Effect of Sn on Physical and Optical Properties of Lithium Zinc Borate Glasses

22.5 Li2O3-(20-x) ZnO-57.5B2O3 – xSnCl2 (x = 0, 0.5. 1) glasses were synthesized using the conventional melt quench technique. Glass composition with larger concentration of lithium oxide exhibits excellent transport properties. Introducing intermediate element like zinc into the matrix will lead towards slight modification in structure by not varying much in its intrinsic properties. Thus, incorporation of Sn as dopant into the matrix has tailored the physical and optical properties of glasses. In this study, it is interesting to note that, the density of the glass has reduced for 0.5 mol % SnCl2 in the matrix and on the contrary, the density of glass was found to be increased for the SnCl2 with 1 mol % concentration. It indicates that, there could be a possibility of slight modification in the structure of glasses which eventually leads towards tuning of the optical bandgap due to existence of Sn.


Introduction
Glass is an inorganic non-crystalline solid that is often transparent, hard, brittle, and chemically inert.Glass find its applications in variety of fields like optics, architecture, medical, house wares etc [1][2][3].Borate glass network consists of BO3 and BO4 structural units and the combination of these units gives rise to di, tri, tetra, and penta borate groups in glass network [4,5].Borate glasses have many advantages over silica glasses and sodalime glasses like lower melting and softening temperature [1,6], better thermal shock resistance, chemical durability and higher electric resistivity [1].Li2O in B2O3 network modify the host structure through the transformation of the structural units of the borate network from [BO4] to [BO3] with the creation of non bridging oxygen(NBO) which forms more ionic bonds which results in higher ionic conductivity and refractive index values [5,7] and also results in the decrease of optical energy band gap for both direct and indirect band gap [8].Addition of transition metals in borate glasses has shown great potential in technological applications like electrooptic, electronic, electrochemical devices and radiation dosimetry [9][10][11][12].Addition of zinc to lithium borate glass will increase the strength and enhance the electron emission [11].Presence of zinc in lithium borate glasses can alter or increase the glass transition temperature [13], thermal expansion coefficient and density.SnO2 is a semiconductor material, and its incorporation into borate glasses improves their electrical conductivity which has very important applications in sensors, optoelectronic devices, and solid state batteries [14].SnO2 acts as a network modifier and also as network former in borate glasses [15].This work discusses about the physical and optical properties of Sn doped in lithium zinc borate glass system and the work is further intended to examine towards transport behavior of such glass systems.
2.Experimental Method: 57.5B2O3 -(20-x)ZnO-22.5Na2O-xSnO(where, x = 0, 0.5,1 mol %) glass composition were synthesized through conventional melt quench technique using precursors of analytical reagent grade chemicals from with a purity of 99.9% was procured from Loba Chemie.The batch composition was measured with an appropriate stoichiometric calculation and the powders were mixed thoroughly and transferred to the porcelain crucibles.These crucibles were placed inside the muffle furnace for obtaining the molten glass at 1050 °C and were constantly stirred for 15 minutes in its melting temperature in order to achieve homogeneous bubble-free glasses; the melt was then poured onto the brass plate through stainless steel washers in order to obtain the button shaped glasses which are coded as LZB, 0.5SnLZB, 1SnLZB.These samples were polished using sand paper of its grade ranging from 220, 400, 600, 800 and 1500 and then was subjected to analyze physical properties and optical properties.

Results and Discussions
3.1 Physical Properties: Density measurements were calculated using Archimedes principle with toluene as immersing liquid with its density around 0.866 g/cm 3 .Density related parameters such as Molar volume, ion concentration, interionic distance, polaron radius, field strength and average boron-boron distance were evaluated using the reference [16,17].Figure 1 indicates the anomalous relationship has been observed with the incorporation of SnCl2 in the matrix, as it is observed that initially at 0.5 mol % of Sn in LZB matrix, the reduction in density suggests that there could be a possibility of creation of more NBO's in the network.Further addition of Sn of 1 mol% in the matrix, the density of glass has increased which suggests that there could be possibility of formation of bridging oxygens in the network.Further, elucidation on the BO's and NBO's in the network can be probed by evaluating the optical properties of glasses.

Optical Properties
Well-polished lithium zinc borate glasses doped with SnCl2 were subjected to UV-Vis-NIR spectrometer.Figure 3 (a) indicates the characteristic absorption profile of the synthesized glasses.Figure 3 (b) and 3 (c) depicts the direct and indirect bandgap of samples which were estimated with a plot of (ߙℎߥ) against energy (ℎߥ).In case of direct band gap, the exponent' n' accounts for 2, while in case of indirect band gap exponent value are ½. Figure 3 (d) indicates the Urbach energy of glasses.The anomalous behavior of glasses with incorporation of Sn at 0.5 and 1 mol% are indicative with the trend in bandgap as, 0.5SnLZB shows least optical bandgap and higher Urbach energy, thereby it is clear that the non-bridging oxygens in the glasses are found to be quite high.On the contrary, it is found that at 1 mol % of Sn in lithium zinc borate glass matrix the bandgap value has increased and there is drastic reduction in Urbach energy.This clearly suggests that, SnCl2 plays a crucial role in establishing network bridging [18].Based on the bandgap with the reference [19,20] , refractive index related physical properties of glass were evaluated.These values very well corellate with the fact that 0.5SnLZB glass has prononunced non-bridging oxygens in the network.The dielectric constant of the glass is found to be high and it clearly indicative that this glass could exhibit excellent charge transport properties as well as nonlinear optical properties [21,22].

Conclusion:
Lithium Zinc Borate Glasses doped with SnCl2 and coded as LZB, 0.5SnLZB, 1SnLZB were synthesized by conventional melt quench technique.These glasses were examined for physical and optical properties.It is interesting to note that the tin plays a role of intermediate in the lithium zinc borate glass matrix for which, at 0.5 mol % of SnCl2, the glass tends to create more NBOs in the network and on the contrary at 1 mol % of SnCl2, there is an increase in density and optical bandgap which enbales glass to bridge the network.Thus, 0.5SnLZB glass has a great potential in charge-transport or nonlinear optical properties.

Figure 1 :
Figure 1: Concentration of Sn versus Density and Molar Volume

Figure 2 Figure 2 (
Figure 2 (a) : Absorption spectra of Lithium zinc borate glasses doped with Sn Figure 2 (b) : Direct bandgap of lithium zinc borate glasses doped with Sn