Chemical Bonding in Colossal Thermopower FeSb2

FeSb2 exhibits a colossal Seebeck coefficient (S ) and a record‐breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb2 are based on either ligand‐field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb2. Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X‐ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single‐electron Sb−Sb bond implied by the covalent model, and the large formal charge and expected d‐orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences. Unraveling bonding: FeSb2 has extraordinary thermoelectric properties, but previous orbital‐based chemical‐bonding models cannot explain the structures and properties of Marcasite‐type materials. Bader topological analysis revealed FeSb2 to be a covalent network structure, and this allows better rationalization of the material characteristics.