Vacancy defect control of colossal thermopower in FeSb2

Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscope study electronic transport neutron, X-ray scattering and first principle calculation we identify atomic defects that control colossal thermopower magnitude nanoprecipitate clusters Sb vacancy ordering which induce additional phonon substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of unit cell Pnnm Pm. The absence along [010] for high defect concentration forms conducting path due Fe d orbital overlap. connection between anisotropy in FeSb2 paves way understanding tailoring giant related materials.