We demonstrate a transition of the thermoelectric transport characteristics in the CoSbX (X = S, Se or Te) systems from a p -type semiconductor to metallic conductor with increasing size of the X constituent. From DFT calculations CoSbS is found as an indirect semiconductor with band-gap of 0.38 eV, while both CoSbSe and CoSbTe appear as metals. For the two metals, the calculations reveal two degenerate electron pockets (located near the U point for CoSbSe and near the T point for CoSbTe) and a hole pocket along the X-Γ-Y points. In line with the theoretical predictions, electrical transport measurements reveal semiconductingtype temperature dependence of resistivity and positive room-temperature Seebeck coefficient (+570 µV K −1 ) for CoSbS, and metallic-type temperature dependence for CoSbSe and CoSbTe with negative Seebeck coefficient (−14 and −7.5 µV K −1 ). The Hall coefficient is positive for CoSbS(Se) and negative for CoSbTe. Room-temperature charge carrier densities were estimated at 3 × 10 18 /~10 21 /~10 22 cm −3 for CoSbS/CoSbSe/CoSbTe. Thermal conductivity is dominated by lattice rather than electronic contribution, the RT value being of the roughly same magnitude for all the three compounds. The temperature dependence of thermal conductivity bear resemblance to a typical semiconductor in the case of CoSbS and to a metallic alloy for CoSbSe and CoSbTe.

We demonstrate a p-type to n-type conductivity transition for thermoelectric CoSbS achieved by precisely controlling the sulfur vapor pressure during the sample synthesis. The p–n transition is experimentally confirmed by both the Seebeck coefficient and the Hall effect measurements. From the crystal structure refinements, the increase in the sulfur vapor pressure in the synthesis is weakly but steadily reflected in the occupancy factor of sulfur in the CoSbS lattice, while the p–n transition is seen as a peak in all the three lattice parameters, a, b, and c. Computationally, the situation could be simulated with first principle DFT calculations on compressed CoSbS. Without compression, DFT presents CoSbS as a p-type semiconductor with an indirect bandgap of 0.38 eV, while the pressure application results in an n-type semiconductor with decreased lattice parameters but the same indirect bandgap as in the uncompressed case. Experimentally, the thermal conductivity is strongly enhanced for sulfur-deficient samples, which could be due to larger phonon mean free paths. The sulfur loading significantly enhances the electrical conductivity while moderately decreasing the Seebeck coefficient such that the overall power factor is improved by a factor of 9 for the n-type sample and by a factor of 6 for the p-type sample, owing to the increased charge carrier density, although the performance is still relatively low. Thus, this study highlights CoSbS as a promising building block for thermoelectric devices based on its bipolar semiconductor nature with the possibility for both p-type and n-type doping with enhanced power factor.

We have characterized the layered CuCr(S,Se)2 system for the spin‐polarized electronic band structures and low‐temperature thermoelectric transport properties. The electronic band structure calculations reveal semiconducting behavior for CuCrS2, CuCr(S0.5Se0.5)2 and CuCrSe2 with an indirect bandgap of 0.42, 0.30 and 0.10 eV, respectively. The systematically decreased bandgap with increasing Se content is in line with the experimental observations showing a semiconductor‐to‐metal transition with increasing Se‐substitution level in the CuCr(S1‐xSex)2 system because of an increase in the charge carrier density. The p‐type Seebeck coefficient shows a linear temperature dependence for the samples, like in degenerate semiconductors or metals. The remarkably large Seebeck coefficient even in metallic samples is due to a relatively large effective mass of charge carriers. As the thermal conductivity is intrinsically low owing to the layered crystal structure and is further decreased for the Se‐substituted samples because of the increased phonon scattering from point defects, the thermoelectric characteristics are promising. The highest dimensionless figure‐of‐merit values were seen for the x=0.5 sample, e. g., 0.04 at 400 K.