Crystal structure, electric and magnetic properties in NaxCoO2 IUMRS-ICAM2003 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 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. 321-324 pp. 198-203 (2000). Maximum entropy method by MEED code Kumazawa S.et al. , J.Appl.Crystallogr. Vol. 26, pp. 453-457 (1993).

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. 1480-1482 (2001). sintering 920℃ 12h

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

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

Rietveld refinements of XRD measurements at 300K

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

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

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

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

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

Rietveld refinements of Synchrotron radiation X-ray measurements

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

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

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

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

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

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

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

Magnetic properties Magnetic parameters

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.