The aim of this work was to investigate the possibility of doping copper sulfide Cu₂S with selected fifth-group elements, potentially having a positive effect on the thermoelectric properties of the resulting materials. For the selected model structures, theoretical calculations and an analysis of the electronic structure and changes in the enthalpy of formation due to doping were performed using the WIEN2k package employing the Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method within density functional theory (DFT) formalism. Polycrystalline materials with the nominal composition of Cu₃₂S₁₅X₁ (X = P, As, Sb, Bi) were synthesized in quartz ampoules, then sintered using the spark plasma sintering (SPS) technique and “SPS melting” method. The chemical and phase compositions of the obtained sinters were studied by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Additionally, investigations of thermoelectric properties, i.e., electrical conductivity, Seebeck coefficient and thermal conductivity in the temperature range 300–920 K, were performed. The results of this study indicate that only phosphorus is successfully incorporated into the Cu₂S structure.

Tetrahedrites, due to their promising thermoelectric properties, are one of the materials being investigated for use in thermoelectric generators. One problem is the lack of n-type tetrahedrites, which would be beneficial for the design of tetrahedrite thermoelectric modules. Preliminary theoretical studies have shown that elements from groups I and II can be introduced into the structural voids of tetrahedrite, acting as donor dopants, and should enable n-type conductivity. Therefore, in this work, an attempt was made to obtain and study magnesium-doped tetrahedrites. A series of samples, Mg_xCu₁₂Sb₄S₁₃, with different magnesium contents were obtained and their phase and chemical compositions were characterized. It was observed that the structural changes occurring upon doping indicate that Mg atoms are likely to be embedded in the structural voids. The experimental studies have been supported by electronic structure calculations indicating that the most likely location of Mg is in the structural voids at the 6b Wyckoff position. Seebeck coefficient and resistivity measurements showed that doping with Mg reduces the concentration of holes, which is consistent with the predicted donor character of the dopant. However, the introduction of magnesium in sufficient amounts to achieve n-type conductivity was not successful.

Tetrahedrite (copper antimony sulfosalt) is promising p-type themoeletric material due to very low intrinsic thermal conductivity and moderately-high power factor, with one of the limitations being lack of n-type variant to create thermoelectric generator. In this paper, DFT calculations have been carried out to study tetrahedrite doped with Li into structural voids, LixCu12Sb4S13 (0 <= x <= 3). Enthalpies of formation shows that introduction of Li into both 6b and 24g sites is energetically favorable. Dopants in those positions differently affect rattling Cu(12e) behaviour, as well as vary in magnitude of induced local disorder. Topological analysis of charge density classifies tetrahedrite as closed-shell, ionic system of interactions with some degree of covalency. Addition of Li increases bond strain and decreases structure stability. Electronic band structure shows that for x>2.0, material becomes n-type, however results are not precisely conclusive on whether structure will be synthesizable, which should be determined experimentally.