  Results of thermo-electric power (S) and electrical resistivity (r)
measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These
are single-phase compounds crystallizing in the hexagonal structure (space
group P63/mmc) at room temperature. Thermo-electric power values at 300K
(S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples,
respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while
the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is
\~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which
qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is
positive throughout 300-5K temperature range and decreases monotonically to
zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes
sign and shows negative values between 90 K and 16 K before approaching zero as
T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are
coincidentally the precursor materials to the reported superconductivity in
this class of materials, indicates a dramatic change in the electronic
structure of these compounds on lowering the Na content.
