Defect luminescence study in tetragonal GeO2 crystals

The perovskite- or rutile-like GeO2 crystals were grown by the "top-seed" method from a fused germanium dioxide solution with sodium bicarbonate. The luminescence was studied under cathode-, x-ray, and photo-excitation. There are two luminescence bands at 550 nm and 400 nm with slow (100-200 mus) and fast (<20 ns) decay, respectively. The luminescence can be excited either by intra-center and electron-hole recombination processes. An analogous luminescence, however with strong non-exponential decay, is observed in glassy materials obtained from the frozen solution. Sodium-germanate glasses melted under oxygen deficient condition also possess similar luminescence, therefore the luminescence is ascribed to oxygen deficient luminescence centers. Such centers are known in literature for germanium dioxide glass in pure and sodium-modified state. The existence of such centers in GeO2 crystals with rutile structure and the lack of those ones in the quartz-like structure are a proof for the existence of these oxygen deficient centers in structural elements with six-fold coordinated germanium both in crystals with rutile structure as well as in germanate glasses.