First-Principles Study of Chromium Defects in α-Al2O3: The Origin of Red Color in Ruby

Using first-principles calculations within hybrid functional, chromium defects in alpha-Al(2)O(3)are investigated, which are believed to be the cause of red color in ruby. It is found t hat the chromium substitution for aluminum (Cr-Al) defect has low formation energy under both Al- and O-rich growth conditions, whereas the formation energy of the Cr substitution for O (Cr-O) defect is much higher, except underp-type and Al-rich growth conditions. In addition, Cr interstitial (Cr-i) also has low formation energy underp-type and Al-rich conditions, and its formation energy is somewhat lower than that of Cr-O. However, natural sapphire is an insulator indicating that the Fermi-level position should be around the midgap. This confirms that Cr is likely to substitute for Al atom. By exploring the optical properties of the Cr(Al)defect to identify the origin of red color in ruby, the absorption energies associated with the transition of Cr-Al(1+) to Cr-Al(0) and Cr-Al(1-) to Cr-Al(0) are 2.12 and 2.73 eV, respectively. The former and latter can be assigned to the observedUandYbands, respectively, which are believed to be the origin of red color in ruby. Further, it is suggested that the emission lines R, R', andBare associated with other defects or other mechanisms.