Masataka Mizuno

Chemical bonding at the Fe/TiX (X = C, N or O) interfaces

Abstract First principles molecular orbital calculations for the Fe/TiX (X=C, N or O) interfaces have been made by the use of the spin-polarized discrete-variational X α method. At the interfaces, no significant charge transfer occurs between the Fe and TiX layers. Ionic interaction is small and covalent bonding is predominant at the interface. The interfacial bond strength is stronger when Fe atoms are located on X atoms, that is, the Fe-on-X geometry of the Baker–Nutting orientation relationship. The Fe/TiX bond strength decreases with the rising atomic number of X. The antibonding of the FeO bonds at the Fe/TiO interface is noteworthy. Although the Fe/TiO interface shows the smallest lattice mismatch among three interfaces, it is expected that the chemical bonding at the interface is weakest. The interfacial bond strength by the present calculation agrees well with the potency of TiX for intragranular ferrite nucleation in steels that was recently found experimentally.

Effect of solute atoms on the chemical bonding of Fe3C (cementite)

Abstract First-principles molecular orbital calculations for Fe3C (cementite) containing solute atoms have been made by the use of the discrete-variational(DV)-Xα method. Metal-metal (M-M) bonds around solute atoms show strong dependence on the atomic number of solute atoms, but metal-C (M-C) bonds are nearly constant irrespective of the solute atoms. Overlap population diagrams show that the Fermi level in pure Fe3C lies in the antibonding band of the M-M bonding. The solute atoms induce the antibonding band to shift; thus the M-M bonds around the solute atoms are changed. The change in the magnitude of the M-M bonds with solute atoms is in good agreement with the variation in Vickers hardness of the Fe3C solid solutions. Since the M–M bond is dominant between (001) planes in Fe3C, it is natural that the M–M bond strength shows strong effects on the hardness which is mainly determined by the (001) slip. The present result is contrary to the previous views; it had been believed that the M–C bond strength ...

Elastic constants and chemical bonding of LaNi5 and LaNi5H7 by first principles calculations

Elastic constants of LaNi5H7 and LaNi5 are calculated by a first principles pseudopotential method using plane-wave basis sets. Some extra calculations using model clusters were made in order to discuss the magnitude of chemical bondings. Inner displacements associated with all deformation modes are taken into account. Elastic constants are smaller and more isotropic in the hydride than those in the host. The electronic mechanism to determine the change in the elastic properties is investigated from the viewpoint of chemical bonding. Strong Ni–H bonds are formed in LaNi5H7 at the expense of Ni–Ni bonds. They play key roles in determining the elastic properties. The isotropic distribution of the Ni–H bonding charge in LaNi5H7 should be responsible for the isotropic elastic constants. Hydrogen atoms are found to relax considerably during the deformation to maintain the Ni–H bond length. When the inner displacements are ignored, the elastic constants of LaNi5H7 are as large as those of LaNi5. However, the remarkable displacement of hydrogen atoms during the elastic deformation plays an essential role in softening by hydrogenation.

Theoretical calculation of positron lifetimes for LaNi5–H system

Abstract Positron lifetime spectroscopy is a powerful tool for the study of vacancy-type defects in solids. Our positron lifetime measurements have revealed that huge numbers of excess vacancies are formed in addition to dislocations during the first hydrogen absorption process of LaNi 5 and excess vacancies becomes mobile and form microvoids by a thermal activation process. For further investigation, theoretical approaches using electronic structure calculations are indispensable. In this work, we have performed theoretical calculations of positron lifetime for LaNi 5 –H system using first principles electronic structure calculations. By comparison between the theoretical and experimental positron lifetimes, one of the defect components during hydrogen absorption can be ascribed to the annihilation at vacancy-clusters composed of two or three Ni vacancies. The positron lifetime of the vacancy-cluster component increases over 400 ps during isochronal annealing after hydrogen desorption. The vacancy-cluster may contain not only Ni vacancies but also La vacancies, since the vacancy-cluster composed of only Ni vacancies cannot yield such a long positron lifetime.

Electronic Structures and Chemical Bonding of TiX2 (X=S, Se, and Te)

A systematic study of the electronic structures and chemical bonding of the titanium dichalcogenide TiX2 (X=S, Se, and Te) layered structures is performed by a first-principles molecular orbital calculation using the discrete-variational (DV)-Xα cluster method. The intra- and interlayer chemical bonding properties are also investigated using the bond overlap population. Valence band structures obtained by the calculation are in good agreement with experimental results obtained by X-ray photoemission spectroscopy. Each peak in the density of states (DOS) is identified from the viewpoint of chemical bonding. There is a considerably strong covalent bonding between Ti and chalcogen atoms in TiX2. The covalency of chemical bonding is shown to increase and the ionicity to decrease in the series of TiS2–TiSe2–TiTe2. Both in intra- and interlayer chalcogen-chalcogen bonding and intralayer Ti–Ti bonding are much weaker than Ti-chalcogen intralayer bonding. However, interlayer interaction is found to be not purely of the van der Waals type.

Importance of metal-metal bondings at the interface of MgO and 3d-transition metals

Abstract First principles cluster calculations of MgO(001)//M(001) (M=Sc to Cu) interfaces have been systematically made using numerical atomic orbitals as basis functions. Detailed analysis of bond overlap populations (BOP) found that the M–Mg covalent bond predominantly determines the interfacial bond-strength. M atoms prefer to be located on top of O atoms, not because the first nearest-neighbor M–O covalent bond is stronger than the M–Mg bond at the same bond-length, but because the M–Mg bond prefers to be at the second nearest-neighbor distance in order to increase the number of effective interfacial bonds from 1 to 4. The equilibrium interfacial bond-length is found to be determined by the balance of M–Mg bond-reinforcement and weakening of the M–M bond. Atomic number dependence of the interfacial bond-strength is also determined by the variation of the M–Mg BOP. It decreases with rising atomic number mainly because of the contraction of M-orbitals. Theoretical electron energy loss near edge structures (ELNES) are given to help interpretation of experimental spectra to be acquired directly from the interface.

Titanium Dioxide/Ultra High Molecular Weight Polyethylene Composite for Bone-Repairing Applications: Preparation and Biocompatibility

Particulate titanium dioxide (crystalline phase is anatase) w ith an average size of 200 nm was incorporated into an ultra high molecular weight polyethylene (UH MWPE) for potential medical applications. Composites with titanium dioxide volumes of 10 and 40 % were produced by a manufacturing process consisting of blending in the theta-composer and com pression molding with hot press. Titanium dioxide particles were well dispersed, and homogene ous distribution in the polymer matrix, achieved after compounding, was retained during subsequent composite processing. Thin-film X-ray diffraction pattern and Fourier transform infrare d spectra, obtained from TiO2/UHMWPE samples exposed for up to 10 days at 36.5°C to a simulated body f luid (SBF), demonstrated that composites of all the compositions examined developed t he surface biological apatite layer equivalent to that for bone-like apatite.

Positron Annihilation Study of Formation of Mg Vacancy in MgO

We have investigated the formation of Mg vacancy induced by ultra-dilute trivalent impurities in MgO by a combination of positron annihilation measurement and theoretical calculations of positron lifetimes. The undoped MgO yields the shortest positron lifetime of 130 ps that is shorter than that of 166 ps previously reported using a single crystal sample. The positron lifetime of the doped samples increases with the increase of the Al or Ga dopant concentration and is saturated at around 170 ps. This result indicates that the previously reported value of 166 ps is ascribed to not the bulk but the vacancy state induced by impurities. The experimental bulk lifetime of 130 ps, which is obtained by employing trapping model, is well reproduced by the theoretical calculation using the semiconductor model. The calculated defect lifetime is about 20 ps longer than the experimental value. This may be due to the lattice relaxation around Mg vacancy associated with the trapping of positrons.

Fermi surface of a shape memory alloy of TiNi

Direct experimental information about the geometry of the Fermi surface of Ti 48.5 Ni 51.5 whose martensitic transformation temperature is 170 K is presented for the first time. The present results have shown four sheets of the Fermi surface with the same topology, but with different shapes and sizes, as those predicted previously by band structure computations. From geometrical features of the Fermi surface, new possible nesting features of the electron surface at R and the hole surface at Γ are suggested for future theoretical computations of the generalized susceptibility and phonon spectra of this alloy system.

Interfacial structures of Y123 and Nd123 films formed on MgO(001) substrates by liquid phase epitaxy

Interfacial structures of c-axis-oriented YBa 2 Cu 3 O 7– y (Y123) and Nd 1+ x Ba 2– x Cu 3 O 7– y (Nd123) films were investigated by high-resolution transmission electron microscopy (HRTEM) in conjunction with geometrical lattice matching and molecular orbital calculations. These films were formed on MgO(001) substrates by liquid-phase epitaxy. Despite the similarity in lattice constants between Y123 and Nd123, the in-plane orientation relationship (OR) to the substrates is different: [100]film//[100]substrate(I) for Y123 and [110]film//[100]substrate(II) for Nd123. From the results of HRTEM observations and image simulations, it was found that the Y123 and Nd123 films are terminated by BaO and CuO-chain layers at the interfaces, respectively. For both the Y123/MgO and Nd123/MgO systems, the OR(I) is assessed to be the most favorable in point of geometrical matching and the OR(II) is the second among the rotational misorientations on the [001]film and [001]MgO. The molecular orbital calculations reveal that the interface with the OR(II) and the CuO-chain layer termination is preferable in terms of covalent bonding for both the systems. Consequently, we suggest that the preferential interfacial structures are delicately determined by a balance of the geometrical and chemical factors at the interfaces, resulting in making the lowest interfacial free energies.