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$$alpha$$'-Na$$_{x}$$CoO$$_{2}$$の変調構造と輸送特性

Modulated crystal structure and physical properties of $$alpha$$'-Na$$_{x}$$CoO$$_{2}$$

宮崎 譲*; 湯葢 邦夫*; 井川 直樹 

Miyazaki, Yuzuru*; Yubuta, Kunio*; Igawa, Naoki

ナトリウムイオン電池の正極材料候補の一つである、層状コバルト酸化物$$alpha$$'-Na$$_{x}$$CoO$$_{2}$$について、中性子回折法によってその詳細な結晶構造を調べた。Rietveld解析の結果、本化合物のNa量は0.76(1)と求められた。基本構造は従来より提案されていた空間群${it C}$2/${it m}$で表され、その超空間群は${it C}$2/${it m}$(${it p}$0${it q}$)00であることが明らかになった。基本構造においては、原子はそれぞれNa(0, 1/2, 1/2), Co(0, 0, 0)、及びO(0.273, 0, 0.813)の座標を占め、対称性により${it y}$方向への変位変調は生じない。一方、Na原子の${it x}$方向への変位変調が$$pm$$0.18と他の成分に比べて顕著に大きいことが明らかになった。

The crystal structure of polycrystalline samples of $$alpha$$'-Na$$_{x}$$CoO$$_{2}$$ (O1-type) has been determined by means of the (3+1)-dimensional superspace group approach using neutron diffraction data. The precise structural modulation including an undulated arrangement of the CoO$$_{2}$$ conduction sheets has been revealed. Due to the cooperative shift of the Na atoms in the crystallographic a-axis, the coordination polyhedron of NaO$$_{6}$$ changes from prismatic to octahedral site by site. A large positive Seebeck coefficient and a reasonably low electrical resistivity imply that the compound exhibits better thermoelectric performance than that of the $$gamma$$-NaxCoO$$_{2}$$ (P2-type) phase at lower temperatures below 300 K.

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