Influence of melting conditions on the water content and structure of K2O-La2O3-Al2O3-SiO2-P2O5 glasses

- The Increase of temperature and melting time produces the enrichment in SiO2.
- SiO2 enters the glass structure as a fully polymerized sub-network.
- La2O3 and Al2O3 contents are not modified under the different melting conditions.
- The water content is drastically reduced and linked to the alumino-phosphate network.
- Aluminium changes from modifier to former role along with decreasing water content.

Glasses of the system of composition K2O-La2O3-Al2O3-SiO2-P2O5 have recently gained attention due to their response to femtosecond (fs) laser irradiation, and the fact that high repetition rate fs-laser writing allows for the production of very efficient waveguides. When doped with rare-earth ions, the glasses can also be used in amplifiers or lasers and, in this respect, a control of the water content in the phosphate-based glasses is of very high importance. Thus, in the present work we have studied the influence of the melting conditions on the final water content of glasses of the above system and on their structure as studied by Raman and Nuclear Magnetic Resonance spectroscopies. Increasing temperature and melting times conducts to the enrichment in SiO2 and a depletion of K2O and P2O5 contents in parallel with a reduction of the water content as measured by FTIR. On the other hand, the structural study showed that the chemical environment of phosphorous and silicon are not much affected by the changes in composition and that aluminium moves from being 6-fold coordinated to 4-fold. Finally, it has also been observed that SiO2 enters in the glasses as fully polymerized, which is consistent with the reduction of water directly on the ratio between oxygen and phosphorus in the glasses.