Theoretical study on near UV and visible optical absorption characteristics of Ti-doped α-Al2O3 single crystals

Abstract A first principles method is used to study the origin of the near-ultraviolet (300∼400 nm) and visible absorption band (400∼600 nm) in Ti-doped α-Al2O3 crystals. The electronic and optical properties of Al2O3 supercells containing substitutional Ti3+, interstitial Ti3+ and various Ti ion pairs were calculated. The results show that the absorption peaks at 388 (reported) and 325 nm (unreported) are both caused by the charge transfer transition from the substitutional Ti3+ 3d orbital to the adjacent Al3+ 3s3p orbital. When interstitial Ti3+ and Al vacancy coexist, the interstitial Ti3+ will relax to the position of the adjacent Al vacancy, forming a defect like substitutional Ti3+. The line-contact Ti3+-Ti3+, face-contact Ti3+-Ti3+ and point-contact Ti4+-Ti3+ ion pairs are the main causes for the multi-peak configuration and small bumps in the visible absorption band. Moreover, two other possible causes for this phenomenon are supplied from the perspective of ligand field theory and thermal activation.