Thermoelectric Properties of Variants of Cu₄Mn₂Te₄ with Spinel-Related Structure

Thermoelectric properties of Cu₄Mn₂Te₄, which is antiferromagnetic with a Néel temperature T_N = 50 K and crystallizes in a spinel-related structure, have been investigated comprehensively here. The phase transition occurring at temperatures 463 and 723 K is studied by high-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC), and its effect on thermoelectric properties is examined. Hypothetically Cu₄Mn₂Te₄ is semiconducting according to the formula (Cu⁺)₄(Mn²⁺)₂(Te^2-)₄, while experimentally it shows p-type metallic conduction behavior, exhibiting electrical conductivity σ = 2500 Ω^-1 cm^-1 and Seebeck coefficient α = 20 μV K^-1 at 325 K. Herein, we show that the carrier concentration and thus the thermoelectric transport properties could be further optimized through adding electron donors such as excess Mn. Discussions are made on the physical parameters contributing to the low thermal conductivity, including Debye temperature, speed of sound, and the Grüneisen parameter. As a result of simultaneously boosted power factor and reduced thermal conductivity, a moderately high zT = 0.65 at 680 K is obtained in an excess Mn\In co-added sample, amounting to 5 times that of the pristine Cu₄Mn₂Te₄. This value (zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides.