Using first‐principles calculations within hybrid functional, chromium defects in α‐Al₂O₃ are investigated, which are believed to be the cause of red color in ruby. It is found that 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 under p‐type and Al‐rich growth conditions. In addition, Cr interstitial (Cr_i) also has low formation energy under p‐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 ${\text{Cr}}_{\text{Al}}^{1+}$ to ${\text{Cr}}_{\text{Al}}^0$ and ${\text{Cr}}_{\text{Al}}^{1-}$ to ${\text{Cr}}_{\text{Al}}^0$ are 2.12 and 2.73 eV, respectively. The former and latter can be assigned to the observed U and Y bands, respectively, which are believed to be the origin of red color in ruby. Further, it is suggested that the emission lines R, R', and B are associated with other defects or other mechanisms.

中文翻译：

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α-Al2O3中铬缺陷的第一性原理研究：红宝石中红色的起源

使用混合功能，铬缺陷内第一性原理计算在α -Al ₂ ö ₃进行了研究，这被认为是红色在红宝石的原因。发现在富Al和富O的生长条件下，铬替代铝（Cr _Al）缺陷的形成能均较低，而铬替代O（Cr _O）缺陷的形成能要高得多。p型和富铝生长条件。另外，在p型和富铝条件下，Cr间隙（Cr _i）的形成能也较低，其形成能比Cr _O的形成能要低一些。。但是，天然蓝宝石是绝缘体，表明费米能级位置应在中间间隙附近。这证实了Cr可能替代Al原子。通过探索Cr _Al缺陷的光学特性来识别红宝石中红色的起源，吸收能量与${\text{铬}}_{\text{铝}}^{\mathrm{1个}+}$ 至 ${\text{铬}}_{\text{铝}}^0$ 和 ${\text{铬}}_{\text{铝}}^{\mathrm{1个}-}$ 至 ${\text{铬}}_{\text{铝}}^0$分别为2.12和2.73 eV。前者和后者可以分别分配给观察到的U和Y波段，据信这是红宝石中红色的起源。此外，建议发射线R，R'和B与其他缺陷或其他机制相关联。