Isovalent substitution effects on thermoelectric transport properties of CoSbX (X  =  S, Se, Te) system

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 semiconducting-type 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  ×  10¹⁸/~10²¹/~10²² 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.