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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 Cu12−xAlxSb4S13 (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 (EF) with EF 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 EF, thus resulting in relatively low DOS near EF. 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 EF, as indicated by the DFT calculations. Although the electronic thermal conductivity (κe) decreased with increasing Al concentration, the magnitudes of the total thermal conductivity (κT) could not be reduced significantly. As a result, a maximum zT of 0.6 at 673 K was obtained for Cu11.9Al0.1Sb4S13. 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.

Zinc antimonide (ZnSb) and its solid solution with CdSb are well-known p-type thermoelectric materials. Electrical transport properties of doped ZnSb exhibit certain anomalies: (a) non-monotonic changes in the electrical properties with temperature and (b) occurrence of a reversible hysteresis loop in electrical transport data when thermally cycled. The objective of this study was to investigate the underlying cause of these behaviors. Ag-doped compositions of (Zn0.625Cd0.375)1−δAgδSb (δ = 0, 0.02, and 0.04) solid solutions have been prepared by melt-synthesis—rapid compaction—annealing process. Measurement of the electrical conductivity (σ), Seebeck coefficient (S), and Hall coefficient (RH) (room temperature to 673 K) displayed the characteristic hysteresis behavior on thermal cycling along with the unusual rise in the charge carrier concentration (n) around 500 K. Aside from that, it was found that cooling rates dramatically influence room temperature properties. Analysis of synchrotron-based x-ray diffraction data by Rietveld refinement indicates that Ag-doping results in the formation of Zn vacancies [Vzn]. Also, a sharp drop in the concentration of Zn vacancies, [Vzn] around 550 K was observed and could be correlated with the changes in n values. This correlation between changes in [Vzn] and n has been used to explain the observed electrical anomalies, which are a consequence of the repeated annihilation and creation of Zn vacancies with temperature changes.

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ZnSb is a promising thermoelectric (TE) material due to its decent TE conversion efficiency and low cost. However, its full potential for TE applications has not been achieved due to difficulties associated with optimal doping and its unique temperature-dependent charge carrier concentration (n). In this work, we propose a codoping strategy that enables precise control of n, leading to dynamic optimization of the charge carrier concentration. The codoping technique works on the principle of charge compensation between the acceptor impurities and charged vacancies (VZn) which are present in this system. Utilizing this technique, we show that the peak zT (TE figure-of-merit) value in ZnSb can be raised to 1.22 (at 645 K). Further improvement in the TE performance is demonstrated by Cd substitution, which facilitates preferential scattering of phonons and thereby reduction of lattice thermal conductivity. This results in a peak zT value of 1.41 (at 550 K), which is comparable to other state-of-the-art materials in this temperature range. Theoretical estimates of the TE conversion efficiencies indicate values of ηmax = 10% for ZnSb and 12.2% for the Cd-substituted systems, highlighting their potential for TE applications.