Browsing by Author "Kim, M.G."

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Correlation Between Local Structures and Partial Electronic Band States in Ferroelectric Bi4?Xlaxti3o12; 0.0?X?1.46
(Ceramics-Silik�ty, 2009) Hidaka, M.; Wijesundera, R.P.; Kumara, L.S.R.; Noguchi, Y.; Miyayama, M.; Choi Sun-Hee; Sung, N.E.; Kim, M.G.
The local structures and the partial band states near a Fermi level (EF) have been studied at room temperature in the ferroelectric phase of Bi4?XLaXTi3O12 (BLT) with 0.00?x?1.46 by means of X?ray absorption spectra (XAS) including EXAFS and XANES. The EXAFS spectra suggest that, as increasing the substituted La ions, the local structures around Ti and Bi ions in perovskite-blocks of (Bi2?XLaXTi3O10)-2 are gradually deformed till x=0.85, and are largely changed by a structural phase transition occurring between x=0.85 and 1.20. The XANES spectra suggest that there is a strong hybridization between Ti-3d(t2g) and O-2p bands and between Bi-6s and O-2p bands in the valence band states, while between Ti-3d(eg) and O-2p bands and Bi-6p and O-3s bands in the conduction band states. All of the XAS show interesting dependency on the concentration of the substituted La ions in BLT. The results suggest that, in BLT, a reducing spontaneous polarization induced by the La-substitution results from the deformation of the local structures around Ti and Bi ions and from modulation of the hybridized band states near EF.
Electronic states and local structures of Cu ions in electrodeposited thin films of Cu and Cu2O from X-ray absorption spectra
(Physica Status Solidi b (basic solid sates physics), 2006) Wijesundera, R.P.; Hidaka, M.; Siripala, W.; Choi Sun-Hee; Sung, N.E.; Kim, M.G.; Lee, J.M.
X-ray absorption spectra near Cu K edges have been obtained for Cu and Cu2O thin films potentiostatically electrodeposited in an acetate bath. The electronic states of Cu ions and the local structures around these ions are studied using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, respectively. The inner strains in micro-size crystals of the electrodeposited Cu thin film are less than those of a commercial Cu foil. The partial electronic states of Cu ions near the Fermi level (EF) and the local structure around these ions show a systematic transformation from a Cu2O single phase (Phase I), to a Cu2O?Cu intermediate phase (Phase II) and a Cu single phase (Phase III) in thin films electrodeposited on titanium substrates for the deposition potential range of ?100 to ?900 mV. Phase II consists of independent Cu2O-like and Cu-like local structures, but not a Cu2O?Cu complex. Phase II is also characterized by a transition region from the semiconductor Cu2O to the metal Cu for the Cu electronic state having a p-like character near the Fermi level in the oxygen O-2p conduction band. (? 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Metal-insulator transition induced by electronic and structural modulations in oxygen-deficient perovskite-type TbBaCo2O5.5
(Physica Status Solidi b (basic solid state physics), 2006) Hidaka, M.; Soejima, M.; Wijesundera, R.P.; Soda, M.; Sato, M.; Choi Sun-Hee; Sung, N.E.; Kim, M.G.; Lee, J.M.
The metal?insulator (M?I) transition at about 340 K (TMI) in TbBaCo2O5.5 has been investigated by X-ray absorption spectra near Tb-LIII, Ba-LII, and Co-K edges. In the metallic and insulating phases, the high-density unoccupied bands of Tb-6d? and Ba-6d? occupy just above a Fermi level (EF), while the positive charge-transfer Tb-5dnLm and Ba-5dnLm bands are just below EF. The narrow positive charge-transfer Co-3d7(eg)L band is just below EF, where L represents a hole induced in the valence band of O-2p. It is considered that the metallic conductivity above TMI results from the (p?d) hybridized holes in the top region of the O-2p valence band and the hopping electrons in the bottom of the O-2p conduction band. The spin-state transition for Co ions in the CoO6 octahedra and the CoO5 pyramids does not occur at TMI. The local structural distortion induced around the Ba ions, but not around the Tb and Co ions, mainly affects the electronic configuration interaction between the Ba-5d and the O-2p orbitals in BaO. (? 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)