Effect of Niobium Oxide Incorporation on Optical Properties of Sodium Bismuth Borate Glasses

: Glasses with the chemical compositions 60% B 2 O 3 –20% Na O–(20 % Bi O 3 x % Nb 2 O 5 (where, x = 0, 5, 10 and 15 mol. %), were prepared by fast quenching of their melts. Characteristics of the obtained solids were checked by XRD, FTIR and optical absorption spectroscopy. XRD data confirmed the amorphous nature of the glass samples. FTIR results revealed no changes in the ratio of the structural units BO 3 :BO 4 when replacing Bi 2 O 3 by Nb 2 O 5 . Moreover, both Bi 3+ and Nb 5+ cations occupied octahedral coordination states and both acted as glass network modifiers. Optical studies affirmed that the lowest value of both direct and indirect optical band gaps was obtained in samples with equal concentration of Nb 2 O 5 and Bi 2 O 3 . All optical absorption bands were elucidated in terms of the terminal oxygen atoms around Nb 5+ and/or Bi 3+ cations. The optical nonlinearity of the glass samples was assessed in terms of absorption coefficient and the value of the optical band gap.


Introduction
Nonlinear optical behavior of amorphous materials has been extensively studied by several researchers because of its promising usage in photonic devices applications (Boyd, 2003;Sutherland, 2003). In this regard, bismuth borate glasses have caught great attention due to their facile and low cost synthesis, enhanced mechanical and chemical stability, in addition to the large range of nonlinear refractive index and nonlinear absorption coefficient (Farouk et al., 2013;Shanmugavelu et al., 2013). Furthermore, borate glasses doped with heavy metal oxides show interesting nonlinear optical properties and have different application in microscopic lens designing (Ehrt, 2000). Bismuth oxide doesn't play the role of a glass former; it is rather a good modifier that enhances optical, thermal and structural characteristics of the glass (Oo et al., 2012). It also increases the ratio of the non-bridging oxygen (NBO) and therefore enhances the glass absorbability (Chen et al., 2008;Pal Singh et al., 2012). The decrease of the number of NBO is associated to an increase in the optical band gap (Novatski et al., 2008). Bismuth is likely occupying octahedral coordination in the Bi 2 O 3 oxide inside the glass network, which contributes in the existence of NBO (Rajendran et al., 2003). The presence of transition metal niobium oxide Nb 2 O 5 in glasses elevates chemical stability and improves the mechanical properties . It was reported that introducing niobium oxide into the glass matrix increases the activation energy and the glass transition temperature T g . Optical properties such as optical band gap and optical basicity were strongly improved when Nb 2 O 5 was introduced into the glass system containing Bi 2 O 3 (Sanghi et al., 2010;Swapna et al., 2017). The largest value of the non-linear refractive index was reported to glasses containing empty d shell transition metal ions such as Nb 5+ (Cardinal et al., 1997). Furthermore, by incorporating Nb 2 O 5 in the glass network, the Nb 5+ cation acts as a network former and improves the third order nonlinear optical susceptibility (Suhara et al., 2012). The usual coordination of Nb 5+ cation in oxide glasses is octahedral. Therefore, niobium atoms are located in octahedral sites. The non-linear refractive index was reported to increase proportionally to the concentration of Nb 5+ cation in the glass system (Couzi et al., 1996). It was assumed that the polarization of the niobium oxygen bond is the main contribution to the optical non-linearity (Cardinal et al., 1997). Recalling that nonlinear optical behavior of glasses is improved by adding lone-pair holder such as Bi 3+ or cation of empty d shell such as Nb 5+ (Chen et al., 2008). The aim of the present work is to study the optical characteristics of sodium bismuth niobium borate glasses in relation to the replacement of bismuth oxide by niobium oxide.

Experimental Work
High purity starting materials, not less than 99%, were used to prepare the suggested compositions, as indicated in Table (1). To find out the role of each oxide in the mixture, the field strength was calculated for each oxide using relation (1): where, F is the field strength of oxide C a O b , c Z and c r are the oxidation state and covalent radius of the cation C and o r is the covalent radius of oxygen ion (Pye, 2005;Rao, 2002;Shelby and Lopes, 2007).
Oxide precursors were weighted by 4-digits balance, mixed well together in porcelain crucibles and then melted for 2 h at 900±50°C, in an electric furnace. Melts were then solidified by rapid quenching in air onto pre-cooled copper plates. X-ray powder diffractometer (Rigaku Miniflex, Japan), with Cu Kα radiation (1.54 Å), was used for XRD analysis in order to check the formation of glasses. In order to obtain information about structural units for the studied samples, infrared spectra were recorded using Fourier Transform Infrared (FTIR) Jasco 5300 spectrometer. Parts of the solidified glass samples were powdered and diluted in dry KBr, pressed in pellets form and measured at room temperature over the range from 4000 to 400 cm −1 . Optical absorption spectra were recorded in the range from 200 to 2500 nm using Genway 6405 UV-VIS spectrophotometer. Figure 1 shows the x-ray diffractograms of all samples, with no observed sharp peaks. Only one broad hump was observed around 2θ = 47°. Such observation refers to the amorphous nature of the prepared solids, with short ordered structure. Each sample, in the studied glasses, contains 60 mol. % of B 2 O 3 which is considered as the glass former oxide with two remarkable structural units BO 3 and BO 4 . In addition to 20 mol. % of both (Bi 2 O 3 + Nb 2 O 5 ), that are considered as intermediate oxides with the ability to act as glass network formers and/or glass network modifiers. From x-ray diffraction tables, the hump observed at 2θ = 47° is a characteristic of the noncrystalline Bi 2 O 5 (Junjiang et al., 2002). This result affirms that some/all Bi 2 O 3 did not participate in the glass matrix as a glass former, but rather as a network modifier.  (Balachander et al., 2013;Fragoso et al., 2005). Such band may indicate that Nb 5+ have octahedral coordination numbers, which indicates that these cations did not share in the glass network but occupy the interstitial vacancies. While 456 cm −1 band is due to the Bi-O valence band vibrations of BiO 6 structural units and/or the angle modification of the boron oxygen bond linkage (El-Maaref et al., 2017;Sutherland, 2003).

FTIR Spectra
Moreover, it is clear that bands of BO 4 became higher and broader than those of BO 3 when B 2 O 3 is replaced by Nb 2 O 5 . In other words, the relative number of compositional group BO 3 decreased while that of BO 4 increased when B 2 O 3 replaced by Nb 2 O 5 , as seen in Fig. 3.   Optical Absorption Spectra Figure 4 shows the relative optical absorption spectra of the studied glasses. All samples have the same cutoff wavelength (λ cutoff = 443 nm), with no observed absorption peaks in UV region (200-400 nm). By careful inspection of Fig. 4, two well defined absorption bands can be observed, the first one is a broad band extended from 700 nm to 1000 nm, its center shifted from 810 to 825 nm (red shift) when Bi 2 O 3 was exchanged by Nb 2 O 5 . The second band appeared in region from 1000 to 2500 nm and is peaked at 1745 nm. The red shift noticed from 810 nm to 825 nm may be attributed to the change in the glass optical basicity because of the variations of OHgroups, in addition to presence of niobium oxide and the consequence change in the energy levels (Duffy and Ingram, 1971;Stehle et al., 1998). Also, it is clear that the increase of Nb 2 O 5 content increased the intensity and broadness of the absorption peaks. Such behavior may indicate that, the content of OH-in the glass network increased by increasing Nb 2 O 5 , the thing which mean low density, high molar volume and good amorphous nature for the studied glass system. By surveying previous related publications, it can be stated that the observed optical absorption may be attributed to deformation in the glass networks due to the interstitial impurities in addition to the octahedral forms of both Bi 3+ and Nb 5+ cations (Stehle et al., 1998). But, according to ligand field theory the positive transition metal ion is always attached to negatively charged ligands (Figgis, 1987), so the observed absorption may be attributed to the presence of the non-bridging oxygen (NBO) ions that surround each Nb 5+ cation. It should take into consideration that the increase in NBO may cause the third-order susceptibility to arise and hence improves the nonlinear behavior of the glass (Almeida et al., 2011). Accordingly the broadness of the absorption band may be explained as consequence of the relative increase in the number of NBO in the ligand, by replacement the poor-inoxygen Bi 2 O 3 by the rich-in-oxygen Nb 2 O 5 . The effect of niobium oxide arises from electronic d-d transitions and the coupling between the d-electrons of Nb 5+ and the vibrations of the negatively charged O 2in the ligands.
On the other hand, it is well known that the analysis of absorption edges gives precise information about the value of optical band gap (Hodgson, 2012;Meyer et al., 1998). It is also known that the value of absorption coefficient α increase as a consequence of light absorption. This change in α is known as the fundamental absorption edge and is related to the absorbed energy by the relations given (Davis and Mott, 1970;Tauc and Menth, 1972) Table (2) for both direct allowed (E opt ) d and indirect (E opt ) i allowed transitions. By inspecting Table (2), it can be concluded that both band gaps showed slight decrease as Bi 2 O 3 replaced by Nb 2 O 5 .
Table (2), shows the variation of both direct and indirect optical absorption energy with increasing the niobium contents in glasses. It is clear that, values of both Sand (E opt ) i are decreased until reach their lowest values at concentration x=10 mol. % and then increased slightly again until reaching constant values. This result agrees with the literature (Sanghi et al., 2010). This behavior may be attributed to the contributions of two factors, the first one is the increasing of the terminal oxygen atoms number which increased by increasing the concentration of Nb 2 O 5 in glass samples. This factor causes the values of both direct and indirect optical absorption energy increased by increasing Nb 2 O 5 in glass samples. The second one is the decreasing fraction of Bi 2 O 3 which in turn decreases the polarizability and decreases refractive index and finally decreases the values of both direct and indirect optical absorption energy gaps. It is obviously, clear that the second factor is more predominant one in these samples. Lowest values of both (E opt ) d and (E opt ) i are obtained when x=10 mol. %. This is the case when the concentrations of both bismuth oxide and niobium oxide are the same, i.e. it is the case, of the highest value of the number of nonbridging oxygen. This result indicates that, the sample with the best nonlinear properties is the one that has concentration x = 10 mol. %, in which the optical band gap has its lowest value. This is because the nonlinearity of the glass system is enhanced by lowering the value of the optical band gap (Meyer et al., 1998).

Conclusion
The present work aimed at studying the effect of replacing bismuth oxide Bi 2 O 3 by transition metal oxide Nb 2 O 5 in the sodium bismuth borate glass, until the total replacement of Bi 2 O 3 by Nb 2 O 5 . Structural analysis results manifested the amorphous nature of the materials and the correlation between the non-bridging oxygen and the optical absorbance of the glass system. Optical spectroscopic studies showed that the presence of niobium oxides increases the absorption coefficients of the material due to the increased amount of non-bridging oxygen in the negative ligand around the positively charged transition metal cation. It also showed that one of absorption bands is completely affected by the existence of niobium oxide due to the electronic d-d transitions and the coupling between the d-electrons of Nb 5+ and the vibrations of the negatively charged O 2in the ligands. Results of optical band gap estimation manifested that the sample with lowest value of both optical band gap and subsequently the value of refractive index is the one in which x=10 mol. %. Which is the one in which the concentrations of both bismuth oxide and niobium oxide are the same, which has the highest number of non-bridging oxygen. This indicates that this sample is expected to have best nonlinear behavior, due to the aforementioned results. (αhv) 1/2 (αhv) 1/2 0% Nb 2 O 5 15% Nb 2 O 5