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

TL;DR

This study uses hybrid functional calculations to accurately model the Al impurity in silica, revealing hole localization and matching experimental EPR parameters, thus improving understanding of defect states in quartz.

Contribution

It demonstrates that screened exchange hybrid functionals effectively describe hole localization and EPR parameters for Al impurities in silica, surpassing standard density approaches.

Findings

Hole localizes on one oxygen atom around Al impurity.

Defect level identified at about 2.5 eV above valence band.

Calculated EPR parameters match experimental data.

Abstract

We performed first-principle calculations based on the supercell and cluster approaches to investigate the neutral Al impurity in smoky quartz. Electron paramagnetic resonance measurements suggest that the oxygens around the Al center undergo a polaronic distortion which localizes the hole being 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 a O 2p orbital. The calculated values of the g tensor and the hyperfine splittings are in excellent agreement with experiment.