  Single dopings of Mn and Fe in Si are investigated using 8-, 64-, and
216-atom supercells and a first-principles method based on density functional
theory. Between the two transition metal elements (TMEs), atom sizes play an
essential role in determining the contraction or the expansion of neighboring
atoms around the TME dopant at a substitutional site. At a tetrahedral
interstitial site, there is only expansion. Magnetic moments/TME at the two
sites are calculated. Physical origins for these inter-related properties are
discussed. A few suggestions about the growth of these Si-based alloys are
given.
