Accurately weighed and mixed raw materials were melted in porcelain crucibles at 1250 _C for 1 h in an electric furnace (Vecstar, F 2, United Kingdom) and after rotating twice the melts were poured on warmed stainless steel plate and the samples were left to cool and then crushed in an agate mortar to fine powders. The fine powders were re-melted for another 1 h to achieve acceptable homogeneous and clear glasses. Finally, the melts were cast into preheated stainless steel molds and the samples were immediately transferred to a muffle furnace adjusted at 400 _C for annealing. After 1 h, the annealing muffle was left to cool to room temperature at a rate of 25 _C/h.