Crystal structure, electric and magneticproperties in Na_xCoO₂

K. Nagasawa, H. Nakatsugawa

Division of Materials Science and Engineering, Graduate School ofEngineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku,Yokohama, 240-8501, Japan

Recentdiscovery of large thermoelectric power in single crystal Na_xCoO₂has stimulated broad interest in application of p-type thermoelectric devices.In this study, we investigated crystal structure, electric and magneticproperties in polycrystalline samples of Na_xCoO₂. Thecrystal structures were analyzed using a Rietveld analysis program RIETAN-2000based upon the x-ray and neutron diffraction patterns. The electron densitydistributions at room temperature were visualized by an elaborate method, whichis a combination of the maximum entropy method (MEM) and the Rietveldrefinement. The four-probe dc resistivity and magnetic susceptibility weremeasured using a home made device and a SQUID magnetometer (MPMS) as a functionof temperature, respectively. Recently, an anomaly temperature was reported inthe temperature dependence of resistivity for x = 0.55, 0.65 and 0.75 samplesby Motohashi et al. In particular, x = 0.75 was confirmed theexistence of the first-order phase transition. In contrast, we found the changeof crystal structure between x = 0.79 and 0.85 samples at room temperature.This means that the crystal structures of x = 0.85 or x > 0.85 samples atroom temperature are in the low temperature phase which was revealed by Tojo etal.