The chemically pure SiO₂, H₃BO₃, Al₂O₃, CeO₂ and Na₂CO₃ were used to prepare all glass samples from the starting materials. Stoichiometric powders were carefully mixed and melted for 50 minutes in a platinum crucible at 1340˚C - 1450˚C. Subsequently, melts were quenched on a metal plate that was pre-heated to 350˚C to avoid cracking progress.

A Shimadzu X-ray type Dx-30 diffractometer is used for X-ray diffraction measurements. The values of the maximum peak and intensity are used to determine the material type that is compared to patterns in the joint committee for powder diffraction 108 standards’ international powder diffraction file (PDF) database (JCPDS). A transmission electron microscope (TEM) of type JEOL-JEM-1011 was used for the determination of the size and shape of the studied samples. Microstructural data was tested using the JSM-7500F field emission scanning electron microscope. The machine operated with an accelerated voltage of 25 kV. All samples were sputter-coated with a thin layer of gold (3 - 4 nm) to avoid sample charging and increase the signal-to-noise ratio.

Figure 1 shows XRD patterns of the glass samples containing different concentration from CeO₂. There is no evidence for crystal formation in the sample of CeO₂ > 1 mol%. At lower concentration, there are weak diffraction peaks which may be situated at 26˚, 32˚ and 34˚. This weak diffraction peaks were identified as the stable sodium silicate phase enriched with NBO atoms [JCPDS 40-0376] [10] [11]. After further increasing of CeO₂, the intensity of these peaks disappeared and started to emerge in the amorphous background in glasses of higher CeO₂ concentration. Based on the present results, the mechanism proposed is that both metasilicate and disilicate phases nucleate concurrently at the very early stages of nucleation [8]. As the content of cerium oxide increases, nucleation and growth of metasilcate becomes restricted. As a results, the relative number of metasilicate (Na₂SiO₃) crystals decreases, and they are no longer observed after addition of one mol% CeO₂, since significant amount of glassy phase is the dominant. We can thus safely state that equilibrium state has not been reached and that metasilicate is a metastable phase and its constituent may be changed with more cerium oxide addition. As a result, another structural species containing CeO₄ are evidenced to be formed. Therefore, disappearance of diffraction peaks in spectra of glasses may be considered as a result of decreasing

concentration of Q² (silicate units of 2NBO) species which perform metasilcate Na₂SiO₃ phases. As a result, phases containing less NBO atoms (disilcate) are the dominant upon CeO₂ addition. Decreasing Q² and increasing Q³ upon increasing CeO₂ concentration might be considered the main reasons for increasing the amorphous nature of the glasses.

It has been found that more addition of CeO₂ to the glass network resulted in spreading of CeO₂ over silicate structural groups to form two-dimensional amorphous phase (silicate precursor) containing nanocrystalline cerium silicate of undefined structure [8] [9] [10].

At extremely high cerium oxide concentration, the structural units of the tetragonal Ce₂Si₂O₇ disilicate began to form at expense of Na₂SiO₃ that resulted in increase of disorder in a glassy phase [12].

Evidences based on XRD data are in a good agreement with that obtained from TEM and EDP, Figure 2 and Figure 3. Both confirmed that well-formed structural species are constructed in its ordered state in glasses of low CeO₂ contents.

Samples of 1 & 20 mol% CeO₂ are presented as examples in Figure 2 and Figure 3. It can be observed from these figures that at lower CeO₂ contents (1 mol%, for example), sub crystallized species are still formed within the glassy state. The crystal size and morphology of the crystalline phases is shown to depend on the glass composition [8] [9], particularly on CeO₂ concentration. The size of the well-formed ordered species in glass of 1 mol% CeO₂ is higher than that of glass containing higher CeO₂ concentration, since it lies in the region of 70 - 110 nm in low ceria glass, since it lies in the region of 70 - 110 nm in low ceria glass.

Cerium has a great capacity to remove NBO from the silicate and serves the role of a glass former, as seen by the reduction in cluster size as CeO₂ level increases. This would be accompanied by sodium silicate clustered phases decreasing in size and content. Furthermore, some of the modified cerium species are detached from the original glass phases to produce smaller species of sphere-like structures that characterize the CeO₂ phases [6] [12], as seen in Figure 3.

Since cerium and sodium oxide can act as modifiers, the micro-crystallized clustered species are clearly visible in glasses containing 1 mol % CeO₂. Ordered species are always represented by such diffractions [6] [12]. The size of the aggregation clustered species, on the other hand, appears to diminish with increasing CeO₂ level (see Figure 3). CeO₂ has the potential to remove more and more NBO and Na ions from the silicate and borate networks in this circumstance, reducing the content and size of the well-formed clusters.

The linkage between distinct species weakens and almost completely separates when CeO₂ concentration increases, as seen by TEM and EDP, forming a sphere-like structure representing changed cerium units. These species, which contain around 20 mol% cerium oxide, may be easily detected with a size less than that of glasses containing 1 mol% cerium oxide (see Figure 3).

Features observed by Means TEM (Figure 3) is supported to a great extent with that detected by atomic force microscope Figure 4 which clarified that irregular amplitude of images of cerium occurs in microparticles size (MPs; a, b) and spherical like shape of cerium oxide is found in nanoparticles (NPs; c, d).

Generally increasing CeO₂ content at the expense of B₂O₃ in borosilicate glasses will lower the size of Na₂SiO₃ units through withdrawing of Na₂O which is taken to form Na₂CeO₄ units of lower size. Formation of the latter affects the nature of glass matrix, since less ordered network is the main product. Macro size species are found in nonspecific shape as presented from TEM micrograph and AFM (Figure 2 and Figure 4(a), Figure 4(b)) [12]. On the other hand, nonosized species are found in spherical like shapes (Figure 3 and Figure 4(c), Figure 4(d)).

In order to extract the structural role of CeO₂, it is important in this study to compare EDS spectra of pure CeO₂ with that of cerium containing glass. Figure 5 represents TEM and EDS spectra of pure cerium oxide. It is shown that most

of line spectra are concentrated around 4 - 6 KeV (Ce bonded to oxygen with covalent bond). Little of Ce are bonded with ionic bond (spectra at about 1 KeV). Coordination of cerium with oxygen atoms gives uniform units of spherical morphology [5]. As seen from TEM micrograph representing pure CeO₂, spherical shaped units are seen to be greatly distributed in a layers manner, Figure 5.

Different observation is observed from EDS spectra of cerium containing glasses, since intensity of Ce line spectra at about 1 KeV is more higher in sample containing 1 mol% CeO₂ (Figure 6) when it compared with sample of both pure CeO₂ (Figure 5) and one containing 20 mol%, Figure 7. This observation supports that both concentration of NBO and the modifier in the silicate network are relatively high. Formation of cerium as former is not evidenced, since there is no resolved Ce peak at higher energy (4 - 6 KeV). Reverse behavior is observed in cerium rich glasses, since the reduction in peak intensity at 1 KeV is shown (for both pure and high CeO₂ glass) and new different cerium spectral peaks (3 lines) are clearly formed in the region of (4 - 6 KeV). This leads that CeO₂ changes its role from modifier to former to form CeO₄ groups. As a result the size and concentration of polycrystalline species (Na₂SiO₃) [12] [13] should be changed.

It is shown from this figure that most of cerium is a modifier. Since intense peak at about 1 KeV is only resolved. There are no peaks that are evidenced between 4 - 6 KeV. In addition, it found that the concentration of Na atoms (18%) is higher

than that of glasses contains higher cerium oxide. The concentration of Na decreases reaches 8% and Ce increase (from 0.48% to 5%) with increasing CeO₂ contents.

In such a case CeO₂ plays the role of glass former, since three peaks are evidenced between 4 - 6 KeV. It can be noted from EDS spectra of the investigated glasses that, the total Na₂O concentration in the glassy phase is highly reduced upon CeO₂ addition. The corresponding Na concentration (16 by atom) of glass of 1 mol% CeO₂ is lowered to 8 for glass of 20 mol% CeO₂. Reverse behaviour is noticed for silicon, its atomic content in low CeO₂ concentration was 9.8 and it increased to reach 16 upon more addition of CeO₂. Increasing Si and decreasing Na concentration is interpreted on basses of formation of more shielded silicate units upon increasing CeO₂ content. In addition some of Na₂O should be with drowning from the original phase and consumed to build CeO₄ groups. This in turn results in reduction of its (Na₂O) content in the original glass phase. These considerations are found to correlate with that obtained by NMR spectroscopy Figure 8. The decreasing trend of the fraction of tetrahedral boron (N₄) detected by ¹¹B NMR spectroscopy (Figure 8) may be referred to most of Na₂O as a modifier is with drowning by CeO₂ to form tetrahedral cerium species. Presence of the latter was highly evidenced from EDS spectroscopy, since new spectral lines at about 5 Kev are grown at expense of EDS lines at 1 KeV.