First-principle investigation of native and impurity defects in MgB2

We use first-principles techniques to investigate the properties of native defects and impurities in MgB2. We focus our investigation on those defects that could have a relevant role in Al and Li co-doped MgB2. This system shows an anomalous dependence of superconductivity on doping concentration, not yet understood. Structural effects such as segregation and clustering have been put forward to justify experimental evidence. Here we show that native defects have very high formation energies, that Al and Li do not form aggregates and incorporate as substitutional defects randomly distributed on the Mg sublattice sites. There exists a competition between segregation and incorporation of Li, the latter being slightly favored energetically. Our results suggest that anomaly in the superconducting behavior must be accounted for by electronic structure effects.