Yasunori Kaneta

Structural behavior of uranium dioxide under pressure by LSDA + U calculations

The structural behavior of UO2 under high pressure up to 300GPa has been studied by first-principles calculations with LSDA+U approximation. The results show that a pressure-induced structural transition to the cotunnite-type (orthorhombic Pnma) phase occurs at 38GPa. It agrees well with the experimentally observed ~42 GPa. An isostructural transition following that is also predicted to take place from 80 to 130GPa, which has not yet been observed in experiments. Further high compression beyond 226GPa will result in a metallic and paramagnetic transition. It corresponds to a volume of 90A^3 per cell, in good agreement with a previous theoretical analysis in the reduction of volume required to delocalize 5f states.

Interplay of defect cluster and the stability of xenon in uranium dioxide from density functional calculations

Self-defect clusters in bulk matrix might affect the thermodynamic behavior of fission gases in nuclear fuel such as uranium dioxide. With first-principles LSDA+U calculations and taking xenon as a prototype, we find that the influence of oxygen defect clusters on the thermodynamics of gas atoms is prominent, which increases the solution energy of xenon by a magnitude of 0.5 eV, about 43% of the energy difference between the two lowest lying states at 700 K. Calculation also reveals a thermodynamic competition between the uranium vacancy and tri-vacancy sites to incorporate xenon in hyper-stoichiometric regime at high temperatures. The results show that in hypo-stoichiometric regime neutral tri-vacancy sites are the most favored position for diluted xenon gas, whereas in hyper-stoichiometric condition they prefer to uranium vacancies even after taking oxygen self-defect clusters into account at low temperatures, which not only confirms previous studies but also extends the conclusion to more realistic fuel operating conditions. The observation that gas atoms are ionized to a charge state of Xe+ when at a uranium vacancy site due to strong Madelung potential implies that one can control temperature to tune the preferred site of gas atoms and then the bubble growth rate. A solution to the notorious meta-stable states difficulty that frequently encountered in DFT+U applications, namely, the quasi-annealing procedure, is also discussed.

Study on Electronic Structure and Optoelectronic Properties of Indium Oxide by First-Principles Calculations

The electronic structure of In2O3 has been studied for the first time using a first-principles calculation method based on the density functional theory. Although the complexity of the crystal structure of In2O3 which contained 40 atoms in its unit cell had prevented studies of its electronic structure, we were able to study it using the characteristic of minimum basis sets of the linear muffin-tin orbital method with atomic sphere approximation. The calculated partial density of states (PDOS) showed that the valence bands were composed mainly of oxygen 2p-like states and the conduction bands consisted mainly of indium 5s-like states with free-electron-like character. The results of PDOS analysis were used to analyze the spectra from X-ray photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy. Calculated results were also used to interpret optoelectronic properties of tin-doped indium oxide.

Stability mechanism of cuboctahedral clusters inUO2+x: First-principles calculations

The stability mechanism of cuboctahedral clusters in nonstoichiometric uranium dioxide is investigated by first-principles LSDA+U method. Calculations reveal that the structural stability is inherited from U6O12 molecular cluster whereas the energy gain through occupying its center with an additional oxygen makes the cluster win out by competition with point oxygen interstitials. Local displacement of the center oxygen along direction also leads the cluster 8-folded degeneracy and increases relatively the concentration at finite temperatures. But totally, elevation of temperature, i.e., the effect of entropy, favors point interstitial over cuboctahedral clusters.

Electronic band structures of Ce3Pt3Sb4 and Ce3Pt3Bi4

One-electron energy band structures for Ce 3 Pt 3 X 4 (X=Sb and Bi), which belong to the valence fluctuation regime with an energy gap, are calculated by a self-consistent LAPW method with the local density approximation. The valence bands consist of the X p and the Pt 5 d states and the conduction bands are derived from the Ce 5 d states. With depressing the valence bands, the empty Ce 4 f bands are located between these bands and thus a narrow band gap appears at the Fermi level. This result explains reasonably the semiconductor-like property of these compounds.

Electronic band structure of the Th3Ni3Sb4 and Th3X4 (X=P, As, Sb)

In order to clarify the origin of the semiconductor like behavior of U 3 Ni 3 Sb 4 , one-electron energy band structures for the isostructural Th 3 Ni 3 Sb 4 and the Th 3 P 4 type Th 3 X 4 (X=P, As, Sb), which belong to the same space group, are calculated by the self-consistent APW method with the local density approximation. The calculated results for Th 3 X 4 show that the compounds are the narrow gap semiconductor or semimetal; the valence bands are derived from the X p states and the conduction bands from the Th 6 d states. Th 3 Ni 3 Sb 4 is also the semiconductor and the valence bands consist of the Ni 3 d and Sb 5 p states. Due to the mixing between the Ni 3 d and Th 6 d states, the empty Th 6 d conduction band is shifted up resulting a gap of 0.36 eV. This result explains reasonably why U 3 X 4 is semimetal but U 3 Ni 3 Sb 4 is semiconductor with a gap of 0.2 eV.

Ab initio investigation on oxygen defect clusters in UO2+x

Oxygen defect clustering in uranium dioxide had been indicated in powder neutron diffraction measurements, and an empirical clustering mechanism had been proposed to explain the data. However, using first-principles LSDA+U calculations, we find that this empirical model, in fact, cannot work. A more physically reasonable model is proposed based on a thermodynamical competition between point defects and cuboctahedral clusters. This mechanism interprets the puzzled origin of the observed asymmetric interstitial O′ and O″ naturally. It also gives a good and consistent agreement with all available experimental data, except the high occupation of the O″ site.

Fermi surface of Ce-monopnictides in various magnetic ordered states

Band structure calculations for the ferro- and ferrimagnetic phases of CeSb and CeBi are made based on the p-f mixing model and by including the full treatment of the anisotropic d-f exchange interaction in Ce ion. Calculated results well correspond to the Fermi surfaces observed by the de Haas van Alphen experiments. The band which hybridizes most strongly with the occupied f-states has not been observed. The mass enhancement factor for this band due to the scattering with magnon like modes is calculated to be about 11.6. This is consistent with the heavy fermion character of the band expected from low temperature specific heat measurement.

Magnetic states controlled by energetic ion irradiation in FeRh thin films

Changes in magnetic properties and lattice structure of FeRh films by 180 keV–10 MeV ion (H, He, and I) irradiation are studied. In spite of the irradiation with different ion species and wide range of energies, the changes in magnetization are dominated by solely a single parameter; the density of energy which is deposited through elastic collision between the ions and the samples. For the low deposition energy density, the magnetization increases with increasing the deposition energy density, while the lattice structure remains unchanged. When the deposition energy density becomes larger, however, the magnetization decreases after reaching the maximum value. The decrease in the magnetization accompanies the crystal structure change from B2 to A1. The present results imply that the magnetic state of FeRh films can be designedly controlled by the energetic ion irradiations.

p-f Mixing and Carrier Number in Ce-Monopnictides

On the basis of band calculation, the p - f mixing parameters and the spin-orbit splittings in La-monopnictides are evaluated, and temperature and pressure dependence of carrier number in Ce-monopnictides are studied. It is shown that in La-monopnictides the strongly correlated electron-hole pair effect is important, while in Ce-monopnictides the occupied bonding 4 f level near the Fermi level plays the most important role in the pressure effect. However, the electron-hole weak pair effect is dominant for the temperature dependence. Various anomalous properties are explained through the above model.