Drastic 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 this transition occurs increases with Ni content. The temperature dependence of the resistivity in the metallic region in the Ni substituted samples shows a T2 dependence, which is qualitatively different from that observed in the pristine sample. The evolution of the Fano asymmetry parameter, extracted by analyzing the lineshape of the IR active in-plane Co-O mode, both as a function of Ni concentration and temperature corroborates the occurrence of the MIT. It is argued that the progressive substitution of the Co4+ ions with Ni increases the probability of double occupancy and therefore the on-site Coulomb interaction energy leading to a shift in the thermodynamically driven MIT to higher temperatures.