Crystallization and phase equilibrium in the sodium barium metaphosphate system

Crystallization of sodium barium metaphosphate glasses has been studied by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and thermal analysis. The glasses studied have compositions given by the general formula (50−x)Na2O–xBaO–50P2O5, where x=0, 10, 20, 30, 40 or 50. These glasses crystallize from the surface in the absence of a nucleating agent, but interior crystallization can be induced by inclusion of a nucleating agent (Pt) in the batch. The nucleated glasses can be converted to glass-ceramics with an appropriate heat treatment. Phase relations have been determined for the pseudobinary system. The phases formed, i.e. NaPO3, Na4Ba(PO3)6, Na4Ba(PO3)3 and Ba(PO3)2, are those predicted by the published equilibrium diagram. In some cases, however, three phases were present in region of the diagram where only two equilibrium phases should exist.