### 3.2 MAS NMR analysis

Figure 3 shows $ {}^{31} $ P MAS NMR spectra before and after the heat-treatment. For the glass No.2, a peak at $ \delta\approx-14 $ ppm is observed, which is assigned to pure $ Q_{P}^{2} $ unit, $ Q_{P}^{2} $ unit connected to one four-coordinated boron or $ Q_{P}^{3} $ unit connected to four-coordinated boron [2, 3], where n in $ Q_{X}^{n} $ means the number bridging oxygen (BO) connected to the X atoms. Comparing the spectra for before and after the heat-treatment in Figure 3 (a), no change is observed on the environment of $ PO_{4} $ network. However, in the glass No. 5, a significant change is observed, that is, a peak at $ \delta\approx-24 $ ppm in the as-prepared glass shifts to $ \delta\approx-29 $ ppm after the heat-treatment. The sharp peak at -29 ppm is assigned to $ Q_{P}^{4} $ in $ AlPO_{4} $ crystal [4]. The relative fractions of P in $ AlPO_{4} $ crystal to total phosphorus were not determined. Comparing as-prepared glasses (Figure 4), the peak shifts to a higher magnetic field side with increasing Al/Na ratio.

Figure 5 shows $ {}^{27} $ Al MAS NMR spectra of the glass No. 5 before and after the heat-treatment, which shows that Al atoms are mainly present as $ AlO_{4} $ unit in glasses. The peak at -39 ppm after the heat-treatment is also assigned to $ Q^{4}_{Al} $ in $ AlPO_{4} $ crystal [4], indicating the precipitation of $ AlPO_{4} $ crystal. According to a rough estimation by the peak area, the relative fractions of the glassy and crystalline Al species are 0.34 and 0.66, respectively.

### 3.3 ICP analysis

Table 2 shows the chemical compositions of the glass phases after phase separation obtained from ICP analysis. These compositions were estimated as follows; $ B_{2}O_{3} $ -rich phase is soluble in hot water, and hence the composition of $ B_{2}O_{3} $ -rich phase is determined from the concentration of the elements in the solution after hot-water treatment. $ SiO_{2} $ -rich phase is insoluble in hot water but soluble in HF solution, and hence the insoluble solids after hot-water treatment were dissolved by HF solution, from which the composition of $ SiO_{2} $ -rich phase was determined.

As shown in Table 2, phosphorus in the $ Al_{2}O_{3} $ -free glass No. 2 is preferentially distributed into $ B_{2}O_{3} $ -rich glass phase after the phase separation. On the other hand, in the $ Al_{2}O_{3} $ -containing glasses No. 3 and No. 5, phosphorus is distributed also into

Figure 5. $ {}^{27} $ Al MAS NMR spectra of glasses before and after the heat-treatment.

SiO $ _{2} $ -rich glass phase. In addition, the amount of phosphorus distributed into SiO $ _{2} $ -rich glass phase increases with increasing Al/Na ratio. Moreover, aluminum is preferentially distributed into SiO $ _{2} $ -rich glass phase.

However, in the case of acid treatment, phosphorus and aluminum were completely eluted in all samples, suggesting that P and Al species both in glassy and $ AlPO_{4} $ crystalline phases were insoluble to hot water but soluble to acid.

Table 2. Analytical composition of the glass phases after phase separation

(a) $ \mathrm{SiO}_{2} $ -rich phase (mol%)
No.	$ \mathrm{Na}_{2}\mathrm{O} $	$ \mathrm{B}_{2}\mathrm{O}_{3} $	$ \mathrm{SiO}_{2} $	$ \mathrm{P}_{2}\mathrm{O}_{5} $	$ \mathrm{Al}_{2}\mathrm{O}_{3} $
2	0.0	4.2	95.7	0.1	-
3	2.1	0.4	93.1	1.1	3.3
5	2.6	0.6	87.9	3.5	5.5

(b) B2O3-rich phase (mol%)
No.	Na2O	B2O3	SiO2	P2O5	Al2O3
2	18.1	73.1	2.8	6.0	-
3	10.6	81.7	2.3	5.3	0.0
5	7.0	89.1	1.7	2.2	0.0

### 4. Discussion

From the results of ICP analysis, it was suggested that phosphorus was preferentially distributed to $ B_{2}O_{3} $ -rich glass phase after phase separation, and with increasing $ Al_{2}O_{3} $ content, however, the amount of phosphorus distributed into $ SiO_{2} $ -rich glass phase increased. According to $ {}^{31}P $ MAS NMR measurements, the peak shifted to a higher magnetic field side with increasing $ Al_{2}O_{3} $ content, which