Localized states in germanate glasses. Ultraviolet absorption tail of crystalline and glassy germanium dioxide and alkali germanate

The optical absorption of crystalline and glassy germanate were measured with a double-beam instrument. Pure GeO₂, Li₂GeO₃ crystal, pure GeO₂, sodium germanate and lanthanum germanate glasses were studied. Glass samples were prepared by melting and some by evaporation during melting. Melting conditions were normal and oxidizing. Intrinsic absorption tails of a GeO₂ crystal with a structure of α-quartz and in a Li₂GeO₃ crystal are situated at 6 eV. Two absorption thresholds were observed in a GeO₂ glass: one is situated at 6 ± 0.2 eV, the other at 4.5 ± 0.2 eV. The intensity of the latter increases in sodium germanate glasses and corresponds to the intrinsic absorption threshold of a tetragonal GeO₂ crystal. This threshold is known from the literature. The absorption spectra and their thermal dependences are well approximated by the Urbach rule in two forms. The threshold at 6 eV in all the materials studied obeys the Urbach-Toyozawa rule. The threshold at 4.5 eV agrees with the so called 'glass' type rule.