The direct influence of Fe 3d orbitals on EFG of β-FeSi2

We have examined the influence of 3d electrons on the electric field gradient (EFG) for the purpose of understanding properties of d-electrons in β-FeSi2. Since EFG is very sensitive to atomic configuration around Fe atom and to the distortion of d-electron states caused from surrounding fields, additionally EFG can be observable in the form of Quadrupole Splittings (Q.S.) obtained from the measurement of Mössbauer spectroscopy. Through the analysis, we find that the average radius 〈r〉 of 3d orbital in β-FeSi2 is rather large, compared with that in no perturbation state. Such an expansion is supposed to be due to the formation of Fe–Si covalent bonds (d–p bonds). Electronic charge density distribution plot and effective charge calculations of Fe atom on the basis of as DV-Xα or Gaussian programs exhibit the presence of covalent bonds between Fe–Si atoms. Consequently, the presence of Fe–Si covalent bonds seems to play important roles in understanding various properties of β-FeSi2.