UVSOR-II BL5U

Electronic Structure of Pseudo-one Dimensional Ba3Co2O6(CO3)0.7
K. Soda1, M. Kikuchi2, S. Ogawa1, H. Miyazaki1,3, M. Inukai1,4, M. Kato1, S. Yagi1,K. Iwasaki1,5 and T. Matsui1,6,1
Graduate School of Engineering, Nagoya University, Nagoya 464-8603 Japan
2
School of Engineering, Nagoya University, Nagoya 464-8603 Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
Venture Business Laboratory, Nagoya University, Nagoya 464-8603 Japan
5Toyota Boshoku Coporation, Kariya 448-8651, Japan
6
EcoTopia Science Institute, Nagoya University, Nagoya 464-8603 Japanmight be expected in one-dimensional fermion
system [4], the anomalous exponents from 0.5 at 20K to 0.8 at 200 K are obtained for EB = 0.01~0.1 eV,suggesting the larger short-range interaction at thelower temperature. A small hump at EB ~ 0.02 eV
observed at T = 20 K might also imply opening of apseudogap or a magnetic ordering, which causes therecently observed reduction in the electric
conductivity at low temperatures [5].
2Ch/eV75601GHABDFE0102015105BINDING ENERGY / eVDIFFERENCEINTENSITY / arb. units

Ba3Co2O6(CO3)0.7

0Fig.1. Valence-band spectra of Ba3Co2O6(CO3)0.7.
1T/K200
100200.05

INTENSITY / arb. units

Cobalt oxides such as NaxCoO2 have attracted muchattention because of their fascinating transport andmagnetic properties. Recently it has been found that abarium cobalt oxycarbonate Ba3Co2O6(CO3)0.7, whichhas pseudo-one dimensional structure with Co-O
chains consisting of face-sharing CoO6 octahedra
along the c axis, shows a fairly large thermoelectric
power factor of 0.9 mWm-1K-2 at 300 K with thethermoelectric power of about +120 V K-1 andmetallic behavior of its electric conductivity above
300 K [1]. In this report, we have investigated its
valence-band electronic structure by photoelectron
spectroscopy to understand the physical properties.
Photoelectron measurements were carried out at thebeamline BL-5U of UVSOR-II. Single crystalline
specimens of Ba3Co2O6(CO3)0.7 was prepared in sizeof 5 x 0.5 x 0.5 mm3 by a flux method [1], and theirclean surfaces was obtained by in situ fracturing thespecimens in perpendicular to the c axis.
Figure 1 shows typical photoelectron spectra
recorded at 20 K with the excitation photon energiesh of 60 and 75 eV as well as their difference
spectrum. Each spectrum is normalized with theintegrated intensity and subtracted the background by
an iteration method [2]. There are features A to H
observed in the spectra; the features A to C areascribed to the hybridized bands of the Co 3d and O
2p states, while the features D, F, G and H are
assigned to the CO3-derived states, Ba 5p spin-orbit
doublets and O 2s state, respectively. The feature E isattributed to the surface components. The remarkable
suppression of the features A and B at h = 60 eV is
due to the Co 3p-3d resonance, which indicates therelatively large Co 3d contribution to the features Aand B while the O 2p one to the feature C.
Figure 2 shows detailed spectra near the Fermi level
EF measured at h = 40 eV and several temperatures
T in comparedison with reference Au spectra.
Ba3Co2O6(CO3)0.7 reveals large reduction in intensity
towards EF but clear finite intensity at EF. This may
suggest the electron doping into the low-spin bands ofCo4+(t2g 3d5), which causes the positive thermoelectric
power of 81 or 141 V K-1 at high temperatures for
the Co4+ concentration x of 0.7 [3], consistent withthe observed value. Although a 1/8-power-law
dependence of the intensity on the binding energy EB

Au
00000.15

Ba Co O (CO )30.1

263 0.7

0.05
0-0.05
BINDING ENERGY / eV-0.1

Fig.2. Valence-band spectra near the Fermi level.
[1] T. Yamamoto et al., Proc. Int. Symp. on EcoTopia
Science 2007, p.145.
[2] K. Soda et al., J. Phys. Soc.Jpn. 60 (1991) 3059.
[3] W. Koshibae et al., Phys. Rev. B 62 (2000) 6869.
[4] J. Voit, Pep. Prog. Phys. 57 (1994) 977.
[5] K. Iwasaki, private communication.