1. 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 (XRD) measurement
Neutron diffraction (ND) measurement
Inductively coupled plasma atomic-emission spectrometry (ICP-AES) analysis
Magnetic susceptibility measurement using MPMS SQUID magnetometer
Analyses
Rietveld analysis by RIETAN-2000 code
Maximum entropy method by MEED code

4. Sample preparation : x=0.77(6), 0.78(3), 0.79(9) and 0.82(6)
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
pressing
4MPa
sintering
920℃ 12h
T.Motohashi.et.al Appl.phy.lett.vol (2001)

5. Powder x-ray diffraction patterns at 300K
RESULTS
Powder x-ray diffraction patterns at 300K
002
x=0.77(6)
202
108
112
008
104
006
102
004
100
103
106
110
114
101
105
200
203
107
201
002
x=0.78(3)
112
008
202
108
104
006
102
004
100
103
106
110
105
114
101
200
107
203
201

6. Deviations in the 008 and 108 peaks
x=0.79(9)
112
002
108
104
006
102
100
106
114
004
103
110
008
105
202
101
200
203
107
201
002
x=0.82(6)
112
108
104
006
114
102
004
100
106
103
110
202
101
105
008
200
203
107
201
Deviations in the 008 and 108 peaks

7. Rietveld refinements of XRD measurements at 300K
Structural parameters, reliability factors and Goodness of fit at 300K

8. Neutron powder diffraction patterns at 10K
x= 0.77(6)
203
103
110
112
107
104
006
201
106
114
200
108
002
101
008
202
100
004
102
105
x=0.78(3)
203
103
112
110
107
201
104
006
008
200
114
108
101
202
106
002
100
004
102
105

9. x=0.79(9)
203
103
110
112
107
104
006
201
106
108
008
114
200
002
101
202
100
004
102
105
x=0.82(6)
203
103
110
112
107
104
006
201
106
114
200
108
101
008
002
100
202
004
102
105

10. Rietveld refinements of ND measurements at 10K
Structural parameters, reliability factors, and Goodness of fits at 10K

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

12. 3D-MEM charge density distributions at 300K
x=0.78(3)
x=0.77(6)

13. Decrease of the Co-O hybridization in the CoO2 layer
x=0.79(9)
x=0.82(6)
Decrease of the Co-O hybridization in the CoO2 layer

14. Magnetic properties
Magnetic parameters

15. SUMMARIES Acknowledgments
The electron density distributions are slightly modulated with increasing x. There found to be the obvious overlapping of the charge density between O-O network and Co ions for x=0.77(6), but no significant overlapping for x=0.79(9).
This is the direct observation of decrease of the Co-O hybridization in the CoO2 layer with increasing x.
In addition, the intermediate spin Co4+ ions in the population ratio at the 2a site increase with increasing sodium content.
Acknowledgments
　 We are grateful to K. Ohoyama for his help in the neutron powder experiment. We are also grateful to the Kanagawa High-Technology Foundation for inductively coupled plasma atomic-emission spectrometry analysis data. The SQUID magnetometer in Ecotechnology System Laboratory, Yokohama National University, was used. 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.