Enhanced thermopower upon Ni substitution in Na0.75CoO2

Thermopower measurements have been carried out on the Ni substituted samples of Na0.75CoO2 in the temperature range 4.2K to 300K. The room temperature TEP increases by 20microV/K even with 1% Ni substitution and systematically increases with increasing Ni content upto 5%. The increase in TEP is accompanied by a decrease in rho, thus increasing the ratio of S^(2)/rho on Ni substitution. At low T, the TEP shows an anomaly in the substituted samples, showing a peak at T ~ 20K.