  The sodium-cobalt oxide NaxCoO2+d is a promising candidate for thermoelectric
applications since it may possess simultaneously large thermoelectric power and
low resistivity. It is difficult to understand the simultaneous appearance of
the two properties within the framework of conventional one-electron models. It
has been suggested that the spin state of cobalt ions plays a crucially
important role in enhancing the thermoelectric power. In order to study the
relationship between the cobalt spin state and thermoelectric properties,
NaxCoO2+d samples with precisely controlled sodium and oxygen contents are
indispensable. Recently, we established an uncon-ventional sample-synthesis
method that enables us to precisely control the Na content in NaxCoO2+d
samples. In this article, we discuss the effects of sodium and oxygen
nonstoichiometry on the thermoelectric properties of NaxCoO2+d.
