Crystallization and microstructure of Na2O-CaO-Nb2O5-P2O5 glasses

Glasses in the system Na2O-CaO-Nb2O5-P2O5 were crystallized during cooling and reheating up to 900 degrees C and crystalline phases were compared. Most of glasses showed surface nucleation, except the glass with a 10 mol% niobium content exhbited bulk nucleation. During cooling phosphates (Ca-3(PO4)(2), Ca2P2O7) and niobophosphates (NbPO5, NbOPO4 and Nb18P2.5O50) crystallized. Sodium niobophosphates recrystallized to Na4Nb8P4O32 or Na2Nb6P4O26 during reheating. Samples with highest amount of sodium formed more NaNbO3, samples with phosphate in excess of 35 mol% formed Na2Nb6P4O26 instead of Na4Nb8P4O32. The microstructure for all samples was inhomogeneous and contained crystallites with a large distribution of sizes.