Mg 2 Si crystallizes in the antifluorite structure (Space Group #225, Fm 3m) as shown in Figure . Mg occupies all of the tetrahedral sites of the face-centered cubic Si sublattice. Consequently, defects that occupy the Mg site will be tetrahedrally coordinated to 4 neighboring Si atoms (Figure ). On the other hand, defects that reside on the Si site will be cubically coordinated to 8 neighboring Mg atoms (Figure ). We also consider an interstitial site that is also cubically coordinated to 8 Mg atoms (Figure ).