Full-PAW calculations of XANES/ELNES spectra of Ti-bearing oxide crystals and TiO-SiO glasses: Relation between pre-edge peaks and Ti coordination

We performed first-principles full-projector-augmented-wave calculations of the Ti and O K-edge x-ray absorption near-edge structure/energy-loss near-edge structure spectra for Ti-doped SiO2 glasses and Ti-bearing crystalline oxides with various Ti coordination. Obtained spectra showed satisfactory agreement with the experimental results. From combined interpretation of the electronic structures and the spectra, we clarified that the pre-edge peaks in Ti K edge used in determining Ti coordination can be accounted mainly from the symmetry of Ti-O polyhedra to dominate on-site mixing of Ti p and Ti d orbitals. We showed that an isolated fourfold coordinated Ti atom in a SiO2 glass produces a strong pre-edge peak in Ti K edge and a narrow peak below the threshold in O K edge of a SiO2 glass, as observed experimentally. This supports the idea that Ti exists in fourfold coordination in a SiO2 glass at low Ti contents. We observed that the main region of a Ti K-edge spectrum depends on the intermediate-range structure around the Ti atom in Ti-doped SiO2 glasses, while it is averaged in the observed spectra. Chemical shifts in the Ti and O K-edge threshold energies for Ti-bearing oxides and Ti-doped SiO2 glasses were also examined, and the O K-edge threshold energy showed clear correlation with the iconicity or covalency of each system.