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FeSb2 is zeta iron carbide structured and crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Fe3+ is bonded to six equivalent Sb+1.50- atoms to form a mixture of edge and corner-sharing FeSb6 octahedra. The corner-sharing octahedral tilt angles are 51°. All Fe–Sb bond lengths are 2.58 Å. Sb+1.50- is bonded in a 4-coordinate geometry to three equivalent Fe3+ atoms.
FeSb2 is Khatyrkite structured and crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. Fe3+ is bonded in a 8-coordinate geometry to eight equivalent Sb+1.50- atoms. All Fe–Sb bond lengths are 2.82 Å. Sb+1.50- is bonded in a 4-coordinate geometry to four equivalent Fe3+ atoms.
Abstract Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscopic study with electronic transport neutron, X-ray scattering, and first principle calculation, we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering, which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell P n n m → P m . The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb 2 paves the way for the understanding and tailoring of giant thermopower in related materials.