Thermoelectric properties of Al substituted tetrahedrite

In this study, aluminum, a p-block element, is substituted at the Cu(1) site, and its effect on the structural and thermoelectric properties of tetrahedrite Cu_12−xAl_xSb₄S₁₃ (x = 0.1, 0.25, 0.5, and 0.75) was investigated. The samples were prepared via solid-state synthesis followed by induction hot pressing. The theoretical calculations, using density functional theory (DFT), showed that the Al substitution results in lowering the band degeneracy near the Fermi level (E_F) with E_F moving towards the bandgap, indicating effective compensation of holes. The projected density of states (PDOS) revealed almost negligible hybridization of Al states with Cu 3d and S 3p states near E_F, thus resulting in relatively low DOS near E_F. The electrical resistivity and Seebeck coefficient increased with increasing Al content due to the compensation of holes and reduction of the charge carrier concentration. However, the Seebeck coefficient values were relatively low due to a low DOS near E_F, as indicated by the DFT calculations. Although the electronic thermal conductivity (κ_e) decreased with increasing Al concentration, the magnitudes of the total thermal conductivity (κ_T) could not be reduced significantly. As a result, a maximum zT of 0.6 at 673 K was obtained for Cu_11.9Al_0.1Sb₄S₁₃. Based on the current study and previously reported results, the paper demonstrates how the phase stability and transport properties of the tetrahedrite are affected significantly by the nature of the substituent at the Cu(1) tetrahedral site.