The structure of glasses in the system of xCeO 2 (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 analy s es. 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 neighbor s 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 refer s to CeO4, CeBO3and Ce(BO2)3 species.
Borate glasses have attracted a great interest [
Addition of a network modifier to B2O3 is reported [
In this regard, few structural studies on binary cerium borate glasses have been done [
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.
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.
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.
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.
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.
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 [
These considerations are further supported through comparison between XRD pattern of pure CeO2 as shown in
11B NMR spectra of alkali modified borate glasses were generally possessed two well separated peaks [
It worthy to note that features of 11BNMR spectra of cerium borate glasses [
glasses. In glasses modified by CeO2, the BO4 and BO3 peaks are being broader and totally overlapped, as presented in
As shown in
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
As noted from
The total fraction of four-coordinated units, B4, could be calculated for the three samples using a deconvolution procedure [
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).
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.
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