Narrow band gap and enhanced thermoelectricity in FeSb2

Abstract

FeSb₂ was recently identified as a narrow-gap semiconductor with indications of strong electron–electron correlations. In this manuscript, we report on systematic thermoelectric investigation of a number of FeSb₂ single crystals with varying carrier concentrations, together with two isoelectronically substituted FeSb_2−xAs_x samples (x = 0.01 and 0.03) and two reference compounds FeAs₂ and RuSb₂. Typical behaviour associated with narrow bands and narrow gaps is only confirmed for the FeSb₂ and the FeSb_2−xAs_x samples. The maximum absolute thermopower of FeSb₂ spans from 10 to 45 mV/K at around 10 K, greatly exceeding that of both FeAs₂ and RuSb₂. The relation between the carrier concentration and the maximum thermopower value is in approximate agreement with theoretical predictions of the electron-diffusion contribution which, however, requires an enhancement factor larger than 30. The isoelectronic substitution leads to a reduction of the thermal conductivity, but the charge-carrier mobility is also largely reduced due to doping-induced crystallographic defects or impurities. In combination with the high charge-carrier mobility and the enhanced thermoelectricity, FeSb₂ represents a promising candidate for thermoelectric cooling applications at cryogenic temperatures.