First‐Principles Study of Chromium Defects inα‐Al₂O₃: The Origin of Red Color in Ruby

Abstract

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 (CrAl) defect has low formation energy under both Al‐ and O‐rich growth conditions, whereas the formation energy of the Cr substitution for O (CrO) defect is much higher, except under p‐type and Al‐rich growth conditions. In addition, Cr interstitial (Cri) also has low formation energy under p‐type and Al‐rich conditions, and its formation energy is somewhat lower than that of CrO. 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 CrAl defect to identify the origin of red color in ruby, the absorption energies associated with the transition of CrAl1+ to CrAl0 and CrAl1− to CrAl0 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.

Ab-initio study of paramagnetic defects in Mn and Cr doped transparent polycrystalline Al2O3 ceramics

“…The results for the formation energies are in reasonable agreement with the HSE functional calculations of Na-Phattalung et al [10], which further signifies the conclusions of Somjit and Yildiz [41] regarding applicability of GGA-PBE functional for the purpose of defect thermodynamics in alumina. Not surprisingly, under Al-rich conditions, the formation energy of oxygen vacancy is lowest amongst these defects; therefore, VO is expected to form in considerable concentrations upon sintering at the lowest limits of oxygen partial pressure.…”

“…Nevertheless, the optical transitions cannot be deduced using reported DOS, given that a more accurate evaluation of the energy levels becomes necessary for describing absorption and emission wavelengths. Moreover, the phononic contribution is not negligible in alumina, as calculated by Na-Phattalung [10] in the case of Cr-doped alumina, and since there is no reason to believe other defects will be any different, calculation of configuration coordinate diagrams becomes essential in this regard. Although single substitutional chromium is a well-known chromophore, considering the energy levels shown by DOS plots, it is possible to say that the other defects reported here are potential chromophores as well and further hybrid functional calculations are necessary to shed light on absorption or emission wavelengths, which may arise due to these defects.…”

“…Spin polarized density functional theory has been applied using the Generalized Gradient Approximation, as parametrized by Perdew, Burke and Ernzerhof (GGA-PBE) [32]. A supercell size of 2×2×1 (120 atoms in case of pristine crystal) has been used under periodic boundary conditions, which is known to be large enough to almost eliminate the spurious defect-defect interactions [10]. Geometry optimizations have been carried out under a constant cell volume with a plane wave cut-off energy of 500 eV, k-point spacing of 0.5 per angstrom (k-mesh grid of 3×3×1) and Gaussian smearing of 0.05 eV with SCF convergence threshold of 10 -5 eV.…”

“…Recent advances on nanofluids containing alumina are also reported to be effective for heat sink applications with considerable increase in thermal conductivity when compared with plain water [8], making alumina a highly versatile material for a wide range of applications. The transparency of alumina ranges from approximately 200 nm to 4000 nm due to its wide band gap of approximately 9 eV [9,10], making this material exceptional for many optical applications, such as lasers (with appropriate dopants), lenses, screens, and windows [5]. Alumina is also an excellent electrical insulator, preventing leakage currents in electric and electronic devices [11].…”

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

“…[43] The obtained calculated lattice parameters are in excellent agreement with the experiment. [44,45] To study the defects in α-Al 2 O 3 , a supercell approach was carried out. [46] A supercell size of 120 atoms, which is a repetition of 30-atom α-Al 2 O 3 unit cell by a 2 Â 2 Â 1, was used for this study.…”