Kenji Koga

Electronic structure and thermoelectric properties of Skutterudites

Thermoelectric properties on Yb filled skutterudite antimonides are calculated by using a realistic band structure and discussed from the point of view of electronic structure. The electronic structure is calculated by the full-potential linearized augmented plane-wave (FLAPW) method with the local density approximation (LDA). In the band structure calculation of YbFe/sub 4/Sb/sub 12/ the 4f electron is treated by two ways: one is as itinerant electron, another is as localized electron in the Yb ion. In the itinerant case Yb ion shows mixed valence between divalent and trivalent, in the localized case trivalent. In both cases the band structure near Fermi level is characterized by Fe: 3d orbital. The band structure of YbCo/sub 4/Sb/sub 12/ is calculated as Yb-filling CoSb/sub 3/, which is a n-type material. The f orbital of the Yb ion is strongly hybridized with the valence band, the conduction band near the bottom affects 6s and 5d of Yb atomic orbital. Electronic transport coefficients are calculated within relaxation time approximation using the linearized Boltzmann equation. Then the electron velocity is calculated by the Fourier interpolation method. The calculated coefficient of thermoelectric power /spl alpha//T is 0.07 /spl mu/V/K/sup 2/ at room temperatures, which is about 1/2 of the experimental value.

Band structure and thermoelectric properties of type-III barium clathrates

The electronic structure of type-III Ba clathrates was calculated by using a full-potential linearized augmented plane-wave (FLAPW) method. In the calculation of Ba₆Al₄Si₂₁ a virtual crystal approximation was used. The calculated band structure shows that Ba₆Al₄Si₂₁ is an intrinsic semiconductor with an indirect gap. The top of the valence band is at the Gamma point and the bottom of the conduction band is on the Lambda axis. The thermoelectric properties are calculated by using the calculated electronic states

Thermoelectric properties of Sn-based clathrates

Inorganic clathrates consisting in group-IV elements have crystal structures with large cages. Electronic structure and transport properties of Sn-based clathrate semiconductors have been studied by the ab-initio band structure calculating method. In this calculation the FLAPW method with the GGA is used and the Ga configuration of occupied sites is determined by the comparison of the total energy E/sub tot/ among various configurations. The band gap of the calculated electronic structure E/sub g/ is 0.16 eV, which agrees with the experimental result, estimated by the temperature dependence of the electric conductivity. However the band gap E/sub g/=0.16 eV does not explain the experimental Seebeck coefficient /spl alpha/. When we assume a lager band gap E/sub g/=0.45 eV and a carrier concentration n/sub e/=8/spl times/10/sup 18//cm/sup 3/, the calculated /spl alpha/ reasonably explains the experimental data.

X-ray spectra of skutterudite and filled skutterudite

X-ray near edge spectra of La-filled and unfilled skutterudites are studied by the first-principle density functional calculation. For this purpose electronic structures of LaCo/sub 4/Sb/sub 12/ and CoSb/sub 3/ are calculated by means of the full-potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation (GGA). As the electronic structure of skutterudite is sensitive to the lattice parameter, the lattice parameters of CoSb/sub 3/ are determined by the minimizing condition of the total energy including the spin-orbit (SO) interaction. The obtained lattice parameters are in good agreement with the experimental values. By using the determined values electronic structures of CoSb/sub 3/ and LaCo/sub 4/Sb/sub 12/ are calculated. The band gap of CoSb/sub 3/ is 110 meV, which is about a half of the result without SO interaction. X-ray absorption and emission near edge spectra for Co-L/sub III/ and Sb K are calculated and discussed.

Structure and stability of nano-metallic particles

The electron microscopic studies have been done for Au and Au-25 at.% Cu nanoparticles (1–10 nm) prepared by the inert-gas vapor-aggregation technique, and those annealed on amorphous carbon films at 723 K under vacuum. The Au-Cu particles with complex structures have been restructured into icosahedron (1–6 nm in diameter) and polycrystalline (>6 nm) particles by annealing, despite coalescence taken place between the particles. The Au particles, however, did not show such drastic structural changes by annealing. The results suggest that the Au-Cu binary state produces a global minimum for the icosahedral structure on the configurational free energy surface.The electron microscopic studies have been done for Au and Au-25 at.% Cu nanoparticles (1–10 nm) prepared by the inert-gas vapor-aggregation technique, and those annealed on amorphous carbon films at 723 K under vacuum. The Au-Cu particles with complex structures have been restructured into icosahedron (1–6 nm in diameter) and polycrystalline (>6 nm) particles by annealing, despite coalescence taken place between the particles. The Au particles, however, did not show such drastic structural changes by annealing. The results suggest that the Au-Cu binary state produces a global minimum for the icosahedral structure on the configurational free energy surface.