ABSTRACT

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 ¹¹B 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 ¹¹B 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.

Keywords:

Borate, Cerium, Former Units, NMR Analysis

1. 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 CeO₂ [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, 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 [20] [21] [22] [23] [24] 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 [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 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 (λ_CuKα = 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

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.

3.2. ¹¹B NMR Spectroscopy

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.

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 [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.

4. Conclusions

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.

Cite this paper

El-Damrawi, G., Gharghar, F. and Ramadan, R. (2018) More Insight on Structure of New Binary Cerium Borate Glasses. New Journal of Glass and Ceramics, 8, 12-21. https://doi.org/10.4236/njgc.2018.81002

References