Research on the Electronic Structure, Ultraviolet Optical Properties, and Raman Response for Ti³⁺-Doped α-Al₂O₃ Crystal by the First-Principles

Sapphire (α-Al₂O₃) is a leading optoelectronic and optical-window material whose transmission spans the deep-ultraviolet, visible and mid-infrared, covering the overwhelming majority of the useful optical spectrum. However, within this broad window the ultraviolet (UV) region is the most defect-sensitive region—the UV response of Ti:sapphire (Ti³⁺-doped α-Al₂O₃) is absorption-dominated, arising from interband transitions and the tail of the Ti 3d impurity bands. The microscopic defect origin of this UV response remains disputed, and how Ti³⁺-Ti³⁺ ion pairing reshapes the electronic and optical response has never been systematically studied. Here we present a first-principles density functional theory study that, for the first time, jointly maps the band structure, UV optical spectra (absorption, reflection and refraction), and temperature-/pressure-dependent Raman scattering of both single Ti³⁺-doped and Ti³⁺-Ti³⁺ ion-pair-doped α-Al₂O₃ in a 120-atom supercell. The results show that intrinsic Al₂O₃ is a wide-band gap insulator with a computed band gap of 7.065 eV, which is reduced to 4.722 eV upon single Ti³⁺ doping and to 3.480-4.588 eV for the three Ti³⁺-Ti³⁺ ion-pair configurations (line-, face- and point-contact). Doping introduces impurity levels near the Fermi level and gives rise to a new absorption peak in the 30-40 nm UV range, while the optical spectra shift systematically with doping concentration. Raman analysis shows that Ti³⁺ doping induces local lattice distortion and activates seven active vibrational modes (181-628 cm⁻¹); the characteristic peak areas grow with temperature, whereas the peak positions shift first to lower and then to higher wavenumbers with increasing pressure (reversing above 1.0 GPa). Together, these results unify how Ti³⁺ pairing governs the UV optical and vibrational response of sapphire, laying the groundwork for defect-engineered broadband optoelectronic materials.