Tunable electronic properties and low thermal conductivity in synthetic colusites Cu{sub 26−x}Zn{sub x}V{sub 2}M{sub 6}S{sub 32} (x ≤ 4, M = Ge, Sn)

We have first synthesized Cu{sub 26−x}Zn{sub x}V{sub 2}M{sub 6}S{sub 32} (x ≤ 4, M = Ge, Sn) with the cubic colusite structure and measured the thermoelectric properties. For both M = Ge and Sn, the samples with x = 0 show moderately large thermopower of +27 μV/K at 300 K. The metallic conduction of p-type carriers and Pauli-paramagnetic behavior are consistent with the electron-deficient character expected from the formal charge Cu{sub 26}{sup 1+}V{sub 2}{sup 5+}M{sub 6}{sup 4+}S{sub 32}{sup 2−}. The substitution of Zn for Cu results in significant increases in both the electrical resistivity and thermopower. The resistivity of the samples with x = 4 displays a three-dimensional variable-range hopping behavior at low temperatures. These facts indicate that the doped electrons fill the unoccupied states in the valence band and thereby the Fermi level moves to the localized electronic states at the top of the band. The lattice thermal conductivity is as low as ∼1 W/Km at 300 K for all samples. The structural and thermoelectric properties of the colusites are discussed in comparison with those of doped tetrahedrite Cu{sub 12−x}Zn{sub x}Sb{sub 4}S{sub 13}.