Absence of germanate anomaly in ternary lithium germanophosphate glasses: modification behavior of mixed glass system of strong and fragile formers

Abstract

Ternary lithium germanophosphate glasses have been synthesized over a wide range of compositions. Thermal and spectroscopic techniques (IR, Raman, and 31P MAS NMR) have been used to investigate their properties and structures. It is found that modification occurs almost exclusively on the P2O5 part of the structure and the GeO₂ network appears unaffected. GeO_6/2 octahedra are not formed in the glasses, except in one GeO₂-rich and Li₂O-poor composition. Thermodynamic arguments are given to rationalize this behavior. The unique behavior of the Tg values and molar volumes have been discussed in terms of a model, which considers modification occurring only on the P₂O₅ part of the glass structure. F_1/2 fragilities of these glasses have been evaluated from Δ T_g and T_g values, and a relation between F_1/2 and the physical dimensionality has been proposed.