Concentration-Dependent Electronic and Optical Properties of Cr-Doped α-Al₂O₃: A First-Principles DFT Study

Chromium (Cr) substitution offers a route to modifying the electronic structure and optical response of α-Al₂O₃, a wide-band-gap oxide with limited intrinsic absorption in the visible region. In this study, the concentration-dependent electronic and optical properties of Cr-doped α-Al₂O₃ are investigated using spin-polarized density functional theory within the generalized gradient approximation of Perdew–Burke–Ernzerhof, as implemented in VASP. Substitutional Cr concentrations of 8.33% and 16.66% are considered. The electronic structure is examined through band-structure and total and partial density-of-states analyses, while the optical response is evaluated using the complex dielectric function, energy-loss function, refractive index, extinction coefficient, absorption coefficient, and Tauc analysis. The calculated band gap decreases from 6.47 eV in pristine α-Al₂O₃ to 1.75 eV and 0.57 eV at Cr concentrations of 8.33% and 16.66%, respectively. The electronic-structure analysis indicates the formation of Cr-derived states within the host band gap, providing a qualitative explanation for the concentration-dependent gap narrowing and the shift of optical absorption toward lower photon energies. The optical results further demonstrate that Cr substitution substantially modifies the calculated light-response characteristics of α-Al₂O₃. These findings highlight the potential of substitutional doping as a means of tuning the electronic and optical properties of alumina. However, the absolute band-gap values should be interpreted cautiously because of the known limitations of the PBE functional and the relatively high dopant concentrations represented by the periodic models.