Structural investigation of hydrous phosphate glasses

Dissolved water has major impact on the physical and chemical properties of phosphate glasses. In the present study we have investigated the structural response to water incorporation for glasses in the system Li2O-MgO-Al2O3-P2O5. Glasses containing 0-8 wt% H2O were synthesised at 500 MPa confining pressure in internally heated gas pressure vessels at 1323 K (LMP, Al-poor glass) and 1423 K (LMAP, Al-enriched glass). Water contents of glasses were determined by pyrolysis and subsequent Karl-Fischer titration (KFT) and/or by infrared spectroscopy. Density varies nonlinearly with water content implying large structural changes when adding up to 2 wt% H2O to the dry glass. Glass transition temperatures measured by differential thermal analysis (DTA) continuously decrease with water content. The trend can be explained by depolymerisation of the phosphate network. Near-infrared spectroscopy shows that even in Al poor glasses only a minority of dissolved water is present as H2O molecules, but the largest amount is present as OH groups formed by hydrolysis of P-O-P bonds. The network is stabilised by aluminium which is predominantly six-coordinated in these glasses as shown by Al-27 MAS NMR spectroscopy. With increase of Al in the glasses, breaking up of the phosphate network through hydrolysis is depressed, i.e. much lower OH contents are formed at same total water content. Network depolymerisation upon addition of H2O is evident also from P-31 MAS NMR spectroscopy. While phosphate tetraheda are crosslinked by two to three bridging oxygen in dry glasses, diphosphate groups are dominant in glasses containing 8 wt% H2O.