Synthesis of [Ca 2 (Co 1-x Cu x ) 2 O 4 ] y CoO 2 Single Crystals and Their Intrinsic Properties
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Synthesis of [Ca2(Co1-xCux)2O4]yCoO2 Single Crystals and Their Intrinsic Properties M. Suzuki1, S. Horii1, M. Sano1, K. Fujie1, K. Otzschi1, J. Shimoyama1,2, and K. Kishio1 1 Department of Superconductivity, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2 PRESTO/JST, Japan ABSTRACT Thin plate-like [Ca2(Co1-xCux)2O4]yCoO2 single crystals were successfully grown by the flux method using SrCl2 solvent. The largest crystal size was approximately 3 × 3 × 0.01 mm3. The copper concentrations, x, in the crystals were found to be depending on each crystal, suggesting a certain solid-solution range of copper in the present system. The obtained crystal showed low resistivity ( ~4.3mΩcm) and high Seebeck coefficient ( ~160µVK-1) at 300K. The crystal showed anisotropic magnetization with larger magnetization under H // c. Irreversible magnetization behavior below ~90K was also found under H // c.
INTRODUCTION Since the discovery of high thermoelectric performance in NaCo2O4 [1], the layered cobalt oxides have been of great interest and several new candidate materials, such as Ca3Co4O9 (= [Ca2CoO3]0.62CoO2, Ca349) [2,3], Bi-Sr-Co-O [4,5], have been developed through the extensive studies. These compounds were consisted of misfit stacking of CoO2 and rock-salt type blocking layers along the c-axis. The most advantageous point of these layered cobaltites is thermal stability up to 1000K or more. They exhibit high Seebeck coefficient, S, low resistivity, ρ, and resulting high power factor, S 2 / ρ, from low temperature ~100K to high temperatures. Very recently, Miyazaki et al. [6] have discovered a new layered cobaltite, [Ca2(Co1-xCux)2O4]0.624CoO2 (x = 0.35), having a similar crystal structure with Ca349. This new compound has a rock-salt type (CaO)-(Co0.67Cu0.35O)-(Co0.67Cu0.35O)-(CaO) sheets at the blocking layer, while (CaO)-(CoO)-(CaO) sheets exist in that of Ca349. Samples with single phase were prepared only from a starting composition of x = 0.35 and synthesis condition by standard solid-state reaction is limited to be 920°C in flowing oxygen atmosphere [6]. Therefore, substitution level of copper for cobalt sites in the block layers is extremely narrow (x ~ 0.35). Reflecting similar crystal structure with Ca349, this new compound showed excellent thermoelectric properties with S ~ 150µVK-1 and ρ ~ 15mΩcm at 300K, which are slightly superior to those of Ca349. This result means that [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 can be a promising candidate material for thermoelectric devices. However, its intrinsic and anisotropic physical properties have not been clarified yet, because a growth method to obtain [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 single crystals has not been established. Based on above
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backgrounds, we have attempted to synthesize [Ca2(Co1-xCux)2O4]yCoO2 single crystals and evaluate their physical properties, such as electronic resistivity, magnetic susceptibility and thermoelectric power in the present study.
EXPERIMENTAL Polycrystalline [Ca2(Co0.65Cu0.35)2O4]0.624CoO2 (nominal com
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