1. Crystal structure, electric and magnetic properties in NaxCoO2
IUMRS-ICAM2003
Crystal structure, electric and magnetic properties in NaxCoO2
K.Nagasawa and H.Nakatsugawa
Division of Materials Science and Engineering, Graduate School of Engineering,
Yokohama National University, Japan

2. INTRODUCTION
A layered cobalt oxides NaxCoO2 have been known as a candidate for thermoelectric materials and a large number of studies have been made on the strongly correlated electron system in the CoO2 layers.
But
Little is known about the relationship between CoO2 layers and Na+ layers with increasing sodium content.
From this point of view, we studied crystal structure, electric and magnetic properties in these materials.

3. EXPERIMENTAL Measurements X-ray diffraction measurement (Cu Kα)
Synchrotron radiation X-ray diffraction measurement in SPring-8 by JASRI
Magnetic susceptibility measurement using MPMS SQUID magnetometer
Analyses
Rietveld analysis by RIETAN-2000 code
Izumi F. and Ikeda T., Mater. Sci. Forum Vol pp (2000).
Maximum entropy method by MEED code
Kumazawa S.et al. , J.Appl.Crystallogr. Vol. 26, pp (1993).

4. Sample preparation : x=0.73 , 0.75 , 0.77 , 0.79 , 0.81 , 0.83
mixing
Na2CO3, Co3O4
calcination
Rapid heat-up (RH) technique
880℃ 12h
Specimen is directly placed in a furnace pre-heated at 750℃
Instantaneously start chemical reaction
Avoiding Na evaporation
calcination
880℃ 12h
pressing
4MPa
T.Motohashi.et.al Appl.phy.lett. Vol. 79, pp (2001).
sintering
920℃ 12h

5. Powder x-ray diffraction patterns at 300K
RESULTS
Powder x-ray diffraction patterns at 300K
x=0.75
002
106
112
008
104
006
110
202
108
102
004
100
103
101
105
114
200
107
201
x=0.77
106
002
110
104
006
112
008
102
202
108
004
100
103
101
105
114
200
107
201

6. Deviations in the 008 and 108 peaks with the change of Na content x
002
x=0.81
106
104
006
102
004
110
100
103
105
101
112
108
008
114
202
107
200
201
x=0.83
106
002
110
104
006
102
004
100
103
112
108
101
105
114
008
202
200
107
201
Deviations in the 008 and 108 peaks with the change of Na content x

7. Rietveld refinements of XRD measurements at 300K

8. Synchrotron radiation X-ray powder diffraction
x=0.75 T=300K
110
112
106
008
202
108
114
200
107
201
x=0.75 T=200K
110
106
112
202
114
108
008
200
107
201
x=0.75 T=100K
110
106
112
114
202
108
200
008
107
201

9. 110
x=0.77 T=300K
106
112
008
202
108
114
200
107
201
110
x=0.77 T=200K
106
112
114
202
200
108
008
107
201
x=0.77 T=100K
110
106
112
114
202
108
200
008
107
201

10. 106
110
x=0.79 T=300K
112
114
202
108
008
200
107
201
106
x=0.79 T=200K
110
112
114
202
108
008
200
107
201
106
110
x=0.79 T=100K
112
114
202
108
008
200
107
201

11. x=0.81 T=300K
110
106
112
114
202
108
008
200
107
201
x=0.81 T=200K
110
106
112
114
202
108
200
008
107
201
x=0.81 T=100K
110
106
112
114
202
108
200
008
107
201

12. Deviations in the 008 and 108 peaks with the change of temperature
x=0.83 T=300K
110
106
112
114
202
108
008
200
107
201
x=0.83 T=200K
110
106
112
114
202
200
108
008
107
201
x=0.83 T=100K
110
106
112
114
202
200
108
008
107
201
Deviations in the 008 and 108 peaks with the change of temperature

13. Rietveld refinements of Synchrotron radiation X-ray measurements

14. Crystal structure of (Space group: P63/mmc)
2b site
2d site
Na2
Na1 Co O

15. 3D-MEM charge density distributions using Synchrotron radiation X-ray diffraction data
　　　x=0.75 T=300K　　　　　　equi-counter surface 1.5 e/Å3
　　　x=0.75 T=300K　　　　　　equi-counter surface 2.0 e/Å3

16. 　　　x=0.75 T=200K　　　　　　equi-counter surface 1.5 e/Å3

17. 　　　x=0.75 T=100K　　　　　　equi-counter surface 1.5 e/Å3

18. Co-O hybridization x=0.81 T=300K equi-counter surface 1.5 e/Å3

19. Co-O hybridization x=0.81 T=200K equi-counter surface 1.5 e/Å3

20. Co-O hybridization
　　　x=0.81 T=100K　　　　　　equi-counter surface 1.5 e/Å3
　　　x=0.81 T=100K　　　　　　equi-counter surface 2.0 e/Å3
formation of the Co-O hybridization in the CoO2 layer with increase of Na content x

21. Magnetic properties
Magnetic parameters

22. SUMMARIES Acknowledgments
In the Rietveld refinement, there are good agreement between x=0.75 and x=0.81, except for (008) and (108) peaks. The deviation of the two reflections is very large relative to that of other reflections.
Electron density distribution based on the synchrotron radiation X-ray diffraction data show the direct observation of increase of the Co-O hybridization in the CoO2 layer with increasing the sodium content.
Acknowledgments
We are grateful to K. Kato and S. Aoyagi for their help in the synchrotron radiation experiments which were performed at the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No.2003B0055-ND1a-np). This work was supported by CASIO foundation for the promotion of science, Yazaki foundation for the promotion of science and engineering and Yokohama manufacturers association foundation.