Interstitials of binary rock salt compounds

The energetic and mechanical stability of interstitial point defects in binary rock salt materials was studied using first-principles methods. A novel, stable, and energetically competitive interstitial site (base-interstitial) was identified for anion interstitials in rock salts. The formation energies of base-interstitial defects were compared with well-explored tetrahedral (body-interstitial) and split interstitials and were found to be highly competitive energetically. For alkali halides, the lowest formation energies are associated with the base-interstitial site together with the <110> split interstitial, and these are therefore the predominant interstitial sites. Electronic band structures are affected by the presence of interstitial defects in rock salt structures. In particular, the Fermi level is shifted below the valence band maxima for the body, base, and split interstitials in metal halides, indicating p-type conductivity. However, the Fermi level remains within the bandgap for metal monochalcogenides, indicating no preferred conductivity for base- and split-interstitial defects. The discovery of a new interstitial site affects our understanding of defects in binary rock salts, including structure and dynamics as well as associated thermodynamic and kinetic properties that are interstitial dependent.