Effect of doping on thermoelectric properties of delafossite-type oxide CuCrO₂

We have studied the effects of doping on the high-temperature thermoelectric properties of the delafossite-type oxide CuCrO₂. The single or double doping of divalent cations for Cr³⁺ ions was carried out to introduce hole carriers. For the first step, we measured the electrical conductivity σ and Seebeck coefficient S of single-doped samples, and calculated the power factor P = σS². Mg-, Zn-, Ca-, Ni-, and Co-doped samples showed a higher power factor than CuCrO₂, while the Fe-, V-, and Mn-doped samples exhibited a lower power factor. The maximum power factor P = 2.36 × 10^-4 W/mK² at 1100 K was obtained with the Mg-doped sample. The above tendencies of the power factor are well explained by the valence states and ionic radii of the dopants. For the next step, Mg and M (M = Zn, Ca, Ni, or Co) double-doped samples were prepared. Since there was no impurity phase in the Mg+Ni cases, we have elucidated the structure and high-temperature thermoelectric properties of CuCr _0.97-xMg_0.03Ni_xO₂ (0 < x ≤ 0.05). The lattice parameter a decreases with increasing x, which is responsible for the increase in the degree of overlap of Cu 3d orbitais. Since the Cu-O bond distance of the double-doped samples is shorter than that of the Mg-doped sample, it was found that hole carriers are introduced into the Cu site by the double doping. The shorter Cu-O bond distance also results in the increase in the overlapping integral between Cu 3d and O 2p orbitals. Because of the increase in the overlapping integral between the electronic orbitais caused by the topological reason and the increase in the number of hole carriers in the Cu site, the double-doped samples exhibited a higher electrical conductivity than the Mg-doped sample. The maximum electrical conductivity 45 S/cm around 1000 K was obtained for the sample of x = 0.04. The Seebeck coefficient of the double-doped samples was higher than that of the Mg-doped sample, in which the total number of hole carriers (i.e., the sum of the hole carriers in the Cu and Cr sites) is decreasing. The Seebeck coefficient of the double-doped samples was higher than 225μV/K from 300 to 1100 K. The thermal conductivity of the double-doped samples (κ > 6 W/ mK) was higher than that of the Mg-doped sample. As a result, the maximum dimensionless figure of merit ZT = σS²T/κ = 0.10 was realized with the sample of x = 0.04 at 1100 K, which was twice as high as that of the Mg-doped sample.