Borate glasses containing CeO2 and PbO are used in wide field of applications. This may be due to their distinction features including wide glass forming region and low thermal expansion  -  . Several investigations have been reported on the structure of lead containing glasses  -  . It was concluded that glass and glass ceramics containing high concentration from PbO are of great importance from the viewpoint of glass formation. This may because PbO in such a case inters the network of the glass as s former.
In the low PbO content (up to 50 mol%), majority of PbO act as a network modifier. The reverse behavior is found in binary CeO2-B2O3 glasses, since cerium oxide acts as a glass former   . Each atom of PbO added is used to convert two BO3 species into two BO4 units. The fraction of BO4 groups increases with increasing PbO contents reaching maximum value ~0.50 - 0.53  -  at equal amount of PbO and B2O3 oxides. Further increase of PbO content decreases the number of BO4 units by forming nonbridging oxygen atoms in glass network. In such situation Pb ions are mainly surrounded by two BO4 tetrahedral units and in part by BO3 units with NBOs  . But major portion of CeO2 inters the network of the glass as a modifier at extremely high contents (40 - 60 mol %)  .
In this paper, the structure of CeO2-PbO-B2O3 glasses is investigated by means of FTIR absorbance and NMR techniques with an aim to determine the structure role of cerium in ternary cerium lead borate glasses. In this regard, there is a remarkable lack and shortage information about the role of ceria in glass ceramics.
2. Experimental details
2.1. Glasses preparation
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.
2.2. 11B NMR measurements
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.
3. Results and discussion
3.1. Binary Borate glasses
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
Figure 1. 11B NMR spectra of cerium free (at the bottom) and of glass contains 50 mol% CeO2.
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.
3.2. Ternary cerium Lead borate glasses
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  -  (figure 3). The wide spectral band of chemical shift exists between 11 - 20 ppm is attributed to BO3 (both in ring and nonring) groups   . 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     . 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  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.
Figure 2. FTIR absorbance spectra of cerium free (at the bottom) and of glass contains 50 mol% CeO2.
Figure 3. 11B spectra for CeO2-PbO-B2O3 glasses at different CeO2 concentrations.
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.
Figure 4. Changes of B4 and N4 fraction as function of CeO2 concentration.
Figure 5. Changes of Ce4 fraction as function of CeO2 concentration.
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.
Glasses in system of xCeO2∙(50 − x)PbO∙50B2O3 with 0 ≤ x ≤ 50 mol% have been investigated, for the first time, by 11B NMR structural technique. It is evidenced that cerium and lead ions play a dual role in the studied system. In low CeO2 content, ≤10 mol, CeO2 plays a modifier role. The structure role of CeO2 is changed from modifier to a glass former at higher content. The fraction of the tetrahedral cerium (Ce4) as a former species is determined from a suggested approach which is based on correlation between structural feature obtained from both NMR and FTIR analysis. Accordingly, Ce4 fraction is determined from the differences between values of B4 and N4.
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