Borate glasses have attracted a great interest     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 CeO2  -  . The great importance of cerium ions is known  -  to come from their interesting characteristics such as physical, optical, catalytic and as well as magnetic properties. Particular technological applications including gamma ray shielding, luminescent, scintillators and dielectric, optical and electronic device are highly related to structural role of CeO2 in glasses.
Addition of a network modifier to B2O3 is reported      to break the B-O bonds and induce the transformation of BO3 triangles to BO4 tetrahedral units. On the other hand, CeO2 plays different role when it is added to B2O3 since CeO2 enters to the glass network as an intermediate oxide. The modification part of CeO2 produces the conversion from BO3 to BO4 groups while the rest of CeO2 can participate in the glass network to form CeO4 tetrahedral groups.
In this regard, few structural studies on binary cerium borate glasses have been done     . 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 CeO2 concentration.
It is aimed in the present study to determine the structural role of CeO2 in cerium rich glasses by using the advantage of NMR spectroscopy, since to our knowledge, no studies in this regard have been carried out.
2. Experimental Details
2.1. Sample Preparation
The glass samples have been prepared by the normal melting method using cerium oxide (CeO2) and boric oxide (H3BO3) 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.
2.2. Measurement Tools
2.2.1. X-Ray Diffraction (XRD)
XRD measurements were carried out on powdered samples at (Metallurgical Institute, El-Tebbeen-Helwan) using a BrukerAxs-D8 Advance powder XRD system with a Cu Kα radiation (λCuKα = 0.1540600 nm).
The range of the diffraction angle (2θ) is changed from 4˚ to 70˚ using a dwell time of 0.4 seconds.
2.2.2. 11B NMR Measurements
Solid-state 11B NMR spectra were performed at Magnetic Resonance unit, Mansura University. The samples were measured with JEOL GSX-500 high-resolution solid-state MAS NMR spectrometer in a magnetic field of 11.74 T at 11B Larmor frequency of 160.4 MHz and spinning rate of 15 KHz. A single pulse length was used of 0.5 - 1.0 ms with a pulse delay of 2.5 s, and an accumulation of 200 - −300 scans. All Samples were grinded to fine powders then filled into standard 4 mm NMR sample tubes.
2.2.3. 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/cm2 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.
3. Results and Discussion
3.1. X-Ray Analysis
Figure 1 presents XRD diffraction patterns for xCeO2(100 − x)B2O3 with x = 30, 40, 50 mol% CeO2. The compositions of 30 and 40 mol% CeO2 were mainly amorphous, whereas composition containing 50 mol% CeO2 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 CeO2 concentration.
containing 30 and 40 mol% CeO2 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% CeO2. The discrete sharp lines observed at ~25.7, 28.6, 30.8, 37.6, 47.49˚ are mainly assigned to crystalline CeO4[PDF nr.810792], CeBO3[PDF nr.210177] and Ce(BO2)3[PDFnr.230877] species. The strong tendency to crystallization may attributed to increasing in network connectivity as result of further concentration of tetrahedral CeO4 as former species    . The tetrahedral CeO4 groups have the priority to combine together with B atoms and form an ordered structural chain in Ce-O-B linkages causing an increment in crystallization.
These considerations are further supported through comparison between XRD pattern of pure CeO2 as shown in Figure 2 and that of cerium borate glass containing 50 mol% CeO2. 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 CeO2. In such a case, the distribution of the accumulated CeO4 units within the amorphous borate structure units may play the role of lowering the crystallinty.
3.2. 11B NMR Spectroscopy
11B NMR spectra of alkali modified borate glasses were generally possessed two well separated peaks      . One is corresponding to BO3 and the other is related to BO4 sites. The broad 11B NMR resonance with peak centered at about ~12 ppm is assigned to different trigonal boron species distributed as boroxol and non-ring BO3 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 of 11BNMR spectra of cerium borate glasses  are quite different from that of―alkali and alkaline earth binary borate
Figure 2. XRD patterns of pure CeO2.
glasses. In glasses modified by CeO2, the BO4 and BO3 peaks are being broader and totally overlapped, as presented in Figure 3. This was especially true for the glass containing 40 mol% CeO2 which displayed asymmetrically broadened peak. The more pronounce broadening leads one to strongly suggest that presence of a BO4 site with 2BO3 and 2CeO4 in next nearest neighbor environments. The difference in bond length and bond angle between B3-O-Ce4 and Ce4-O-B4 linkages is considered as the main reason for spectral broadening in glass containing 40 mol% CeO2. On the other hand, glass containing 50 mol% CeO2 showed more symmetric and less broadening behavior than that of glass containing 40 mol% CeO2. This can be discussed on the bases of presence a BO4 site coordinated with more CeO4 units in next nearest neighbor environment. In this regard, BO4 site with three or four CeO4 sites may efficiently be formed. Enhancement of CeO4 species around central B atom will result in increasing the symmetry elements in the whole glass network, since the majority of bonds are of Ce4-O-Ce4 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 BO4 surrounding with 4BO3, since the observed chemical shift is appeared around 0 ppm. In such a case, we can suggest that, chemical shielding of BO4 coordinated with 4BO3 is similar to BO4 coordinated with 4CeO4, since both possess the same value of chemical shift.
As shown in Figure 3, the lower value of CeO2 (30 mol%) will affect the spectral feature, since two distinguished peaks are resolved. One characterizing BO3 units and the other can be argued to be due to BO4 site with 3BO3 and one CeO4 sites. Presence of CeO4 with lower site around BO4 results in changing the
Figure 3. 11B NMR spectra of binary cerium borate glasses as function of CeO2 concentration.
position of spectral peak to more positive value since shielding of BO4 atoms from BO3 sites differs from that with CeO4. Thus, chemical shifts of the studied system have obviously been varied from 12 ppm for glass containing 30 mol% CeO2 to 0 ppm in glass enriched with CeO2.
3.3. FTIR Analysis
FT-IR spectra of samples with x = 30, 40, 50 mol% CeO2 are displayed in Figure 4. Three fundamental absorption bands     have been observed in the borate glass system. The first band located between 600 - 800 cm−1 is assigned to symmetric bending vibrations of BO3 bonds. The second band from 800 - 1200 cm−1 can be attributed to BO4 stretching vibrations, while the third band from 1200 - 1600 cm−1 is assigned to B-O stretching vibrations of triangular BO3 units.
As noted from Figure 4, structural changes appeared in a wide and asymmetric broadening, with increasing CeO2 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 CeO2 through presenting more CeO4 units in the glass structure and their abilities to shield and coordinate with BO4 units. As a consequence, increasing of CeO2 at the expense of B2O3 results in increasing of Ce4-O-Ce4 bonds at expense of B4-O-B3. In such a case, the glass structure is mainly consisting of Ce4-O bond in CeO4 groups. Moreover, formation of some mixed vibrations from Ce4-O with B4-O and B3-O may also be suggested. The results of these measurements are quite in good agreement with those obtained from NMR of the same glasses.
Figure 4. FTIR absorbance spectra of binary cerium borate glasses a function of CeO2 concentration.
The total fraction of four-coordinated units, B4, could be calculated for the three samples using a deconvolution procedure     . This can be done by obtaining the relative area of each band corresponding to the structural units of both triangular BO3 and tetrahedral BO4 & CeO4 units.
As a result, the value of B4 is then defined as the ratio of the area related to the sum of structural groups containing BO4 and CeO4 four coordinated units to the area related to total units (BO3 + BO4 + CeO4). Figure 5 represents an example for the deconvolution in Gaussian band of 50CeO2・50B2O3 glass sample.
Figure 6 illustrates the change in the total fraction B4 versus CeO2 content of
Figure 5. Deconvolution of infrared spectrum of the composition 50CeO2・50B2O3 as an example for the analysis.
Figure 6. B4 fraction of binary cerium borate glasses as function of CeO2 concentration.
the studied glass samples. It can be observed from this figure, there is a linear dependence between B4 and the change of CeO2 composition. The role of CeO2 in increasing B4 may be attributed to the considerable frequent increasing in tetrahedral units in glass network at expense of BO3 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 CeO4 and both BO3 and BO4 groups is being the most abundant within glass network. Such argument becomes clearly visible from FTIR spectra, particularly in glass of 50 mol% CeO2, where the shoulder at about 1600 cm−1 is assigned to Ce-O vibration in phase rich with cerium borate mixed units.
The structural features of cerium borate glasses correlated with CeO2 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% CeO2. In such a glass, the principal crystalline phase is assigned to crystalline CeO4, CeBO3 and Ce(BO2)3 species which is mainly referred to both CeO4 and BO4 as dominated units.
・ FTIR spectroscopy and NMR spectroscopy have confirmed that CeO2 in binary borate glasses plays mainly the role of glass former in the form of CeO4 units. The formation of expected ordered Ce4-B-Ce4 linkage impairs the conversion of triangular BO3 units into BO4 tetrahedra and causes a wide broadening in the spectrum.
・ Increasing of the total fraction of all four coordinated units (B4) is highly associated with increasing in CeO4 concentration and this would consequently result in formation of more ordered structures.
・ The change in chemical shifts of 11B nuclei from 12 ppm to 0 ppm with increasing CeO2 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.
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