Structure and High Temperature Thermoelectric Properties of Delafossite-Type Oxide CuFe1-xNixO2 ($0 \leq x \leq 0.05$)

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We have investigated crystal structure of delafossite-type oxide CuFe1-xNixO2 ($0 \leq x \le 0.05$) and measured its thermoelectric properties at high temperatures ranging from 300 and 1100 K. The lattice parameter $a$ of the Ni2+-doped samples is nearly equal to that of CuFeO2, while the lattice parameter $c$ of the Ni2+-doped samples increases. Nearly constant $a$-axis is due to the decrease of (Fe/Ni)–O distance and simultaneous increase of O–(Fe/Ni)–O angle. Increase of the $c$-axis is due to the increase of Cu–O distance in the Ni2+-doped samples. The valence states of the Fe- and Cu-sites are calculated from bond valence summation. The valence state of the Fe-site in the Ni2+-doped samples is larger than that of CuFeO2, an indication of hole doping in the Fe-site. This increase of hole carriers enhances the electrical conductivity $\sigma$. The highest electrical conductivity is 18 S/cm. Although the Seebeck coefficient $S$ decreased by Ni2+ doping, the $S$ is still high value ($S>250$ μV/K). The thermal conductivity $\kappa$ of CuFe1-xNixO2 is relatively high ($\kappa>4$ W/mK). The maximum dimensionless figure of merit $ZT=\sigma S^{2}T/\kappa=0.14$ is obtained with the sample of $x=0.01$ at 1100 K, being higher than that of the polycrystalline $\gamma$-Na0.7CoO2. There is no significant evaporation of the constituent elements after the heat cycles.
2007-08-15