Phosphate glasses generally have lower glass-transition temperatures and greater thermal-expansion coefficients than silicate and borate glasses, and so are candidate materials for specialty glass-metal sealing applications. Phosphate glasses also have desirable optical properties (rare-earth stimulated-emission cross sections, low thermo-optical coefficients, ultraviolet transparency, etc.) and so are used as laser glasses. Recent developments of novel compositions for biomedical applications and as fast-ion conductors have also revived interest in the study of phosphate glass structures.
The structure/property relationships in simple phosphate glasses have not received nearly the attention paid to silicate and borate-based systems. This is partially due to the difficulty in preparing anhydrous compositions with more than 50-mol% P2O5. Ultraphosphate compositions (where the [O]/[P] ratio is less than 3) are highly reactive to ambient water. In addition because P2O5 sublimes at about 300°C, ultraphosphate glasses have not been studied to the extent that metaphosphate ([O]/[P] = 3) and polyphosphate ([O]/[P] > 3) compositions have.