular dynamics simulations and X-ray diffraction experiments. In this study, we have clarified that network structures consisting mainly of As4 units exist at lower pressures and that the correlation between the As4 units is the origin of an intermediate-range order, which is seen as a prepeak of the static structure factor. When the pressure increases, the intermediate-range order disappears and structural change occurs gradually. At approximately 7 GPa, the pair distribution function and the bond angle distribution show some similarities to those of the simple cubic structure as seen in the high-pressure phase of crystalline As.

Dynamic asymmetry of self-diffusion in liquid ZnCl2 under pressure: An ab initio molecular-dynamics study

The static and dynamic properties of liquid ZnCl2 under pressure are investigated by ab initio molecular-dynamics simulations. The pressure range covers ambient to approximately 80 GPa. The ZnCl4 tetrahedra, which are rather stable at ambient pressure, are shown to deform and collapse with increasing pressure while maintaining an almost constant nearest-neighbor distance between Zn and Cl atoms. The average coordination number of Cl atoms around Zn atoms increases monotonically with pressure, from four at ambient pressure to seven at approximately 80 GPa. Although the self-diffusion coefficients of Zn and Cl atoms, d(Zn) and d(Cl), are almost the same at ambient pressure, the difference between them increases with pressure. At around 10 GPa, d(Zn) is about two times larger than d(Cl). Under further compression, this dynamic asymmetry becomes smaller. The microscopic mechanism of the appearance of the dynamic asymmetry is discussed in relation to the pressure dependence of the local structure.

Concentration Dependence of the Dynamic Properties of Liquid TlxSe1-x Based on Ab initio Molecular-Dynamics Simulations

The concentration dependence of the dynamic properties of liquid TlxSe1-x alloys is investigated by ab initio molecular-dynamics simulations. In the concentration range \(x=0.0\) to 0.67, the highest diffusivity occurs at approximately \(x=0.2\) with the existence of fairly long Se chains in the liquid. Upon the further addition of Tl atoms, the self-diffusion coefficients of both Se and Tl decrease despite the shortening of the Se chain length. This behavior is due to the suppression of self-diffusion by the increase in the strength of electrostatic interactions between Tl and Se atoms. The concentration dependence of the ionic conductivity is discussed on the basis of the fluctuation–dissipation theorem as well as the Nernst–Einstein relation.