Hole localization in Al doped silica: A DFT + U description.

Despite density functional theory (DFT) being the most widely used ab initio approach for studying the properties of oxide materials, the modeling of localized hole states in doped or defective oxides can be a challenge. The electronic hole formed when silica is doped with aluminum is such a defect, for which a DFT description of the atomic and electronic structures has previously been found to be inconsistent with experiment, while Hartree-Fock provides a consistent description. We have applied the DFT + U approach to this problem and find that the structural distortions around the dopant are consistent with experimental data as well as earlier cluster calculations using Hartree-Fock and perturbation theory. A hole state is found 1.1 eV (1.6 eV experimentally) above the top of the valence band with localization of spin on the oxygen atom which shows the elongated Al-O distance. A formation energy of 5.7 eV is found. We discuss implications for using DFT+U to model defective oxide systems with O 2p holes.