Structural, Magnetic and Thermoelectric Properties of Delafossite-type Oxide, CuCr_1-xMg_xO₂ (0 ≤x ≤0.05)

We report the crystal structures, magnetic susceptibilities and thermoelectric properties of a delafossite-type oxide, CuCr_1-xMg_xO₂ (0 ≤x ≤0.05) at temperatures in the range from 4 to 1100 K. The lattice parameter, c, linearly decreases with increasing Mg concentration in the range 0 ≤x ≤0.03. This decrease is mainly caused by the shrinking of O–Cu–O dumbbells which connect the CdI₂-type (Cr/Mg)O₂ slabs. Magnetic susceptibility measurements indicate that Cr³⁺ is in the high spin state in the paramagnetic phase above 25 K. The electrical resistivity, ρ, of CuCr_1-xMg_xO₂ exhibits semiconducting behavior (dρ/dT < 0) in the range from 350 to 1100 K, which decreases through the partial substitution of Mg²⁺ for Cr³⁺ with 0 ≤x ≤0.03. Positive and high Seebeck coefficients of CuCr_1-xMg_xO₂ at high temperatures are consistant with the theoretical values predicted by Koshibae, who considered the spin entropy flux for the high-temperature Seebeck coefficent. From the linear S vs ln σ plot, considerable contribution from the band structure and carrier concentraton to the Seebeck coefficient is indicated. The power factor, S²σ, reaches its maximum value at around x = 0.03 in this system. The thermal conductivity, κ, for CuCr_1-xMg_xO₂ ranges from 6 to 10 W·m^-1·K^-1 at 300 K, slowly decreasing with increasing temperature up to 1000 K. In the present system, the maximum dimensionless figure of merit, ZT=S²T/ρκ= 0.04, is realized for the case of x = 0.03 at 950 K.