The glass was prepared by the standard melt-quenching method.
Appropriate amounts of ZnO, Na2CO3, and (NH4)2HPO4were thoroughly
mixed and placed in a Pt–Au5 crucible. The batch was first treated at
600 °C during 2 h (with a heating rate of 1°/min) to allowfor the sodium
carbonate and ammonium salt decompositions.