The structure and energetics of native and O-related defects were investigated (figures 7 to 10). First-principles total-energy calculations revealed that the non-stoichiometry could be very small. The O impurities had a high solubility under O-rich growth conditions.

However, the presence of excess Mg during growth could sharply reduce O incorporation by forming MgO precipitates. The O impurities were found to exist in atomic form in MgB2. Interstitial O atoms were very mobile, with an activation energy of 1.3eV; thus indicating that they could be annealed out ex situ in Mg vapor.

Solubility, Diffusion, and Precipitation of Oxygen Impurities in MgB2. Y.Yan, M.M.Al-Jassim: Physical Review B, 2003, 67[21], 212503 (4pp)

Figure 6

Structure of Type-B Stacking-Fault, Seen along [100]