Katsuyuki Matsunaga

First-principles plane-wave pseudopotential calculations were performed to study electronic structures, structural relaxation, and energetics of intrinsic vacancies and interstitials in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ In the presence of the intrinsic point defects, extra levels appeared in the band gap. Considering various charge states for the intrinsic point defects, it was found that each point defect is most stable in its fully ionized state. From the formation energies of individual po

Inorganic semiconductors generally tend to fail in a brittle manner. Here, we report that extraordinary "plasticity" can take place in an inorganic semiconductor if the deformation is carried out "in complete darkness." Room-temperature deformation tests of zinc sulfide (ZnS) were performed under varying light conditions. ZnS crystals immediately fractured when they deformed under light irradiation. In contrast, it was found that ZnS crystals can be plastically deformed up to a deformation strai

We have developed Tersoff potential parameters for boron in order to simulate cubic boron carbonitride systems by molecular dynamics. Combined with parameters for C and N available from the literature, our parameters are shown to reproduce the lattice parameters and bulk moduli of boron nitride and boron carbonitride (C 0.33 (BN) 0.67 ) with good accuracy. By simulating several systems of formula (C x (BN) 1- x ) over a wide range of carbon contents ( x =0 to 1), we observed the same trends in t

First-principles plane-wave calculations were performed for hydroxyapatite (HAp) in order to investigate the electronic structure and vacancy formation mechanisms. The HAp unit cell contains $\mathrm{P}{\mathrm{O}}_{4}$ tetrahedra and OH groups formed by covalent P-O and H-O bonds. Ca ions play a role for connecting the $\mathrm{P}{\mathrm{O}}_{4}$ tetrahedra and OH groups in an ionic character of bonding. It is found that $\mathrm{O}{\mathrm{H}}^{\ensuremath{-}}$ vacancies with accompanying ${\

First-principles calculations are performed for Mg(2+) and Zn(2+) substitution in hydroxyapatite (HAp) and octacalcium phosphate (OCP), because the foreign ions are known to play an important role for bone formation. In order to study their possible location in the system of HAp in contact with the aqueous solution, OCP is considered as a structural model of the transition region between HAp and the solution. It is found that, when the foreign ions substitute for Ca sites, the surrounding oxygen

We have performed molecular dynamics simulations of amorphous Si 3 N 4 containing boron (Si‐B‐N). We have examined short‐range atomic arrangements and self‐diffusion constants of amorphous Si‐B‐N systems with various boron contents. Our simulations show that boron atoms are threefold coordinated by nitrogen atoms and that nitrogen atoms are bonded to both silicon and boron atoms in the amorphous network of Si‐B‐N. Also, the self‐diffusion constant of nitrogen in Si‐B‐N is much decreased compared

A bonding mechanism of large-mismatched metal/oxide heterointerfaces, classified as incoherent interfaces, is investigated by first-principles calculations. As a model system, incoherent $\mathrm{Ni}∕\mathrm{Zr}{\mathrm{O}}_{2}(111)$ interfaces are selected, and the interfacial bonding characters and their relevance to the interface strength are analyzed. It is found that the chemical bonds of the interfacial atomic pairs are strongly dependent on the atomic configurations in the interface struc

First-principles calculations are performed to investigate atomic and electronic structures of Sr(2+) ions substituting for Ca(2+) in octacalcium phosphate (OCP). The defect formation energies are evaluated from total energies of supercells and ionic chemical potentials of Sr(2+) and Ca(2+) determined under the chemical equilibrium with aqueous solution saturated with hydroxyapatite (HAp). The defect formation energy depends on the solution pH and the substitutional Ca sites in OCP, and the esti

First-principles plane-wave pseudopotential calculations were performed to study electronic structures, structural relaxation, and energetics of point defects in Ti-doped ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ Substitutional and interstitial Ti ions with charge compensating intrinsic defects were considered, and their formation energies were evaluated under various atomic chemical potentials. It was found that substitutional ${\mathrm{Ti}}^{4+}$ ions with charge compensating Al vacancies were most

Vacancies, impurities, and foreign ions dissolving in calcium phosphate bioceramics play an important role in the biological properties of the materials. However, little is known about the thermodynamic stability of the defects. In this regard, point defects in hydroxyapatite (HAp) and octacalcium phosphate (OCP) were calculated in a first‐principles manner, and the chemical‐potential dependence of the defect formation energies was revealed. In particular, because calcium phosphates are usually

The behavior of grain boundary sliding in pure and yttrium-doped Al2O3 was directly measured at a high temperature, using bicrystal experiments. For this purpose, we fabricated Al2O3 bicrystals containing a random grain boundary with or without yttrium ions. High-resolution transmission electron microscopy observations and energy dispersive x-ray spectroscopy analyses showed that bicrystals were successfully joined at an atomic scale, and doped yttrium ions segregated along the grain boundaries.

Point defects, dislocations, grain boundaries and interfaces are always involved in ceramic microstructures and play important roles for physical and chemical properties of ceramics. Currently, proper control of these crystal defects is inevitable to tailor ceramic materials with superior properties. This article reviews recent research projects on distinct properties and phenomena in ceramics due to crystal defects. In particular, we would like to emphasize importance of central core regions of