Site-selective doping of transition metal cations into NaCo2O4

We have investigated site-selective doping of Ni in order to experimentally explore the roles of the Na and Co sites of NaCo₂O₄. The selective doping is carried out by changing preparation procedures of the oxide. Cation "substitution" is intended by incorporating Ni(NO₃)₂ into a Co-deficient starting mixture of Na₂CO₃ and Co₃O₄ prior to the first heat treatment. Cation "exchange" is, on the other hand, carried out by heating a mixture of Ni(NO₃)₂ and a single phase sample of NaCo₂O₄ which has been prepared beforehand from a stoichiometric starting mixture. Comparison with control samples without addition of Ni(NO₃)₂ reveals that the "substituted" sample shows a significant decrease in the thermopower with keeping the electrical conductivity almost intact, while the "exchanged" sample shows the lower electrical conductivity with leaving the thermopower unchanged. These results strongly suggest that Ni cations occupy different sites in the Ni-doped samples, disturbing Co-Co spin exchange at the Co site, and compensating holes at the Na site.