Thermoelectric properties and crystal chemistry of promising oxide candidate Na/sub x/CoO/sub 2/

Promising p-type thermoelectric properties of a layered transition metal oxide Na/sub x/CoO/sub 2/ are discussed in terms of crystal chemistry and the conduction mechanism. Highly anisotropic spatial distribution of 3d orbitals coupled with an unusually short Co-Co interatomic distance in the ab plane, along which edge-sharing CoO/sub 6/ octahedra connecting each other to form the CoO/sub 2/ sheets, strongly suggest that Co3d d/spl epsiv/-d/spl epsiv/ (t/sub 2g/-t/sub 2g/) direct overlapping is plausible for establishing hole conduction in the valence band with a fairly covalent character. An extremely low thermal conductivity (/spl les/1 W/mK) of the oxide is also revealed, implying contribution of the randomly distributed Na vacancies and an inherent advantage of materials with low-dimensional crystal structure. In spite of insufficient densification, sintered bodies fired under atmospheric pressure shows ZT/spl ap/0.5. Possible influence of the ionic conduction of Na/sup +/ on the thermoelectric properties of the oxide is also examined.