Crystal structure, electric and magnetic properties in NaxCoO2 K.Nagasawa and H.Nakatsugawa (d03ga312@ynu.ac.jp) (naka@ynu.ac.jp) Division of Materials Science and Engineering, Graduate School of Engineering, Yokohama National University, Japan

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.

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

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.vol79.1480 (2001)

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

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

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

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

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

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

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

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

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

Magnetic properties Magnetic parameters

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.