Thermoelectric properties of delafossite-type oxide CuFe_1-xNi_xO₂ (0 ≤ x ≤ 0.05)

We have studied crystal structure and high temperature thermoelectric properties of delafossite-type oxide CuFe_1-xNi_xO₂, (0≤ x≤ 0.05). There is no significant difference in the lattice parameter α among the Ni-doped samples and CuFeO₂ due to the decrease of Fe-O distance and simultaneous increase of O-Fe-O angle by the Ni doping. The lattice parameter c is slightly longer in the doped samples than that of CuFeO₂, reflecting the increase of Cu-O distance. The decrease of the Fe-O distance indicates hole doping in the Fe-site. Because of the hole doping, the electrical conductivity σ is enhanced to 18 S/cm. The Seebeck coefficient S decreases slightly by the Ni doping, but remains high (S > 250 μ V/K). The obtained Seebeck coefficient at high temperature agrees well with theoretical values based on the generalized Heikes formula that takes the spin entropy flux into account. The thermal conductivity κ of CuFe_1-xNi_xO₂ is relatively high (κ>4 W/mK). The maximum ZT value, ZT = σS²T/K = 0.14 is obtained with the sample of x = 0.01 at 1100 K.