Correlations between Structural and Optical Properties of Peroxy Bridges from First Principles

This work aims at addressing the issue of the optical signature of peroxy bridges by using first-principles methods that combine density functional theory, GW (where G and W stand for one-particle Green function and screened Coulomb potential, respectively), and the solution of a Bethe-Salpeter equation on a bulk amorphous SiO2 model. We demonstrate that the presence of bridges induces broad and weak absorption bands between 3.2 and 7.5 eV. By analyzing the Si-O-O-Si dihedral angle distributions and the corresponding electronic structure, we show that the low overlap between O 2p states involved in the optical transitions together with the dihedral angle site-to-site disorder are at the origin of this weak and broad absorption. Moreover, the energy difference between the two first optical transitions depends linearly on the energy difference between the two first occupied defect-induced electronic states, i.e., depends on the dihedral angle of the bridge. This behavior may explain the longstanding controversy regarding the optical signature of peroxy bridges in amorphous SiO2. Because the correlation is independent of the specific hosting hard material, the results apply whenever the dihedral angle of the bridge has some degree of freedom