Hirohiko Adachi

Electronic effect on the ductility of alloyed TiAl compound

Abstract Alloying effects on the electronic structure of a TiAl compound have been investigated systematically using the discrete variational (DV) Xα cluster calculation. It has been shown that the existence of directional Al p-Ti d covalent bonds causes this compound to be brittle. The addition of those elements into TiAl which weaken p-d interactions but enhance d-d interactions is most effective in improving the ductility.

Theoretical x‐ray absorption spectra of SF6 and H2S

We have performed molecular orbital calculations for SF6 and H2S, using the discrete–variational Xα method. Energies and cross sections of virtual states are obtained for theoretical spectra of sulfur K, LII,III and fluorine K x‐ray absorptions for the SF6 molecule. They are in good agreement with the experimental results. Through the same calculation procedures, the theoretical spectra of sulfur K and LII,III absorptions for H2S are derived. The obtained molecular orbitals represent the experimental spectra below the ionization energy very well. SF6 has distinct shape resonances above the ionization energy, in contrast to H2S which has pre‐edge peaks as a main structure. For the SF6 molecule, shape resonances come from the scattering at the steep change of attractive potential of surrounding fluorine atoms. It is demonstrated that phase shift due to the scattering causes the resonances without a potential barrier. Various spectral differences among the sulfur K, LII,III and the fluorine K absorptions in ...

Electronic structure behind the mechanical properties of β-sialons

Abstract High-purity β-sialons (Si 6− z Al z O z N 8− z ) have been fabricated by hot-isostatic-pressing and their mechanical properties were examined. Youngs modulus and Vickers hardness decreased almost linearly with the increase of z -value. The decrease was ascribed to the lattice softening, which was confirmed by Raman spectroscopy. At high temperatures, creep rate of sintered materials and also sintering rate of powders were significantly enhanced with the increase of z -value. Molecular orbital calculations for model clusters were performed by means of discrete-variational (DV) X α method. The electronic structures were quantitatively evaluated by Mulliken population analysis. Strength of covalent bonding expressed as overlap population was found considerably decreased at around the solute atoms, especially at Al atoms, which has been proposed as the electronic mechanism behind the above properties of β-sialons.

Calculation of electronic structure and photoabsorption spectra of monosilane molecules SiH4, SiF4, and SiCl4

The electronic structure, ionization potentials, and photoabsorption spectra of monosilane molecules SiH4, SiF4, and SiCl4 were calculated using the discrete variational (DV) Xα method. Valence molecular orbitals (MOs) of SiH4 consist (from the lowest) of two occupied bonding MOs between Si and H, a1 and t2. Inner valence MOs of SiF4 and SiCl4 consist of the bonding MOs between Si and halogen, a1 and t2, and outer valence MOs consist of bonding MOs a1 and t2, and the MOs e, t2, and t1 localized on halogen. The lowest unoccupied MOs of SiH4 include two antibonding states t2 and a1, and two localized states, e and t2. The lowest unoccupied MOs of SiF4 and SiCl4 are antibonding states a1 and t2 between Si and halogen. Calculated ionization potentials agree well with measured photoelectron spectra.Calculation of the photoabsorption spectrum for Si 2p core excitation for SiH4, SiF4, and SiCl4 shows that peak positions and intensities agree well with measured photoabsorption spectra in both gas and solid phases...

DV-Xα calculation on resonances in X-ray absorption spectra of SF6

Abstract We have performed molecular orbital calculations for SF 6 , using the discrete-variational Xα method. The theoretical energies for the resonances near the X-ray absorption edges for the sulfur atom are in good agreement with the experimental results. The resonances correspond to the wavefunctions which have very large amplitude at the sulfur atom. The present results indicate that the resonances originate from the interference between the wavefunctions in the central sulfur atom those scattered by the surrounding fluorine atoms. Other types of resonances are also found and their wavefunctions are spread over the molecule.

Calculation of the electronic structure of the polar ZnO(0001) surface by the DV-Xα cluster method

Abstract Discrete variational (DV) Xα cluster model calculations are performed for surface electronic structures of ZnO. Two types of cluster models are investigated for the polar (000 1 ) oxygen surface of ZnO, one of which is a stoichiometric face and the other is a reconstructed face in polarity by removal of some O ions from the surface. The electronic states of these two clusters are calculated and compared with that for the bulk cluster model also calculated in the present study. It is found that the stoichiometric surface cluster has a too large Fermi energy ( E F ) of about 110 eV, suggesting instability. The reconstructed surface cluster provides a reasonable E F , in the vicinity of which the levels mainly constructed by O 2p orbitals are localized. Effects of exterior electrostatic field on the electronic state of the clusters are taken into account by using an embedded cluster model in a point charge lattice. Further, calculations have also led to a model for a hydroxylated (000 1 ) face. The result shows that the levels mainly consisting of surface O 2p orbitals localized near E F in the reconstructed surface cluster, largely shift to lower energy by the formation of a covalent O-H bond.

Discrete-variational Dirac-Slater calculation on XPS for the valence levels of UF6

Abstract Relativistic molecular orbital calculations of the valence electronic structure of UF 6 were done using the discrete-variational Dirac-Slater method (DV-DS). Theoretical ionization energies of the UF 6 valence levels were evaluated by means of Slaters transition-state method. The ionization energies and relative X-ray photoelectron spectrum intensities of UF 6 valence levels calculated in the present work are in good agreement with the experimental X-ray photoelectron spectrum.

Application of the DV-Xα cluster method to calculations of the electronic structure of silicate and phosphate glasses

The electronic structure of simple silicate and phosphate clusters, which are assumed to exist in rapidly quenched Li2Oue5f8SiO2 and Li2Oue5f8P2O5 glasses, has been calculated by the discrete variational (DV)-Xα molecular orbital method. The calculations have been carried out for several sizes of clusters, namely, tetrahedral units (SiO44− and PO43−), dimers (Si2O76− and P2O74−), and trimers (Si3O96− and P3O93−). For both systems the valence electronic state is characterized by an occupied valence band, with O 2p components, and an unoccupied band of Si or P orbitals, separated by an energy gap. The energy gap between the highest occupied molecular orbital (HOMO) and the loest unoccupied molecular orbital (LUMO) decreases with increasing cluster size. A mulliken population analysis show that the negative effective charge on the non-bridging oxygens is decreased with decreasing energy gap. The bond order of the Siue5f8O and Pue5f8O bonds, and thus the strength of these bonds, is increased with increasing cluster size.

Theoretical study on the UV excited states of UF6 using the discrete-variational Dirac—Slater calculation method

Abstract We have performed the relativistic molecular orbital calculations for the ground and UV excited states of UF 6 , using the discrete-variational Dirac—Slater method. For the ground state of UF 6 , the formation of the Uue5f8F bonding is explained by the relativistic MOs, in contrast to the non-relativistic MOs which fail to give the bonding character between the U and F atoms. The theoretical excitation energies for the charge transfer transitions are in good agreement with the experimental UV absorption spectrum. The changes in the bond overlap population of the excitation can explain the experiments that the excitation in the A—X band leads to fluorescence, while that in the B—X band to the dissociation of Uue5f8F bonding. This suggests that the difference in the character of the de-excitation processes is mainly determined by the bonding nature for the initial and the final levels on the excitation.

Cluster model calculations of oxygen chemisorption on (001) surfaces of bcc V, Cr and Fe

Abstract The discrete variational (DV) Xα cluster model is used to study the chemisorptions on (001) surfaces of bcc V, Cr and Fe. The adatom induced levels and the electronic charge densities are calculated and compared with experimental data such as UPS and LEED analysis. The oxygen induced levels and the change in valence-state density explain well the UPS spectra of oxygen on Fe surface. The results suggest that the four-fold hollow site of (001) surface is more stable than the on-top site for oxygen chemisorption, and that the first layer distance of the surface is expanded by chemisorption, consistent with the experiment.