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 measur ed with a double-beam instrument. Pure GeO2, Li2GeO3 crystal, pure GeO 2, sodium germanate and lanthanum germanate glasses were studied. Glas s samples were prepared by melting and some by evaporation during melt ing. Melting conditions were normal and oxidizing. Intrinsic absorptio n tails of a GeO2 crystal with a structure of alpha-quartz and in a Li 2GeO3 crystal are situated at 6 eV. Two absorption thresholds were obs erved 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 germana te 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 approxim ated 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.