Ground state changes induced by Ni substitution in NaxCoO2

Abstract We report on the effect of Ni substitution at the Co site on the physical properties of Na x CoO 2 system by investigating the series Na x Co 1− y Ni y O 2 ( x =0.75, 0 y T 3/2 , 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( T 0 / 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.