We report thermoelectric (TE) properties of dense samples of colusites Cu26V2M6S32 (M = Ge, Sn), most of which are composed of earth-abundant elements; Cu and S. The combination of p-type metallic conduction and large thermopowers greater than 200 μV/K leads to high TE power factors of 0.61 and 0.48 mW/K2 m at 663 K for M = Ge and Sn samples, respectively. Furthermore, the lattice thermal conductivity is smaller than 0.6 W/Km over the temperature range from 350 K to 663 K due to the structural complexity. As a consequence, the values of dimensionless TE figure of merit ZT for M = Ge and Sn reach 0.73 and 0.56 at 663 K, respectively. Thus, the colusites are promising candidates for environmental friendly TE materials usable in the range of 500–700 K.

The colusite Cu26V2Sn6S32 has high potential as a thermoelectric material at medium-high temperatures because of a large Seebeck coefficient (S ≃ 220 μV/K) and rather small electrical resistivity (ρ ≃ 100 μΩm) at 660 K. To improve the thermoelectric performance, we have tuned the hole carrier density p by substituting Zn for Cu in Cu26−xZnxV2Sn6S32 (x = 1–3) and starting with Cu and Sn deficient compositions in Cu26−yV2Sn6S32 (y = 1, 2) and Cu26V2Sn6−zS32 (z = 0.25–1), respectively. Powder x-ray diffraction and electron-probe microanalysis showed that the Zn-substituted samples and Sn-deficient (z ≥ 0.5) samples are formed in a single phase, whereas the Cu26−yV2Sn6S32 samples are composed of two phases with slightly different compositions. Within these samples, the value of p at 300 K varies in the range between 3.6 × 1020 and 2.8 × 1021 cm−3. The relation between p and S led to the effective mass m* of 4–7m0 for the hole carriers. The large S of the colusite is therefore ascribed to the heavy mass carriers of the valence band top. The decreases in p with x and y reduced the dimensionless thermoelectric figure of merit ZT, whereas the increase in p with z raised ZT from 0.56 (z = 0) to 0.62 (z = 0.5) at 660 K.

We report the thermal conductivity and specific heat measurements at temperatures down to 0.3 and 0.4 K, respectively, for synthetic Cu-S based minerals, namely, tetrahedrite Cu 10 Zn 2 Sb 4 S 13 with rattling Cu atoms and colusite Cu 23 Zn 3 V 2 Sn 6 S 32 without the rattling mode. Because both are semiconducting and diamagnetic, thermal conductivity and specific heat are predominated by the lattice contribution. For the tetrahedrite, thermal conductivity exhibits a plateau at approximately 4 K and T 1.6 dependence at T < 0.7 K. The specific heat C divided by T 3 displays a broad peak at around 4 K, almost identical to the so-called boson peak generally observed in structural glasses. This peak indicates the presence of an optical mode with the characteristic temperature θ of 20 K, involving the out-of-plane motion of the threefold-coordinated Cu atom. For the colusite, in contrast, thermal conductivity shows a significant peak at 15 K and specific heat reveals an optical mode with a higher θ of 90 K. The contrasting behaviors of the two Cu-S based minerals indicate that the out-of-plane motion of Cu in the tetrahedrite is the source of the glasslike thermophysical properties.