Humihiko Takei

Origin of the metallic conductivity in PdCoO2 with delafossite structure

Abstract Valence state, valence band structure and specific heat of metallic oxide PdCoO 2 with the delafossite structure, which has alternative layers of Pd, Co and O ions with a formation of each triangular lattice, are first investigated using polycrystalline samples by means of photoelectron spectroscopy and specific heat measurements. The lowest electrical resistivity in PdCoO 2 among the whole conductive oxides is attributable to a half-filled band formed by s–d hybrid orbitals originated from the monovalent Pd ions and coordinated oxygens.

Flux growth of single crystals of spinel ZnGa2O4 and CdGa2O4

Single crystals of spinel ZnGa2O4, which has been reported to be a promising material as a transparent electronic conductor, have been grown from PbOB2O3 flux. Inclusion free and optical clear single crystals of maximum dimensions of 10 mm along the edge have been obtained. The lattice constant of the obtained single crystal is 8.332(3) A and the molar ratio of GaZn is strictly stoichiometric and homogeneous along any directions, typically as Zn0.96−1.07Ga1.94−2.02O4. High crystallinity has been proven and the space group of Fd3m has been confirmed from X-ray precession photographs. Single crystals of spinel CdxGa2O4−x have also been obtained from the same flux system. Due to the volatility of CdO, the obtained crystals are Cd-poor in composition, ranging from 0.7 to 0.9 in x. The lattice constant of Cd0.9Ga2O3.9 is 8.615 A.

Growth of MgSiO3 orthoenstatite single crystals by the top-seeded solution growth (TSSG) method

Single crystals of orthoenstatite (MgSiO3) have been grown by Top-Seeded Solution Growth (TSSG) method in the system MgOSiO2Li2CO3MoO3V2O5. The crystals are pulled along the c-axis, and are surrounded by the {1 0 0}, {0 1 0}, {2 1 0}, {1 1 1} and {2 1 1} faces. This morphology is the same as that observed in the flux-grown crystals. The tip surfaces of the crystals show that the growth speed of {1 1 1} face is faster than that of {2 1 1}. Imperfections such as inclusions and holes, are seen concentrated in the central portion, inherited from the seed interface. The faster growth along the c-axis than other crystallographic directions is responsible for the appearance of these imperfections.

Thermal Expansions of Pure and Al-doped CsLiB6O10 Crystals for Nonlinear Optical Applications

Changes in X-ray powder diffraction patterns of pure and Al-doped CsLiB6O10 (CLBO) crystals for nonlinear optics upon heating are observed between room temperature and 500°C. The crystals undergo linear elongation by about 2×10-5 deg-1 along the a-axis, and contraction by about -2×10-5 deg-5 is observed along the c-axis. Such extraordinary behavior on heating is a reflection of the structure of CLBO which costists mainly of a combination of hexagonal B3O4 rings, and a marked difference in linkage of the rings appears between the a- and c-axes. No significant changes in the thermal expansion coefficients are observed on Al-doping, though considerable expansions along the a- and c-axis are observed.

Study of the oxidation and reduction kinetics of copper iron oxide [CuFeO2] in the Cu-Fe-O system

The effect of surrounding atmosphere on the phase relations of CuFeO{sub 2} during crystal growth was investigated under CO{sub 2}, Ar, Ar + 0.5% O{sub 2}, Ar + 1% O{sub 2}, and Ar + 2% O{sub 2}, respectively, using scanning electron microscopy (SEM) together with electron probe microanalysis (EPMA) and X-ray powder diffraction (XPD). CuFeO{sub 2} was found to have partially decomposed into CU{sub 2}O and Fe{sub 2}O{sub 3} before melting, and crystal growth was progressed by means of the traveling-Cu{sub 2}O solvent. Under CO{sub 2} and Ar, Cu{sub 2}O was found to be reduced to Cu. No reduction of Cu could be found under Ar + 0.5% O{sub 2}, Ar + 1% O{sub 2}, and Ar + 2% O{sub 2} atmospheres, whereas the reduction of CuO was observed under an Ar + 2% O{sub 2} atmosphere.

Electronic Band Structure of SrCuO2

The energy band structure of a quasi one-dimensional cuprate SrCuO 2 is studied by angle-resolved photoemission spectroscopy and the linear augmented-plane-wave band structure calculation within local density approximation. It was found that the band structure has strongly one-dimensional nature with a finite dispersion along the Cu-O chains. The calculated band structure of SrCuO 2 is in good agreement with the angle-resolved photoemission result except for the region near the Fermi level.