Thermoelectric Properties of Delafossite-Type Oxide CuFe1-xNixO2 (0 <= x <= 0.05)

We have studied crystal structure and high temperature thermoelectric properties of delafossite-type oxide CuFe1-xNixO2 (0 <= x <= 0.05). There is no significant difference in the lattice parameter a 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 sigma is enhanced to 18 S/cm. The Seebeck coefficient S decreases slightly by the Ni doping, but remains high (S > 250 mu 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 kappa of CuFe1-xNixO2 is relatively high (kappa > 4 W/mK). The maximum ZT value, ZT = sigma(ST)-T-2/kappa = 0.14 is obtained with the sample of x = 0.01 at 1100 K.