Electronic and optical properties of doped crystals: First-principles calculations

Based on density functional theory, the geometric and electronic structures of the Ti doped α-Al2O3 system (Ti:α-Al2O3) are calculated by using a first-principles method. Due to the Ti-doping, both the lengths and angles of bonds in the local geometries are distorted, from which a trigonal–triclinic structural transition may be expected. With increasing Ti-doping, an insulator–semimetal transition is observed in the Ti:α-Al2O3 system, mainly due to a complete spin polarization of electrons at the Fermi level. The dielectric function and absorption edge are further calculated. It is found that the absorption edge is decreased, and is much lower than that of pure Al2O3 crystals, which is in good agreement with the experiment. The results indicate the potential applications of the Ti:α-Al2O3 optical system.