More Insight on Structure of New Binary Cerium Borate Glasses

The structure of glasses in the system of xCeO2(100 − x)B2O3, x = 30, 40, 50 mol% CeO2 has been explored for the first time by correlation between data obtained from XRD, FTIR and B NMR analyses. NMR spectroscopy and FTIR spectroscopy have confirmed that transformation rate of BO3 to BO4 groups is reduced by CeO2 addition. The concentration of Ce4-O-Ce4 is increased at the expense of both B4-O-Ce4 and B3-O-B4 linkages. Boron atoms are mainly coordinated with Ce4 sites as second neighbors due to increasing CeO4 species with further increase of CeO2 concentration. Increasing B4 fraction is considered due to forming of CeO4 with rate higher than that of BO4 units. The change of chemical shift of B nuclei upon exchanging B2O3 with CeO2 confirms that the central boron atoms would be coordinated with tetrahedral cerium atoms as second neighbors. The X-ray diffraction of cerium rich glass is clearly indicated that the formation of crystalline phases refers to CeO4, CeBO3 and Ce(BO2)3 species.


Introduction
Borate glasses have attracted a great interest [1] [2] [3] [4] due to their desirable properties such as low melting temperature, high transparency, and good thermal stability.In addition, these glasses are promising host network to incorporate high concentrations from rare earth elements such as CeO 2 [5]- [12].The great importance of cerium ions is known [13]- [19] to come from their interesting characteristics such as physical, optical, catalytic and as well as magnetic properties.Particular technological applications including gamma ray shielding, G. El

Sample Preparation
The glass samples have been prepared by the normal melting method using cerium oxide (CeO 2 ) and boric oxide (H 3 BO 3 ) as starting materials.The appropriate amount of high purity chemical compounds were well mixed together to obtain fine powder.The batch mixture was then transferred to an alumina crucible and fused in an electric furnace.The melting process was carried out at different temperatures ranging between 900˚C and 1450˚C depending on the glass compositions.The melt was stirred several times until a complete homogenization was obtained.Each melt was then poured on stainless steel plate and pressed by another plate to take the final shape.
The range of the diffraction angle (2θ) is changed from 4˚ to 70˚ using a dwell time of 0.4 seconds.

11 B NMR Measurements
Solid-state 11

Fourier Transform Infrared Spectra (FTIR)
FTIR Spectra of powdered glasses were obtained in the wavenumber range of 400 to 4000 cm −1 using a Fourier transform IR spectrometer (Mattson 5000, Fine Measurements Laboratory, Mansura University, Egypt) with a resolution of 2 cm −1 .Each sample was mixed with KBr by the ratio 1:100 in weight and then subjected to a pressure of load of 5 tons/cm 2 to produce a homogeneous pellet.
The infrared absorbance measurements were carried out at room temperature immediately after preparing.
The spectra were corrected for the background and the dark current noises using two points baseline corrections then were normalized by making the absorption of every spectrum varying from zero to one reported in arbitrary units.

FTIR Analysis
FT-IR spectra of samples with x = 30, 40, 50 mol% CeO 2 are displayed in Figure 4. Three fundamental absorption bands [23] [30] [31] [32] have been observed in the borate glass system.The first band located between 600 -800 cm −1 is assigned to symmetric bending vibrations of BO 3 bonds.The second band from 800 -1200 cm −1 can be attributed to BO 4 stretching vibrations, while the third band from 1200 -1600 cm −1 is assigned to B-O stretching vibrations of triangular BO 3 units.
As noted from Figure 4, structural changes appeared in a wide and asymmetric broadening, with increasing CeO 2 content.Particularly, these changes are notable in the intensity of band ranged between 800 to 1200 cm −1 .Such tendency is possible to the main role played by CeO 2 through presenting more CeO 4 units in the glass structure and their abilities to shield and coordinate with BO 4 units.
As a consequence, increasing of CeO The total fraction of four-coordinated units, B 4 , could be calculated for the three samples using a deconvolution procedure [5] [25] [31] [32].This can be done by obtaining the relative area of each band corresponding to the structural units of both triangular BO 3 and tetrahedral BO 4 & CeO 4 units.
As a result, the value of B 4 is then defined as the ratio of the area related to the sum of structural groups containing BO 4 and CeO 4 four coordinated units to the area related to total units (BO 3 + BO 4 + CeO 4 ). Figure 5    where the shoulder at about 1600 cm −1 is assigned to Ce-O vibration in phase rich with cerium borate mixed units.

Conclusions
The structural features of cerium borate glasses correlated with CeO 2 role have been investigated via different tools.The following conclusions can summarize the observed new features.
• XRD results revealed that, crystallization would take place in glasses with 50 mol% CeO 2 .In such a glass, the principal crystalline phase is assigned to crystalline CeO 4 , CeBO 3 and Ce(BO 2 ) 3 species which is mainly referred to both CeO 4 and BO 4 as dominated units.• FTIR spectroscopy and NMR spectroscopy have confirmed that CeO 2 in binary borate glasses plays mainly the role of glass former in the form of CeO 4 units.The formation of expected ordered Ce 4 -B-Ce 4 linkage impairs the conversion of triangular BO 3 units into BO 4 tetrahedra and causes a wide broadening in the spectrum.
• Increasing of the total fraction of all four coordinated units (B 4 ) is highly associated with increasing in CeO 4 concentration and this would consequently result in formation of more ordered structures.
• The change in chemical shifts of 11 B nuclei from 12 ppm to 0 ppm with increasing CeO 2 content from 30 to 50 mol% is considered due to formation of more shielded borate units via B-O-Ce bonds.Each born atom can be coordinated with 3 or 4 Ce atoms in the second coordination sphere.

Figure 4 .
Figure 4. FTIR absorbance spectra of binary cerium borate glasses a function of CeO 2 concentration.
represents an example for the deconvolution in Gaussian band of 50CeO 2 •50B 2 O 3 glass sample.

Figure 6 Figure 5 .
Figure 6 illustrates the change in the total fraction B 4 versus CeO 2 content of [25]rawi et al. and dielectric, optical and electronic device are highly related to structural role of CeO 2 in glasses.Addition of a network modifier to B 2 O 3 is reported [20] [21] [22] [23] [24] to break the B-O bonds and induce the transformation of BO 3 triangles to BO 4 tetrahedral units.On the other hand, CeO 2 plays different role when it is added to B 2 O 3 since CeO 2 enters to the glass network as an intermediate oxide.The modification part of CeO 2 produces the conversion from BO 3 to BO 4 groups while the rest of CeO 2 can participate in the glass network to form CeO 4 tetrahedral groups.In this regard, few structural studies on binary cerium borate glasses have been done[5][9] [12][25].Changing in cerium environment around boron central atoms explains the dominant role of cerium oxide when it enters to the glass matrix as a glass former.Cerium thus behaves more as a glass modifier in low cerium content and plays the dual role at higher CeO 2 concentration.
DOI: 10.4236/njgc.2018.8100213 New Journal of Glass and Ceramics luminescent, scintillators It is aimed in the present study to determine the structural role of CeO 2 in cerium rich glasses by using the advantage of NMR spectroscopy, since to our knowledge, no studies in this regard have been carried out.
and 40 mol% CeO 2 is clearly evidenced, since a broad hump characterizing this feature is indicated in the XRD spectra of the glasses.On the other hand, sharper XRD peaks are developed on the spectra of glass contains 50 mol% CeO 2 .The discrete sharp lines observed at ~25.7, 28.6, 30.8, 37.6, 47.49˚These considerations are further supported through comparison between XRD pattern of pure CeO 2 as shown in Figure2and that of cerium borate glass containing 50 mol% CeO 2 .Both spectra offer sharp diffraction lines ranged between 25˚ and 35˚.On other hand, the intensities of diffraction patterns of cerium borate glass are appeared to be lower than that of pure CeO 2 .In such a case, the distribution of the accumulated CeO 4 units within the amorphous borate structure units may play the role of lowering the crystallinty.Figure 2. XRD patterns of pure CeO 2 .New Journal of Glass and Ceramics glasses.In glasses modified by CeO 2 , the BO 4 and BO 3 peaks are being broader and totally overlapped, as presented in Figure 3.This was especially true for the glass containing 40 mol% CeO 2 which displayed asymmetrically broadened peak.This can be discussed on the bases of presence a BO 4 site coordinated with more CeO 4 units in next nearest neighbor environment.In this regard, BO 4 site with three or four CeO 4 sites may efficiently be formed.Enhancement of CeO 4 species around central B atom will result in increasing the symmetry elements in the whole glass network, since the majority of bonds are of Ce 4 -O-Ce 4 type and limited bonds are formed with borate units.The symmetric of bonds around boron units will consequently change the value of chemical shift to become nearer to BO 4 surrounding with 4BO 3 , since the observed chemical shift is appeared around 0 ppm.In such a case, we can suggest that, chemical shielding of BO 4 coordinated with 4BO 3 is similar to BO 4 coordinated with 4CeO 4 , since both possess the same value of chemical shift.As shown in Figure 3, the lower value of CeO 2 (30 mol%) will affect the spectral feature, since two distinguished peaks are resolved.One characterizing BO 3 units and the other can be argued to be due to BO 4 site with 3BO 3 and one CeO 4 sites.Presence of CeO 4 with lower site around BO 4 results in changing the Figure 3. 11 B NMR spectra of binary cerium borate glasses as function of CeO 2 concentration.New Journal of Glass and Ceramics position of spectral peak to more positive value since shielding of BO 4 atoms from BO 3 sites differs from that with CeO 4 .Thus, chemical shifts of the studied system have obviously been varied from 12 ppm for glass containing 30 mol% CeO 2 to 0 ppm in glass enriched with CeO 2 .
[10]re 1 presents XRD diffraction patterns for xCeO 2 (100 − x)B 2 O 3 with x = 30, 40, 50 mol% CeO 2 .The compositions of 30 and 40 mol% CeO 2 were mainly amorphous, whereas composition containing 50 mol% CeO 2 was partially crystallized.As it can be a notable, the amorphous structure of the glass network Figure 1.XRD patterns of binary cerium borate a function of CeO 2 concentration.New Journal of Glass and Ceramics containing 30 3.2.11R Spectroscopy 11 B NMR spectra of alkali modified borate glasses were generally possessed two well separated peaks [10] [26] [27] [28] [29].One is corresponding to BO 3 and the other is related to BO 4 sites.The broad 11 B NMR resonance with peak centered at about ~12 ppm is assigned to different trigonal boron species distributed as boroxol and non-ring BO 3 units.While the more intensive and sharper peak located around 0 ppm is due to resonance of four-coordinate boron species.It worthy to note that features of11BNMR spectra of cerium borate glasses[10]are quite different from that of-alkali and alkaline earth binary borate Figure 6.B 4 fraction of binary cerium borate glasses as function of CeO 2 concentration.New Journal of Glass and Ceramics the studied glass samples.It can be observed from this figure, there is a linear dependence between B 4 and the change of CeO 2 composition.The role of CeO 2in increasing B 4 may be attributed to the considerable frequent increasing in tetrahedral units in glass network at expense of BO 3 units.This reveals that the former role of cerium may become more dominant in glass riches with cerium oxide (50 mol%).In such a case, the linkage between CeO 4 and both BO 3 and BO 4 groups is being the most abundant within glass network.Such argument becomes clearly visible from FTIR spectra, particularly in glass of 50 mol% CeO 2,