Formation of lithium clusters and their effects on conductivity in diamond: A density functional theory study

We have applied Density Functional Theory (DFT) within the Generalized Gradient Approximation to study the behavior of lithium impurities in the diamond lattice. It was found that, although isolated interstitial lithium atoms in tetrahedral sites have a donor state relatively close to the conduction band at 0.35 eV, their behavior is critically affected by adjacent interstitials. Lithium atoms that occupy adjacent interstitial tetrahedral sites cluster spontaneously and induce carbon–carbon bond breaking in their neighborhood. The corresponding charge distribution and electronic density of states analyses show that charge localization takes place at the carbon atoms around the lithium clusters and deeper mid-gap states are introduced in the diamond band gap. The formation energy was found to be 1.88 eV and 3.09 eV, for clusters of two and three lithium atoms, respectively, considered in this study. The calculations also indicate that Li clustering would not take place if Li ions instead of atoms were incorporated interstitially.