1. High Temperature Thermoelectric Properties of Oxygen-Deficient Ca3Co4O9
H.Nakatsugawa1), K.Nagasawa1) and Y.Okamoto2)
1) Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama , Japan
　　　　　　　　　　　　　　2) National Defense Academy, Hashirimizu, Yokosuka, Kanagawa , Japan
Conventional Research
Introduction
Since the discovery of a large thermoelectric power in the layered compounds NaxCoO2 and Ca3Co4O9,　misfit-layered cobalt oxides particularly have attracted considerable interest as candidates for thermoelectric (TE) materials.　A recent precise structural study by Masset et al.　revealed that the crystal structure of Ca3Co4O9 consists of an alternate stacking of a distorted three-layered rock salt (RS)-type Ca2CoO3 block layer (BL) and a CdI2-type CoO2 conducting sheet parallel to the c-axis.　However, detailed study of the temperature dependence of oxygen nonstoichiometry δ and its effect on the crystal structure, magnetic transitions, electronic structure and TE properties have not been reported. In this study, we investigate the neutron powder diffraction technique and also the measurements of TE properties, magnetic susceptibility and photoacoustic spectroscopy (PAS) to clarify both the TE properties and the valence state of Co ions.
Y.Miyazaki, Solid State Ionics 172, 463 (2004)
R.Asahi et al., Phys. Rev. B66, (2002)
J.Sugiyama et al., Phys. Rev. B68, (2003)
J.Shimoyama et al., Jpn.J.Appl.Phys. 42, L194 (2003)
Sample Preparation
Experimental Details
Modulated structure
symmetry operation
Neutron diffraction
: HERMES installed at a JRR-3M reactor in JAERI (λ=1.8205Å)
Sample A : Ca3Co4O9
Sample B : Ca3Co4O9-δ
Rietveld analyses
: program PREMOS 91, adopting a superspace group C2/m(1 p 0)s0
[CoO2] sheet : 3D diffraction peak (hkl), lattice constants (a, b1, c), space group C2/m
calcined in air at 920℃ for 12h
calcined in air at 920℃ for 12h
RS-type BL : 3D diffraction peak (hml), lattice constants (a, b2, c), space group C21/m
modulation function ：
[RS]b1/b2 [CoO2] : 4D diffraction peak (hklm), lattice constants (a, b1, b2, c), σ=(0, b1/b2, 0)
sintered in pure O2 at 920℃ for 24h
sintered in pure O2 at 920℃ for 24h
Magnetic susceptibility
: SQUID (Quantum Design, MPMS), ZFC & FC, H = 1000Oe, T = 4～380K
diffraction condition
3 times
3 times
： h + k + m = 2n
mean-field-approximation theory ：
furnace cooled to room temperature
furnace cooled to room temperature
constraint condition
density: 2.595g/cm3 (relative density: 55.2%)
Oxygen-deficiency
： [Ca2Coy1O3.08-δ]p Coy2O2
[CoO2] sheet :
(2n+1)b2* : Co1: Ay=Bx=By=Bz, O1: Ay=Bx=Bz
annealed in pure Ar at 740℃ for 48h
2nb2* : Co1: Ax=Ay=Az=Bx=Bz, O1: Ay=Bx=Bz
Electronic resistivity & Seebeck coefficient
： T = 80 ～ 900K
RS-type block layer :
(2n+1)b2* : Ca, O3: Ay=Bx=Bz, Co2, O2: Ay=Bx=By =Bz,
quenched into cold water
Photoacoustic spectroscopy
2nb2* : Ca, O3: Ay=Bx=Bz, Co2, O2: Ax= Ay=Az=Bx =Bz,
： λ = 400 ～ 1600nm, 5nm step, T = 300K
density: 2.549g/cm3 (relative density: 54.5%)
interatomic distances (MODPLT)
crystal structure (PRJMS)
Rietveld analysis (PREMOS 91)
Sample A
Rwp= 5.37%
0010
0020
0030
Lattice constants : a = (4)Å, b1 = (7)Å, c = (3)Å
　　　　　　　　　　　β= (5)°, b2 = (8)Å
　　　　　　　　　　　b1/b2 = (5)
ideal density: (7)g/cm3
[CoO2] sheet
RS-type
block layer a c b1 b2 O1
Co1
Co2, O2
Ca, O3
[CoO2] sheet
RS-type
block layer a c b1 b2 O1
Co1
Ca, O3
Co2, O2
Sample B
Rwp= 4.69%
0010
0020
0030
Lattice constants : a = (6)Å, b1 = (1)Å, c = (2)Å
　　　　　　　　　　　β= (8)°, b2 = (9)Å
　　　　　　　　　　　b1/b2 = (6)
ideal density: (7)g/cm3
photoacoustic
spectroscopy
magnetic susceptibilities
thermoelectric properties
LS → IS ?
IS → HS ? FC
ZFC
Co3.18+
Co3.48+
Sample A
Sample B
ZFC FC
Co3.11+
Co3.41+
1.55eV
1.24eV
1.04eV
Sample B
Sample A
Sample B
Sample A
Sample A
Sample B LS
HS？ LS
HS？
Summary
●　Sample A : [Ca2Co3.48+O3.08]0.62Co3.18+O Sample B : [Ca2Co3.41+O3.08－δ]0.62Co3.11+O2 ( δ=0.09 )
●　a, b1 and c increase, but b2 , ideal density and sample density decrease & relaxation in modulated structure
● magnetic transition temperature ( K) may be identified LS → IS and IS → HS transitions
●　broad peak positions of the absorption spectra can be confirmed in the wavelength range from 800 to 1200nm
Acknowledgments
This study was also partly supported by Takahashi Industrial and Economic Research Foundation. LS
HS？