  Iron diantimonide is a material with the highest known thermoelectric power.
By combining scanning transmission electron microscope 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 Pnnm to Pm. 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 FeSb2 paves the way for the understanding and
tailoring of giant thermopower in related materials.
