Variable-range-hopping conduction and low thermal conductivity in chalcogenide spinel Cu_yFe₄Sn₁₂X₃₂ (X = S, Se)

Low-temperature thermoelectric properties are reported for polycrystalline samples of chalcogenide spinel Cu_yFe₄Sn₁₂X ₃₂(X = S, Se; y = 8.0 to 4.0). For all samples with X = S, the electrical resistivity ρ behaves similarly to that of a doped semiconductor at high temperatures and shows a variable-range-hopping-type (VRH) temperature dependence at low temperatures. The values of ρ and the thermopower S decrease concomitantly with decreasing initial Cu composition y. For y = 6.0 of the sample with X = Se, ρ shows VRH behavior in a wide temperature range. Because of the monotonic increase of S as T^1/2 and the exponential decrease of ρ with increasing temperature due to the the VRH conduction, the sample with X = Se can be a good high-temperature thermoelectric material. Furthermore, both systems show a low thermal conductivity of 1.5 W/Km because of their complex structures.