Ground state changes induced by Ni substitution in Na_xCoO₂

Abstract

We report on the effect of Ni substitution at the Co site on the physical properties of Na_xCoO₂ system by investigating the series Na_xCo_1-yNi_yO₂(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 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(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.