Hybrid‐Functional Study of Native Point Defects and Ti/Fe Impurities in α‐Al2O3

Abstract

First‐principles calculations with the most accurate hybrid functional are used to investigate the effect of native point defects and Ti/Fe impurities on electrical and optical properties of α‐Al2O3. For dominant native point defects, the Al vacancy (VAl), O vacancy (VO), and Al interstitial (Ali) defects are considered, and it is found that only VAl defect provides the absorption line in the visible light region. Regarding Ti‐doped or Fe‐doped sapphire, it is found that their colors should be related to intra‐atomic d–d transitions. For the Ti–Fe pair, it is found that the ground‐state configuration is TiIII–FeIII, and its binding energy is very high, indicating that this pair can exist in high concentration. The calculated optical absorption energy associated with Ti–Fe pair is ≈1.92 and 1.84, which is appropriate for sapphire to be blue. It is, therefore, suggested that the intervalence charge transfer (IVCT) mechanism is the cause of blue color in blue sapphire.