The glass samples were prepared by mixing and fusing the desired amount of CeO₂, PbO, and H₃BO₃ compounds in alumina crucibles. The melting process was carried out at different temperatures depending on the glass compositions. The glasses were prepared with a wide variety of the cerium oxide concentration which is varied from 2.5 to 50% CeO₂. The melt was swirled frequently and then poured on stainless steel plate and pressed by another plate to get disc like shape.

All samples were measured with JEOL GSX-500 high-resolution solid-state MAS NMR spectrometer in a magnetic field of 11.4 T. ¹¹B MAS NMR spectra were recorded at a frequency of 160.4 MHz and spinning rate of 15 KHz. The glass samples were measured with a single pulse length of 0.5 - 1.0 ms and a pulse delay of 2.5 s, and an accumulation of 200 - 300 scans is obtained.

NMR spectra of both 50CeO₂-50B₂O₃ and 50PbO-50B₂O₃ binary glasses are presented in figure 1. It is clear from this figure that there is a great difference between the features of the two spectra. In case of PbO-B₂O₃ glass (free from cerium), well resolved rsonance peaks characterizing BO₃ (both in ring and nonring) and BO₄ groups are

clearly distinguished. The fraction of boron tetrahedral units is ~0.53 which means that 53% from the total boron is found in its tetrahedral coordination with oxygen atoms. This suggests that the majority of PbO is consumed as a glass modifier. It converts two BO₃ units to two BO₄ groups. On the other hand, broader and anti-symmetric NMR band of CeO₂-B₂O₃ glass are clearly evidenced by the effect of CeO₂. It can be seen from the ¹¹B spectrum of the glass that the bands characterizing BO₃ is diluted by CeO₂. As a result, a wide spectrum containing overlapped peaks characterizing BO₃ and BO₄ is obtained, see figure 1. The fraction of tetrahedral boron is much lower than that of lead borate glass, since the determined value of B₄ = 0.34.

Similar observation can be also found in FTIR absorbance spectra of the two glasses, see figure 2. It can be seen from the spectrum of cerium free glass that individual resonances representing BO₃ and BO₄ are markedly distinguished. Higher CeO₂ concentration (50 mol%) leads to appearing of broader and anti-symmetric FTIR resonance band. This wide band is assigned to mixed Ce₄-O, B₄-O and B₃-O stretching vibration mode.

There are two main spectral regions which characterize the NMR resonance modes of the ternary glass network. The first is dominant in glass of 0 ≤ CeO₂ ≤ 10 mol%, since both resonance spectra characterizing separated BO₃ and BO₄ groups are clearly appeared [12] - [15] (figure 3). The wide spectral band of chemical shift exists between 11 - 20 ppm is attributed to BO₃ (both in ring and nonring) groups [12] [13] . The second

appears at 0 ppm which is due to tetrahedral BO₄ units. An extra increase in CeO₂ at expense of PbO concentration will result in overlapping resonance spectra characterizing both BO₃ and BO₄ groups. This may reflect change in structure role of cerium oxide in this composition region, since it acts as a glass former [1] [13] [16] [17] . As a direct result, tetrahedral CeO₄ species are suggested to be formed upon a frequent replacement of CeO₂ with PbO. Increasing concentration of CeO₄ groups is accompanied with a decrease in the fraction of BO₄ units. This is because part of modifier oxide is consumed to modify CeO₂ network. The same feature is reported in detail in our previous work [18] on the same glasses investigated by FTIR spectroscopy.

Figure 4 represents the changes of B₄ and N₄ fractions with CeO₂ concentration. The B₄ can be showed to change with different rates upon addition of CeO₂ concentration.

First, both B₄ and N4 value in both cases changes slightly around fixed value with introducing CeO₂ up to ~10 mol%. This little difference between the two fraction (B₄ and N₄) may be considered due to the same role of both lead and cerium as glass modifier, since substitution of PbO by CeO₂ hasn’t remarkable effect on both values. Further substitution of PbO by CeO₂ (in the region >10 mol %) will result in decreasing in both B₄ and N₄ with different rates. Moreover, the differences between them increase with increasing CeO₂ contents. Increasing differences between B₄ and N₄ may lead to conclusion that the ability of CeO₂ to act as a glass former is increased with its content. Thus the formation of CeO₄ groups as the most dominate species in this region is considered as the main reason of reduction in the tetrahedral (BO₄) groups in the glass network. As a direct effect, B₄ is abruptly decreased upon more addition of CeO₂ (see figure 4). The difference between B₄ and N₄ for each composition gives a quantitative concentration of CeO₄ fraction as a glass former. Figure 5 presents the change of CeO₄.

fraction with CeO₂ contents. Ce₄ is observed to increase linearly with increasing CeO₂ concentration. The linear correlation between Ce₄ and CeO₂ concentration leads to confirm that most of all CeO₂ inters the glass network as a strong glass former.

Based upon the above considerations, we suggest that Ce (in high CeO₂) glasses has strong ability to form its own structural units and preferentially bridge to BO₃ rather than increasing number of tetrahedral BO₄ groups. For this reason both B₄ and N₄ values are continuously decreased with increasing CeO₂ contents.