Colossal thermopower from defect-induced in-gap states in FeSb₂

The thermoelectric properties of FeSb2 are attracting attention due to its colossal thermopower and record-high thermoelectric power factor. Although this colossal thermopower was attributed to phonon-drag effect, the factors that affect the phonon-drag effect remains unclear and further. By changing the amount of defects in different FeSb2 crystals we show that thermopower maxima around 10 K change between relatively small 14 μV/K and colossal values of about 20 mV/K. The effect of crystallographic defects and impurities on the colossal thermopower coefficient is studied in single crystals of FeSb2. From the Hall effect, all the crystals show clear two-bands behavior. Defect-induced vacancy band with low Hall mobility dominates the thermal transport. Defects influence considerably phonon mean free path and low mobility band carrier concentration, revealing the source of the in-gap states that govern thermopower size. Our results explain the strong sample dependence of Seebeck coefficient in FeSb₂ crystals and give a possible way to furtherly improve the thermal performance of this correlated semiconductor.