Behavior of the packing densities of alkali germanate glasses

Packing densities are more suitable than the mass densities for structural studies. A comparison of the corresponding data on alkali germanate glasses with those of alkali berate glasses corroborates the suggestion of the formation of GeO5 and/or GeO6 units in systems (Me2O)(x)(GeO2)(1-x) in that range of composition where the maximum packing density occurs. The packing densities of both the alkali germanate and berate glasses strongly obey a common behavior while the data of the related crystals are widely spread. Simple structural rules like those used for the berate glasses are suggested for an explanation of the germanate anomaly. However, a specific characteristic of some germanate crystals is the occurrence of three-fold coordinated O sites accompanied by clusters of GeO6 units. There is no evidence for a significant role of such clusters in the glass structures though minor indications for such features were detected by Raman spectroscopy. (C) 1999 Elsevier Science B.V. All rights reserved.