Point defects in particle-irradiated single crystals of tetragonal GeO2

Reactor irradiation of single-crystal tetragonal Ge${\mathrm{O}}_2$ predominantly generates two spin-1/2 paramagnetic defects. The ESR parameters for one of the centers are almost isotropic with $g$ values slightly less than $g_e$. This center is also generated by 1.5-MeV electron irradiation, and is attributed to an $F^{+}$ center. The second defect is holelike with rhombic $g$ values. A model consisting of an $\mathrm{O}_2^{}{}_{}{}^{-}$ (peroxy) molecular ion, located at a cation vacancy with two adjacent anion vacancies, is proposed for this holelike center. These two centers, correspond to defects reported earlier by Purcell and Weeks from polycrystalline studies; however, improved models are obtained from single-crystal measurements. Optical-absorption bands are observed at 2.5 and 3.8 eV for spectra polarized parallel to the $c$ axis. The intensity of the 2.5-eV band correlates with the $F^{+}$ ESR resonance. Optical excitation into the 3.8-eV band results in its bleaching and the accompanied growth of the 2.5-eV band; the changes are reversed by excitation into the 2.5-eV band. The band at 2.5 eV is assigned to a transition of the $F^{+}$ center and the reciprocal relationship between the 2.5-eV band and the 3.8-eV band strongly suggests that the latter is a transition of the $F$ center.