Ground state changes induced by Ni substitution in NaxCoO2

We report on the effect of Ni substitution at the Co site on the physical properties of NaxCoO2 system by investigating the series NaxCo1−yNiyO2 (x=0.75, 0<y<0.15). An upturn in the resistivity is observed in all Ni substituted samples as the temperature is lowered, suggestive of the occurrence of a metal–insulator transition (MIT). The temperature at which the transition occurs increases with Ni content. The temperature dependence of the resistivity in the metallic region in the Ni substituted samples is T3/2, which is qualitatively different from that observed in the pristine sample while in the insulating region the temperature dependence of resistivity is proportional to exp(T0/T)1/2. Thermopower measurements were also carried out in the 4–300 K temperature range in a few of the samples. At low temperatures the thermopower shows an increase in the Ni substituted samples that is consistent with hopping conduction with electron correlation included.