Hybrid‐Functional Study of Native Point Defects and Ti/Fe Impurities in α ‐Al 2 O 3

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 ‐Al 2 O 3 . For dominant native point defects, the Al vacancy ( V Al ), O vacancy ( V O ), and Al interstitial (Al i ) defects are considered, and it is found that only V Al 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 Ti III –Fe III , 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.