Glasslike versus Crystalline Thermophysical Properties of the Cu-S based Minerals: Tetrahedrite and Colusite

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 Cu10Zn2Sb4S13 with rattling Cu atoms and colusite Cu23Zn3V2Sn6S32 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 4K and T-1.6 dependence at T < 0.7 K. The specific heat C divided by T-3 displays a broad peak at around 4K, 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. 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 15K and specific heat reveals an optical mode with a higher. 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.