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.

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

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 [9] [10] [12] . 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.

¹¹B NMR spectra of alkali modified borate glasses were generally possessed two well separated peaks [10] [26] [27] [28] [29] . 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 [10] are quite different from that of―alkali and alkaline earth binary borate

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

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.

FT-IR spectra of samples with x = 30, 40, 50 mol% CeO₂ 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 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.

The total fraction of four-coordinated units, B₄, 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 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

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% CeO_2, where the shoulder at about 1600 cm⁻¹ is assigned to Ce-O vibration in phase rich with cerium borate mixed units.