Fig. 3. $ {}^{31} $ P MAS NMR spectra of the glasses, (a) No. 2, and (b) No. 5. ▼: spinning side band

It is revealed from the compositional analyses that phosphorus is preferentially distributed to $ B_{2}O_{3} $ -rich phase after the phase separation of $ TiO_{2} $ -free glass, and $ TiO_{2} $ addition initiates the phosphorus distribution into $ SiO_{2} $ -rich phase. According to $ {}^{31}P $ MAS NMR, phosphorus in $ TiO_{2} $ -free glass is mainly present in $ Q_{P}^{2} $ unit, and in the $ TiO_{2} $ containing glasses, $ Q_{P}^{1} $ or $ Q_{P}^{0} $ units including P-O-Ti bonds are formed. Since $ Q_{P}^{n} $ ( $ n = 0 \sim 2 $ ) units are negatively charged, they must be coordinated by positively-charged species such as $ Na^{+} $ to maintain electrical neutrality. In the $ TiO_{2} $ -free glass, $ Q_{P}^{2} $ units are compensated by $ Na^{+} $ ion, and hence phosphorus is distributed to $ Na^{+} $ -abundant $ B_{2}O_{3} $ -rich phase. In the $ TiO_{2} $ -containing glass, however, $ Q_{P}^{1} $ or $ Q_{P}^{0} $ units distributed to $ SiO_{2} $ -rich phase are not compensated by $ Na^{+} $ ions because $ Na^{+} $ ions are absent in $ SiO_{2} $ -rich phase. It is therefore expected that $ Q_{P}^{1} $ or $ Q_{P}^{0} $ units are compensated by $ Ti^{4+} $ ions acting as network modifiers with the coordination numbers > 4. In the case of $ Al_{2}O_{3} $ addition [3], phosphorus distribution into $ SiO_{2} $ -rich phase is also observed, in which the formation of positively-charged $ Q_{P}^{4} $ units including P-O-Al bonds is confirmed. The effect of $ Al_{2}O_{3} $ and $ TiO_{2} $ on the phosphorus distribution seems to be the same, and the distribution mechanism, however, seems to be quite different, because $ Q_{P}^{n} $ units formed are different between the cases of $ Al_{2}O_{3} $ and $ TiO_{2} $ addition. It is revealed that the distribution of phosphorus is dependent on the states of $ PO_{4} $ units, and furthermore, the state of $ PO_{4} $ units is dependent on the neighboring ions of $ Na^{+} $ , $ Al^{3+} $ and $ Ti^{4+} $ . To clarify more detailed mechanism of phosphorus distribution, distribution mechanism of $ Al^{3+} $ and $ Ti^{4+} $ is also required.

### 4. CONCLUSION

Sodium borosilicate glasses containing $ P_{2}O_{5} $ and $ TiO_{2} $ were prepared by a conventional melt-quenching method, and the effect of $ TiO_{2} $ addition on the behavior of phosphorus associated with the phase separation were investigated. After the heat treatment, phase separation by spinodal decomposition was confirmed by SEM observation, and at $ TiO_{2} $ content $ \geq 5 $ mol%, precipitation of $ TiO_{2} $ crystal was also found when heated near $ T_{x} $ . $ {}^{31} $ P MAS NMR spectra suggested that in $ TiO_{2} $ -free glasses, phosphorus was essentially present in $ Q^{2}_{P} $ species, and $ Q^{1}_{P} $ or $ Q^{0}_{P} $ species formed after adding $ TiO_{2} $ . From ICP analysis, in $ TiO_{2} $ -free glasses, phosphorus was preferentially distributed into $ B_{2}O_{3} $ -rich phase after the phase separation. $ TiO_{2} $ addition initiated the phosphorus distribution into $ SiO_{2} $ -rich phase, and with increasing $ TiO_{2} $ content, the amount of phosphorus distributed into $ SiO_{2} $ -rich phase increased. It was revealed that the distribution of phosphorus depended on the existing state of $ PO_{4} $ units.

Fig. 4. IR spectra of the insoluble solids after acid treatment.