Ab initio simulation of the oxygen vacancy bistability in pure and Ge-doped α-quartz

The influence of germanium impurities on formation energy and transition energies between the ground singlet and the first excited triplet states of the oxygen vacancy in α-quartz, is studied theoretically using an ab initio embedded cluster approach in an unrestricted Hartree-Fock (UHF) linear combination of atomic orbitals (LCAO) framework; electron correlation effects are evaluated a posteriori using the correlation-only functional proposed by Perdew and Wang. Impurities lower the energy of the singlet state and destabilize the triplet. Consequently, defect formation energy decreases and transition frequencies increase proportionally to the number of Ge atoms neighbouring the vacancy. In the triplet state, the transition to the puckered configuration is considered; the role of Ge impurities is, in this case, to decrease the activation barrier for the transition and to enhance the difference in stability between puckered and normal configurations of the vacancy. The puckering process leads to the creation of two stable E' centres and is, therefore, interesting in connection to Bragg gratings writing by UV irradiation of silica glasses.