Influence of modifying cations on the structure and texture of silicate–phosphate glasses

The studies concern silicate–phosphate glasses belonging to NaCaPO4SiO2, NaCaPO4SiO2AlPO4, KCaPO4SiO2, KCaPO4SiO2AlPO4, (Na,K)CaPO4SiO2 and (Na,K)CaPO4SiO2AlPO4 systems of systematically changed [SiO4]4−/[PO4]3− proportion.27Al MAS NMR investigations have shown that aluminum occurs exclusively in the tetrahedral position, i.e. it is a framework-forming cation.Microscopic and EDX investigations have made it possible to find out that in all the glasses studied liquation takes place – disperse phase in matrix. It has been established that addition of aluminum dramatically changes the composition of the matrix and disperse phase. In the case of glasses which do not contain aluminum the disperse phase is almost pure silicate phase, whereas the matrix is enriched in calcium and phosphorus. In the case of glasses containing aluminum the disperse phase is phosphate calcium phase and the matrix – mainly silicate phase. Such composition of the matrix and disperse phase is observed only in the case of the glasses containing high amounts of NaCaPO4 or KCaPO4. In the glasses containing aluminum, when the content of NaCaPO4 exceeds 25% and that of KCaPO4 is higher than 35% inversion of the matrix and disperse phase compositions takes place.Significant influence of Al+3 and modifying cation (Na+ and K+) on the glass structure has been established. The presence of aluminum causes the elimination of PO bonds. This phenomena can be observed easily when only one modifying cation is present.