Hybrid functional calculations of the Al impurity in α quartz: Hole localization and electron paramagnetic resonance parameters

We performed first-principles calculations based on the supercell and cluster approaches to investigate the neutral Al impurity in smoky quartz. Electron paramagnetic resonance measurements suggest that the oxygen atoms around the Al center undergo a polaronic distortion that localizes the hole on one of the four oxygen atoms. We find that the screened-exchange hybrid functional successfully describes this localization and improves on standard local-density approaches or on hybrid functionals that do not include enough exact exchange such as B3LYP. We find a defect level at about 2.5 eV above the valence band maximum, corresponding to a localized hole in an O 2p orbital. The calculated values of the g tensor and the hyperfine splittings are in excellent agreement with experiment.