Structural Role of Cerium Oxide in Lead Silicate Glasses and Glass Ceramics

Silicate glasses and glass ceramics in the system CeO2-PbO-SiO2 have been studied as a function of the structure factors R and K. The latter two factors are defined as: R = (CeO2 + PbO)/SiO2 and K = (SiO2/CeO2) molar ratios. In this glass, PbO is fixed at 50 mol% and CeO2 increases at the expense of SiO2. NMR investigations have revealed that increasing R which is accompanied with decreasing K leads to reasonable decrease in the shielding of silicon atoms. The chemical shift (δ) showed an increasing behavior due to increasing non-bridging oxygen atoms (NBO) in silicate network. It is evidenced that NBO in cerium free glass is much lower than that of glasses containing CeO2. Increasing R is clearly leading to higher chemical shift and higher NBO. This reflects that CeO2 has an effective structural role, since it would be consumed in all cases as an intermediate oxide. The main portions from CeO2 and PbO inter as glass modifiers which are consumed to form NBO atoms. A limited portion of CeO2 acts as glass former which consumed to form tetrahedral cerium containing NBO due to modification by PbO as a modifier oxide. Increasing R = [(CeO2 + PbO)/SiO2] from 1 to 2.34 leads to a frequent increase of NBO in the average glass network. FTIR spectroscopy of the glasses showed a clear shift of the main absorbance peak toward the low wavenumber with increasing R which confirms the increasing silicate units containing NBO atoms. XRD of the investigated materials revealed the presence of some nanostructures from cerium silicate crystalline phases. Formation of separated phases containing micro clusters is found to depend on NBO concentration, since NBO can facilitate process of phase separation. Majority of modifier are consumed to form NBO in the glass network and the rest are aggregated or separated to form silicate phase riches with cerium cations. In such case, some of silicon atoms are electrically compensated with both Pb and Ce cations.


Introduction
Glass ceramics containing PbO are useful to be studied because of their importance in several fields of applications [1] [2] [3] [4] [5].PbO can easily resist devitrification process and enhances durability of the glasses.Recently, the continuous progress of the materials containing cerium and lead oxide has been requiring immediate attention in viewpoint of both academic [6] [7] and bio-applications [8] [9] [10] [11].Structural role of PbO and CeO 2 as network intermediates is documented in several types of glasses [3] [4] [5].This dual role is depending on their content, glass composition and the type of glass.PbO can modify silicate network similar to alkali oxide particularly at low concentration [12] [13].In such situation, the modifiers are consumed to form silicate units containing NBO atoms [12] [13] [14].In glasses enriched with lead, PbO 4 species would be formed and directly affects the glass structure through forming Pb-O-Si bonds [5] [13].
Structure of cerium borate and borosilicate glasses has been recently investigated via FTIR and NMR spectroscopy [6] [7] [15] [16] [17].The obtained results have proved that cerium ions have a role of a network intermediate.Although several investigations have been done to characterize micro-structure of lead silicate glasses, the role of PbO and CeO 2 in studied silicate glass ceramics has not been fully explored.Our aim, in this study is to offer more information on the short range structure of cerium lead silicate glass ceramics in a wide range of composition.In particular, we focus on the short range ordered structure around the Si atoms via 29 Si NMR techniques.In addition, the role of cerium ions as an agent for phase separation and crystallization has also to be studied.
In this regard, glasses are investigated with SEM, FTIR, XRD spectroscopies and the obtained data is compared with that of base glass, free from CeO 2 .

Experimental
Glass samples containing different concentrations from CeO 2 , PbO and SiO 2 have been prepared by mixing the desired components in silica crucibles.The crucible and its content was transferred into an electric furnace and the temperature is raised gradually to reach the limit suitable for melting.The melting temperature is ranged between 1350˚C -1450˚C, depending on the material composition.The melt was swirled severally to ensure homogeneity and to get bubble free matrix.Finally, the melt was poured between two stainless steel plates.The obtained samples are transferred to another furnace and annealed at 350˚C to reduce internal stress.The samples are obtained in disc like shape of 2 mm thickness and 5 mm radius.

29 Si NMR Measurements
Fine powdered samples of different compositions have been investigated by using JEOL GSX-500 high-resolution solid state MAS NMR spectrometer of magnetic field of 11.74 T (Mansoura University-EGYPT).Spectra of silicon nuclei were recorded at a frequency of 99.3 MHz.A spinning rate of 7 kHz has been applied by using zirconia sample holder.An electric Pulse of 2.62 μs length and of 30 s recycle delay are used.Several scans (10,000 -12,000) were acquired to get high resolution NMR spectra.

Infrared Spectra (IR)
The FTIR absorption spectra were obtained, by KBr pellets technique, at room temperature in the range 400 -4000 cm −1 using Mattson 5000 FTIR spectrometer with a spectral resolution of 2 cm −1 .The glass powder of 0.02 g was mixed with a 0.2 g of KBr and pressed to form a thin disc.At least three samples of each glass were analyzed.The spectrum of each sample is obtained due to collected 20 scans.The obtained spectrum was normalized to the spectrum of blank KBr pellet; i.e. a pure KBr spectrum was subtracted from each glass spectrum.In addition, the spectra were corrected to the background and dark currents using two-point baseline correction.Then the spectra were normalized by making the absorption of each spectrum varies between 0 and 1 arbitrary unit.In addition such normalization is necessary to eliminate the concentration effect of the powder sample in the KBr disc.

Results and Discussion
NMR spectra of glasses having different R values (1, 1.2.1.86 and 2.34) are shown in Figures 1(a)-(d).As shown from this figure, there is a remarkable shift in center of the main peak position of 29 Si NMR spectra with increasing R, i.e. increasing CeO 2 concentrations.The NMR resonance peak centered at −87.3 ppm in cerium free glass (spectrum a) is shifted clearly toward much higher value (−75.4 ppm) in glass of 20 mol% CeO 2 (R = 2.34).Increasing chemical shift of the silicate nuclei with increasing (PbO + CeO 2 ) concentration is attributed to the modification role of both cerium and lead oxides.In addition, decreasing of SiO 2 concentration as a result of increasing CeO 2 and decreasing K will result in increasing NBO per SiO 2 groups.
In terms of Q n notation, (Q is silicon atom and n is the number of bridging bonds between Si and oxygen atoms), chemical shift value of base glass (−87.3 ppm) is attributed to mixture of Q 3 and Q 2 (silicate unit containing three and two bridging oxygen atoms as a major portion).In addition, little of Q 2 [OPb] configurations may also be present [18]   species respectively.In addition few silicate units containing cerium oxide in the second coordination sphere are suggested to be present.
The concentration of different silicate units (Q n , n = 0 -4) could be quantitavely obtained by an integration process which is applied to all NMR spectra of silicon nuclei.Figure 2 is presented as an example for the deconvolution process.From spectral analysis, the corresponding percentage of Q 1 = 17%, Q 2 = 55%, Q 3 = 20%, and Q 4 = 8% characterize binary lead silicate glass are simply evidenced.On the other hand, the values of Q 0 = 29%, Q 1 = 44%, Q 2 = 23%, Q 3 = 4% and Q 4 = 0% are obtained values characterize ternary cerium lead silicate glass of R = 1.86.This leads one to confirm that the major portion from CeO 2 and PbO can be introduced as glass modifier, since silicate units contain a mixture from Q 1 and Q 0 types are formed with higher concentration than that of Q 2 and Q 3 in glass free from CeO 2 .In such glass, some of Q n would contain Ce or    To determine the structural role of CeO 2 as an effective modifier, it is useful to compare FTIR spectra of cerium containing glasses with that of free CeO 2 .As an example, Figure 4 shows FTIR spectra of cerium free glass and for glass samples in glass free from cerium oxide is shifted progressively towards 860 cm −1 , since the activity of Q 2 and Q 1 species containing Ce linkages are present in glass containing 5 and 15 mol%.In addition, week envelope represents Q 0 is appeared at about 770 cm −1 in the spectra of glass containing 20 mol%CeO 2 .These changes are clearly evidenced in glass of higher CeO 2 oxide (15 and 20 mol% CeO 2 ), see Figure 4.As is shown from this figure, band characterizes Q 0 (at 770 cm −1 ) [20] [21] [22] [23] is simply resolved which confirms that loos silicate units would be formed in high CeO 2 to SiO 2 molar ratio.
The nature of XRD pattern is known to depend upon the content of NBO in the main glass forming units such as [Q n SiO 4 ] and [Q n (PO) 3 ] species [7] [8] [9].
In this regard, the relative ratio of modifier to former is high enough to yield pyrosilicate (Q 1 , 3NBO) and orthosilicate (Q 0 , 4NBO) units which have been documented from NMR results.The nonbridging oxygen should be electrically balanced by positive ions (Ce 2+ ).Accumulation of Ce 2+ cations around NBO in silicate network will result in producing crystalline clusters of cerium orthosilicate type [7] [8] [9].These considerations are supported from comparison between XRD pattern of vitreous CeO 2 and of that of cerium silicate glass.Both spectra offer sharp diffraction lines at the same position.The intensity of the diffraction pattern of cerium silicate glass is appeared to be lower than that of the pure CeO 2 .This may because the clusters formed from Ce and O ions are distributed within amorphous silicate units which play the role of lowering the total crystallite species.This argument is further supported, since the sharp diffraction peak is superimposed on a broad hump (between 20˚ -40˚) which is a characteristic feature of the main amorphous glass network.This observation is simply strengthened from XRD spectra of cerium free glass, Figure 5, since amorphous distribution of glass constituents is the dominant.
Glass contains SiO 2 and PbO doesn't greatly affect the process of crystallization or clustring, but the main changes were found to depend on CeO 2 .Figure 6 showed that there is a great difference between morphology of glass containing

Conclusion
NMR investigation has revealed that increasing of CeO 2 at the expense of SiO 2 at a constant concentration of PbO increases chemical shift (δ) of 29 Si nuclei through increasing (NBO) in silicate network.NBO atoms in cerium free glass are much lower than those of glasses containing CeO 2 .Higher concentration of CeO 2 leads to higher chemical shift and higher NBO.The structural role of CeO 2 is definitely determined as a modifier oxide in the investigated lead silicate net- [19].The main NMR resonances centered at −83.3, −77.8, and −75.4 ppm in glasses of R = 1.2, 1.68 and 2.34 (CeO 2 = 5, 15 and 20 mol% CeO 2 ) show that silicate network is frequently deshielded by the effect of increasing the modifier oxide concentration.In glass of R = 1.2 and

Figure 1 .
Figure 1.NMR spectra of silicon nuclei of cerium free, R = 1 (a) and of glasses containing 5 and 15 and 20 mol% CeO 2 , R = 1.2, 1.86 and 2.34 ((b)-(d), respectively).containing 5 mol% CeO 2 , Q 2 species are the dominant.On the other hand, glasses containing 15 and 20 mol% CeO 2 (R = 1.68 and 2.34) contain Q 1 and Q 0 Pb or both in the second coordination sphere of silicon.As a result, Si-O-Ce and Ce-O-Pb configurations are formed.These linkages are known to deshield the silicon nuclei in comparison with Si-O-Si linkages.

Figure 3
Figure 3 presents the change of the average isotropic chemical shift (δ) of 29 Si nuclei upon increasing R. It can be shown from this figure that δ increases (from −87.3 to −75.4 ppm) with increasing R values i.e.CeO 2 contents.This change leads to increasing NBO.Beside, some Si-O-Ce or Si-O-Pb are formed at expense

Figure 2 .
Figure 2. Separation of NMR spectrum of 50PbO-50SiO 2 glass into different component bands.

Figure 5 .
Figure 5. XRD patterns of glass free from CeO 2 (below) and that of glass containing 20 mol% CeO 2 together with vitreous CeO 2 .

Figure 6 .
Figure 6.SEM of glass containing 20 mol% CeO 2 (a) and of glass free from CeO 2 (b).

20 mol% CeO 2 ,
Figure 6(a) and of cerium free glass, Figure 6(b), since crystalline clusters from cerium silicate phases are clearly resolved in the morphology of sample containing 20 mol% CeO 2 .