Thermal, Structural and Crystallization Study of Niobium Potassium Phosphate Glasses

New glass compositions were investigated in the binary system KPO₃-Nb₂O₅. The glass forming domain was determined by melt-quenching of the starting nominal compositions and it has been experimentally observed that glass samples can be obtained between the molar compositions 95KPO₃-5Nb₂O₅ and 50KPO₃-50Nb₂O₅. Chemically stable compositions from 80KPO₃-20Nb₂O₅ to 50KPO₃-50Nb₂O₅ were characterized by DSC for determination of characteristic temperatures Tg, Tx, Tp and Tf. Glass transition temperatures strongly increase with Nb₂O₅ content whereas thermal stability against devitrification progressively decreases. Thermal data were used to suggest a structural model in which NbO_x polyhedra are inserted inside the phosphate chains of PO₄ units. For higher Nb₂O₅ contents, NbO_x units progressively link together to form amorphous NbO_x clusters, responsible for the yellow color and lower thermal stability against devitrification. For the composition 50KPO₃-50Nb₂O₅, it has been found that the first crystallization peak is related with precipitation of hexagonal Nb₂O₅ in the glass matrix whereas the high temperature exothermic peak is due to both phase transition of hexagonal niobium oxide to monoclinic niobium oxide and precipitation of niobium potassium phosphate K₂Nb₆P₄O₂₆.

Keywords:
glasses; phosphate; niobium; crystallization