B NMR Spectroscopy of Lead Borate Glasses : Additive Effect of Cerium Oxide

Glasses and glass ceramics in the system xCeO2∙(50 − x)PbO∙50B2O3 (0 ≤ x ≤ 50) have been studied, for the first time, by NMR and FTIR techniques. Effect of CeO2 substitution with PbO on NMR parameters has been discussed in terms of changing both boron and cerium coordination. The quantitative fraction of four coordinated boron (N4) has been simply determined from B NMR spectroscopy. On the other hand, the fraction of total tetrahedral structural units B4 (BO4 + PbO4 + CeO4) is obtained from FTIR spectral analysis. It is not possible to get the fraction of cerium oxide directly from the applied spectroscopic tools. Therefore, a simple approach is applied, for the first time, to determine CeO4 fraction by using the different criteria of both B NMR and FTIR spectroscopy. The fraction of B4 species is equal to N4, within the experimental error, of the same glasses in the composition region of up to 10 mol% CeO2. On the other hand, there is a clear difference between both N4 and B4 values in glasses of higher CeO2 content (>10 mol%). The related difference showed a linear increasing trend with increasing the content of CeO2 in the glass. This was discussed on the bases of structural role of CeO2 which acts as a glass former in the region >10 mol%, while, at lower concentration, it consumed as a glass modifier.


Introduction
Borate glasses containing CeO 2 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 [1]- [4].Several investigations have been reported on the structure of How to cite this paper: El-Damrawi, G., Gharghar, F., Ramadan, R. and Aboelez, M. (2016) 11 B NMR Spectroscopy of Lead Borate Glasses: Additive Effect of Cerium lead containing glasses [2]- [6].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 CeO 2 -B 2 O 3 glasses, since cerium oxide acts as a glass former [3] [4].Each atom of PbO added is used to convert two BO 3 species into two BO 4 units.The fraction of BO 4 groups increases with increasing PbO contents reaching maximum value ~0.50 -0.53 [7]- [10] at equal amount of PbO and B 2 O 3 oxides.Further increase of PbO content decreases the number of BO 4 units by forming nonbridging oxygen atoms in glass network.In such situation Pb ions are mainly surrounded by two BO 4 tetrahedral units and in part by BO 3 units with NBOs [11].But major portion of CeO 2 inters the network of the glass as a modifier at extremely high contents (40 -60 mol %) [3].
In this paper, the structure of CeO 2 -PbO-B 2 O 3 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.

Glasses Preparation
The glass samples were prepared by mixing and fusing the desired amount of CeO 2 , PbO, and H 3 BO 3 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% CeO 2 .The melt was swirled frequently and then poured on stainless steel plate and pressed by another plate to get disc like shape.

11 B 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. 11 B 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.

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 ≤ CeO 2 ≤ 10 mol%, since both resonance spectra characterizing separated BO 3 and BO 4 groups are clearly appeared [12]- [15] (Figure 3).The wide spectral band of chemical shift exists between 11 -20 ppm is attributed to BO 3 (both in ring and nonring) groups [12] [13].The second appears at 0 ppm which is due to tetrahedral BO 4 units.An extra increase in CeO 2 at expense of PbO concentration will result in overlapping resonance spectra characteriz-ing both BO 3 and BO 4 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 CeO 4 species are suggested to be formed upon a frequent replacement of CeO 2 with PbO.Increasing concentration of CeO 4 groups is accompanied with a decrease in the fraction of BO 4 units.This is because part of modifier oxide is consumed to modify CeO 2 network.The same feature is reported in detail in our previous work [18] on the same glasses investigated by FTIR spectroscopy.First, both B 4 and N4 value in both cases changes slightly around fixed value with introducing CeO 2 up to ~10 mol%.This little difference between the two fraction (B 4 and N 4 ) may be considered due to the same role of both lead and cerium as glass modifier, since substitution of PbO by CeO 2 hasn't remarkable effect on both values.Further substitution of PbO by CeO 2 (in the region >10 mol %) will result in decreasing in both B 4 and N 4 with different rates.Moreover, the differences between them increase with increasing CeO 2 contents.Increasing differences between B 4 and N 4 may lead to conclusion that the ability of CeO 2 to act as a glass former is increased with its content.Thus the formation of CeO 4 groups as the most dominate species in this region is considered as the main reason of reduction in the tetrahedral (BO 4 ) groups in the glass network.As a direct effect, B 4 is abruptly decreased upon more addition of CeO 2 (see Figure 4).The difference between B 4 and N 4 for each composition gives a quantitative concentration of CeO 4 fraction as a glass former.Based upon the above considerations, we suggest that Ce (in high CeO 2 ) glasses has strong ability to form its own structural units and preferentially bridge to BO 3 rather than increasing number of tetrahedral BO 4 groups.For this reason both B 4 and N 4 values are continuously decreased with increasing CeO 2 contents.

Conclusion
Glasses in system of xCeO 2 •(50 − x)PbO•50B 2 O 3 with 0 ≤ x ≤ 50 mol% have been investigated, for the first time, by 11 B NMR structural technique.It is evidenced that cerium and lead ions play a dual role in the studied system.In low CeO 2 content, ≤10 mol, CeO 2 plays a modifier role.The structure role of CeO 2 is changed from modifier to a glass former at higher content.The fraction of the tetrahedral cerium (Ce 4 ) 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, Ce 4 fraction is determined from the differences between values of B 4 and N 4 .
NMR spectra of both 50CeO 2 -50B 2 O 3 and 50PbO-50B 2 O 3 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-B 2 O 3 glass (free from cerium), well resolved rsonance peaks characterizing BO 3 (both in ring and nonring) and BO 4 groups are

Figure 1 .
Figure 1. 11B NMR spectra of cerium free (at the bottom) and of glass contains 50 mol% CeO 2 .

Figure 4
represents the changes of B 4 and N 4 fractions with CeO 2 concentration.The B 4 can be showed to change with different rates upon addition of CeO 2 concentration.

Figure 2 .
Figure 2. FTIR absorbance spectra of cerium free (at the bottom) and of glass contains 50 mol% CeO 2 .
Figure 5 presents the change of CeO 4 .

Figure 4 .
Figure 4. Changes of B 4 and N 4 fraction as function of CeO 2 concentration.

Figure 5 .
Figure 5. Changes of Ce 4 fraction as function of CeO 2 concentration.