Absence of Germanate Anomaly in Ternary Lithium Germanophosphate lasses: Modification Behavior of Mixed Glass System of Strong and Fragile Formers

Ternary lithium germanophosphate glasses have been synthesized over a wide range of compositions. Thermal and spectroscopic techniques (IR, Raman, and $^{31}P$ MAS NMR) have been used to investigate their properties and structures. It is found that modification occurs almost exclusively on the $P_2O_5$part of the structure and the $GeO_2$ network appears unaffected. $GeO_{6/2}$octahedra are not formed in the glasses, except in one $GeO_2$-rich and $Li_2O$-poor composition. Thermodynamic arguments are given to rationalize this behavior. The unique behavior of the $T_g$values and molar volumes have been discussed in terms of a model, which considers modification occurring only on the $P_2O_5$ part of the glass structure.$F_{1/2}$fragilities of these glasses have been evaluated from \Delta $T_g$and $T_g$values, and a relation between $F_{1/2}$ and the physical dimensionality has been proposed