Polaronic transport and thermoelectricity in Fe_1-xCo_xSb₂S₄ (x = 0, 0.1, and 0.2)

Correlated electron materials such as FeSb₂ possess the highest thermoelectric power factor in nature and thermoelectric power up to 45 mV K^-1. We present a study of Co-doped magnetic semiconductor Fe_1-xCo_xSb₂S₄ (x = 0, 0.1, and 0.2), which crystallize in an orthorhombic structure with Pnma space group, similar to FeSb₂. In contrast to Fe_1-xCo_xSb₂ and Fe_1-xCr_xSb₂ where electronic transport is dominated by thermal activation and variable range hopping, our results indicate polaronic transport due to the large discrepancy between activation energy for conductivity, E_ρ (146 ~ 270 meV), and thermopower, E_S (47 ~ 108 meV), in Fe_1-xCo_xSb₂S₄. Bulk magnetization and heat capacity measurements of FeSb₂S₄ show a broad antiferromagnetic transition (T_N = 46 K) followed by an additional weak transition (T^* = 50 K). The T_N and T^* slightly decrease with increase in x. This is also reflected in thermal conductivity measurement, indicating strong spin-lattice coupling. Fe_1-xCo_xSb₂S₄ shows high value of thermopower (up to ~ 624 μV K^-1 at 300 K) and significantly smaller thermal conductivity, a feature desired for potential applications based on FeSb₂ materials.