Sodium borosilicate glasses containing P 2 O 5 and Al 2 O 3 were prepared, and the behavior of phosphorus associated with the phase separation and the effect of Al 2 O 3 addition have been investigated. After the heat treatment, phase separation by spinodal decomposition was observed in the all samples. In the Al 2 O 3 -free glasses, phosphorus was preferentially distributed into B 2 O 3 -rich glass phase after the phase separation. With increasing Al 2 O 3 content, the amount of phosphorus distributed into SiO 2 -rich glass phase increased. According to 31 P MAS NMR measurements, in the Al 2 O 3 -containing glasses, Q 4 units free from terminal oxygen atoms were formed. It was suggested that the distribution of phosphorus depend on the PO 4 units.

Sodium borosilicate glasses containing phosphorus were prepared, and the effect of Al 2 O 3 and TiO 2 addition on the distribution behavior of phosphorus atoms associated with the phase separation was investigated. After heat treatment, a phase separation by the spinodal decomposition was commonly confirmed by SEM observation. In the additive-free sample, phosphorus was preferentially distributed into B 2 O 3 -rich phase after the phase separation, and after adding Al 2 O 3 or TiO 2 , the amount of phosphorus distributed into SiO 2 -rich phase increased. It was also suggested that TiO 2 had a stronger influence on the phosphorus distribution into SiO 2 -rich phase than Al 2 O 3 . 31 P MAS-NMR revealed that most of phosphorus atoms in the additivefree glass were present in PO 4 units consisting of two terminal oxide ions, and the terminal oxide ions decreased and increased after the additions of Al 2 O 3 and TiO 2 , respectively. It was suggested that this structural change brought about a change in the distribution of phosphorus.

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