The structure of alkali germanophosphate glasses by Raman spectroscopy

A series of alkali germanophosphate glasses ((R2O)x(GeO2:P2O5)1−x where R=Na, K and Rb) with variable GeO2:P2O5 ratios (8:1, 6:1, 4:1 and 2:1) have been investigated using Raman spectroscopy. The glass network may be treated as being made up of separate germanate and phosphate components. Addition of alkali cations indicates that the alkalis preferentially modify the phosphate part of the network. Network depolymerization occurs by formation of Q2 and Q1 PO4 tetrahedra. At high Ge:P ratios the glasses exhibit a density anomaly. The anomaly is attributed to the formation of small three-membered GeO4 rings. However, alkali cation size and mass are contributing factors to the shape of the density anomaly in the K2O and Rb2O containing glasses. Depolymerization of the 2:1 alkali germanophosphate glasses is predominantly by formation of Q2 PO4 tetrahedra. At low Ge:P ratios (2:1) density trends are linear. No ring transition is observed in these glasses. Furthermore, the presence of [5]Ge cannot be ruled out, but if present, does not play a major role in generating a ‘germanate anomaly’.