Electromagnetic and Thermoelectric Characteristics of Na x CoO 2 of Precisely Controlled Na Nonstoichiometry
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Electromagnetic and Thermoelectric Characteristics of NaxCoO2 of Precisely Controlled Na Nonstoichiometry Teruki Motohashi, Maarit Karppinen, and Hisao Yamauchi Materials and Structures Laboratory, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT Electromagnetic and thermoelectric characteristics of a sodium-cobalt oxide, NaxCoO2, were investigated. A precise control of Na nonstoichiometry was successfully facilitated by an unusual synthesis method named “rapid heat-up” technique. With increasing x, the absolute value of resistivity (ρ) monotonically decreased, while the value of thermoelectric power (S) increased, giving rise to a drastic enhancement in the thermoelectric power factor, i.e. S2 / ρ. Simultaneously enhanced thermoelectric power and reduced resistivity of the present compound are difficult to be understood within the framework of a conventional band picture. Moreover, for samples with the solubility limit Na content, i.e. x = 0.75, we discovered an unconventional electronic transition at Tm = 22 K, which may be induced by the strong-correlation effect of 3d electrons. INTRODUCTION In 1997, Terasaki et al. [1] reported that a sodium-cobalt oxide NaxCoO2 (x ~ 0.5, i.e. NaCo2O4) showed large thermoelectric power (S) and low resistivity (ρ) simultaneously. The carrier concentration has been found to be much higher in this compound than in some known thermoelectric semiconductors such as Bi2Te3 and PbTe. The large thermoelectric power, i.e. S > 50 µV / K at 300 K, reported for this compound is difficult to be understood within the framework of conventional one-electron pictures. It is suggested that the strong correlation effect plays a crucially important role in the observed large S value for NaxCoO2 [2]. In such a correlated-electron system, a precise control of chemical composition is indispensable because a large modification in the electronic state could be induced by a slight change in the carrier concentration and/or local crystal structure: as an example serves high-Tc superconductivity induced by hole doping into insulating CuO2 planes. Since NaxCoO2 is known to show a wide range of Na nonstoichiometry within 0 < x < 1 [3], it is of interest to study the effect of Na nonstoichiometry on the electromagnetic and thermoelectric characteristics of NaxCoO2. In this contribution, we report results of physical property measurements for NaxCoO2 with a precisely controlled Na
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nonstoichiometry. RESULTS AND DISCUSSION Establishment of a “rapid heat-up” technique So far it has been recognized that the control of Na content is difficult in NaxCoO2 when prepared by firing a powder mixture of Na2CO3 and Co3O4 utilizing a conventional technique, since a large deficiency in the Na content may result. Recently, we established a sample-preparation method called “rapid heat-up” technique that enables us to precisely control the Na content in NaxCoO2 samples [4]. In this technique, a powder mixture of Na2CO3 and Co3O4 of an appropriate ratio is directl
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