Structure and high temperature thermoelectric properties of delafossite-type oxide CuFe_1-xNi_xO₂ (0 ≤ x ≤ 0.05)

We have investigated crystal structure of delafossite-type oxide CuFe _1-xNi_xO₂ (0 ≤ x ≤ 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 Ni ²⁺-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 Ni ²⁺-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 σ. The highest electrical conductivity is 18 S/cm. Although the Seebeck coefficient S decreased by Ni ²⁺ doping, the S is still high value (S > 250 μV/K). The thermal conductivity κ of CuFe_1-xNi_xO₂ is relatively high (κ > 4 W/mK). The maximum dimensionless figure of merit ZT = σS²T/κ = 0.14 is obtained with the sample of x = 0.01 at 1100K, being higher than that of the polycrystalline γ-Na _0.7CoO₂. There is no significant evaporation of the constituent elements after the heat cycles.