Structural, Physical and Optical studies of Sc3+ doped Lithium Aluminum Borate glasses

Optically transparent lithium aluminum borate containing scandium glasses with composition 70B2O3 – 20Li2O – (10 – x) Al2O3 – xSc2O3 (where x= 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 in mol %) were fabricated by conventional melt quenching technique. The non-crystalline nature of the samples was monitored by X-ray diffraction and it confirms non-crystalline nature. Structure of the fabricated glasses are assessed using the Fourier transform infrared spectra. The absorption spectra of these glasses have been recorded in the wavelength range 200 nm – 1100 nm. Direct and indirect optical energy band gaps were calculated from Tauc’s plots. The physical parameters such as refractive index (RI), optical basicity and interaction parameters were determined using proper formulas to know the characteristics of fabricated glass matrix. The results obtained indicate that 0.3 mol% Sc2O3-containing BLASc glass is more suitable for w-LEDs and lasers applications.


INTRODUCTION
Due to its transparency, chemical inertness, and environmental friendliness, glass is one of the most fascinating materials and has drawn significant interest in both science and technology [1][2][3].Among all glass hosts, borates have several special qualities that make them suited for use in fiberglass, including decreased thermal expansion, resilience to thermal shock, increased toughness and strength [3].However, the negative qualities of borate glass include its hygroscopic nature, but it can be reduced by adding alkali oxides such as Li2O, Na2O, and K2O.It can also increase several aspects of borate glasses.Among all alkali oxides, Li2O has been used as a glass forming in a variety of compounds to increase physical property and viscosity while lowering melting point in the borate system.In addition to making borate glass an excellent network modifier, adding amphoteric oxide Al2O3 also enhances the host glass's chemical stability, mechanical strength, and emission qualities [4][5][6].Al2O3-Li2O3-B2O3 glasses are of great interest to researchers because, depending on the concentration of Al2O3 in the glass matrix, Al2O3 acts as a glass former or modifier in the glass formations.
In this research, the impact of Scandium on the lithium aluminum borate glass system is examined.Scandium is utilized as a lightweight material for highperformance applications in the military equipment, electrical, aviation, automotive, and transportation industries.X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) will be used to analyze the structural, optical, and morphological features.UV-Vis spectroscopy have been used to study the optical properties, such as absorbance and optical band gap.Sc2O3 doping to LAB glass matrix was limited in the current investigation to the range of 0 to 0.7 since this range has good glass forming capabilities.

EXPERIMENTAL
Boric acid, lithium carbonate, aluminum oxide, and scandium oxide were among the analar grade chemical agents that were added in the proper amounts to each glass composition.Each time, a porcelain crucible was used to melt the powdered substance at a high temperature (at 1040 °C) and produce a bubble-free, uniform liquid.The molten liquid was poured on to a brass moulds, cooled to ambient temperature (30 °C), and then annealed for two hours at 300 °C.The samples are polished using different grades of emery papers to produce well-polished and homogenous thickness samples.The chemical formulae used in the experiment was as follows: 70B2O3 -20Li2O -(10 -x) Al2O3 -xSc2O3 (where x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7 in mol%).The glass samples were given the names BLA, BLASc0.1,BLASc0.2,BLASc0.3,BLASc0.4,BLASc0.5, BLASc0.6, and BLASc0.7 depending on how much scandium was doped into them.The XRD-6100 SHIMADZU X-ray diffractometer's Cu-Ka radiation (a = 0.15406 nm) with a 2θ angle range of 10 0 to 80 0 was used to determine the present samples' amorphous nature.The Shimadzu UV-1800 spectrophotometer was used to record the absorbance spectra between 200 and 1100 nanometers.With KBr pellets working between 500 and 4000 per centimeter on a Brukar AT-IR spectrophotometer, FT-IR spectra of all glass samples have been recorded.

XRD studies
The fabricated glass samples' X-Ray Diffractograms, which were captured at room temperature between 2θ values 10° and 90° at a rate of 2°/min as depicted in figure 1.It is evident that there is no sharp crystalline peaks, which clearly indicates the non-crystalline nature of glass.[7].  1 displays several physical characteristics of the fabricated samples based on observed densities and computed molar volumes [7 -9].The physical characteristics of the glasses were identified, and they provided a detailed grasp of the atomic arrangements inside their network architectures [10].

3.2.1, Density
One of the most crucial analyses for comparing the investigated glass samples is density measurement.Toluene was used as the immersion solvent for the Archimedes principle density measurements on the created glass matrix.Glass matrix's molar volume can be calculated using the correct relation by knowing its density.As the percentage of scandium rises the density of the manufactured glass matrix falls where as its molar volume increases.This could be the result of NBO becoming bridging oxygen.The molecular volume grows as expected as the mol% of scandium in the glass matrix increases.This is because the amount of NBO in the glass samples changes when the scandium concentration rises, and the molecular volume rises as a result of an increase in bond length in the glass network.The negative relationship between the two values has been confirmed, figure 2 depicts how density and molar volume change as scandium concentration changes.

3.2.2, Optical basicity
Theoretical optical basicity (Λth), was estimated using the relevant formula [11] and the results are reported in Table 1.The creation of innovative functional materials with improved optical properties may benefit from an understanding of the optical basicity properties of glasses.As the concentration of scandium rises, the optical basicity values increase, indicating a higher capacity for oxide ions to donate electrons to neighboring cations and it is as depicted in figure 3   The Clasius-Mosotti relation was used to determine the electronic polarizability [13], typically, polarization rises with the quantity of NBO atoms.As the concentration of scandium rises, the electronic polarizability for manufactured glass matrix rises which is depicted as in figure 3. Technology-wise, the majority of the physical and chemical properties of glasses that are governed by electronic polarizability are equally important to that of optical and electronic materials.The charge overlap between adjacent ions is what Yamashita and Kuroswa's interaction parameter (A) (means physically) refers to [14].Table 1 provides a summary of the Ath values that were acquired.Figure 3 depicts the relationship between optical basicity Λth and the interaction parameter Ath for current glasses, demonstrating a strong linear association between them.

3.3, FT-IR Spectroscopy
FTIR spectroscopy is a method that uses mid-infrared radiation to analyse the molecular structure and composition of substances by creating a distinctive fingerprint for identification and study.The way FTIR operates is by identifying particular light frequencies that are related to the energy of the vibrational bonds inside molecules.The FT-IR spectra of fabricated glass matrix is as depicted in figure 4 and assignments for various bands as in table 2.  Vibrations of B-O-B in the BO4 and BO3 groups [15] The NBOs of the BO4 groups [15] The BO4 group vibration's of B-O bond stretching [16] B-O bonds stretching vibration in the of meta-borate groups [16,17] Hydrogen bonding [17] Presence of OH group [17,18] OH group vibration of OH group [19]

3.4, Optical absorption study
The absorbance spectra of glass samples in the wavelength range 200 nm to 1200 nm are shown in figure 5.The notable absorption peak for the doped glasses is located at 277 nm.The strengths of the absorption bands varies as Sc2O3 concentration rises and it is maximum when mol% of Sc2O3 is 0.4.peak match with absorption spectroscopy measurements satisfactorily [20].For manufactured glasses, the optical absorption coefficient (α) has been computed using relevant relation

3.4.1, Direct and indirect optical energy band gaps
The optical band gaps (Eg), which are calculated using Davis and Mott's equation.Tauc's plots of these glass matrices are depicted in Figures 6 and 7. Using these plots, the optical energy gap for direct and indirect transitions was estimated.The evaluated energy gap values are shown in Table 3.According to Figure 10, a plot of Eg versus mol % of Sc2O3 content demonstrates that Eg declines as Sc2O3 content rises.The structural changes that are occurring in glasses is the main reason for Eg decrease as the Sc2O3 content rises.In general higher negative charge is present on non-bridging oxygen atoms.The non-bridging oxygen atoms with bigger magnitude of negative charge makes it simpler for their electrons to be excited to the conduction band of cations, which reduces the optical band gap [22,23].Additionally, we assume that adding Sc 3+ ions to the existing glass will cause them to occupy interstitial sites and function as modifiers, which will ultimately cause the breakdown of glass network.As a result, the band edge changes with increasing Sc2O3 content to longer wave lengths.Additionally, as NBO rises, the degree of electron localization also increases.This improves the glass network's donor centers and lowers the optical band gap [23].Using the Dimitrov and Sakka relation, the refractive indices for the direct (nd) and indirect (nind) energy gaps of fabricated glass samples was found [27].With an increase in scandium concentration, the index of refraction for the current glass matrix rises.The results are as shown in table 3.

Figure 1 :
Figure 1: X-ray diffraction spectrum of fabricated glass samples 3.2, Physical Parameters Table1displays several physical characteristics of the fabricated samples based on observed densities and computed molar volumes[7 -9].The physical characteristics of the glasses were identified, and they provided a detailed grasp of the atomic arrangements inside their network architectures[10].3.2.1,Density One of the most crucial analyses for comparing the investigated glass samples is density measurement.Toluene was used as the immersion solvent for the Archimedes principle density measurements on the created glass matrix.Glass matrix's molar volume can be calculated using the correct relation by knowing its density.As the percentage of scandium rises the density of the manufactured glass matrix falls where as its molar volume increases.This could be the result of NBO becoming bridging oxygen.The molecular volume grows as expected as the mol% of scandium in the glass matrix increases.This is because the amount of NBO in the glass samples changes when the scandium concentration rises, and the molecular volume rises as a result of an increase in bond length in the glass network.The negative relationship between the two values has been confirmed, figure2depicts how density and molar volume change as scandium concentration changes.3.2.2, Optical basicityTheoretical optical basicity (Λth), was estimated using the relevant formula[11] and the results are reported in Table1.The creation of innovative functional materials with improved optical properties may benefit from an understanding of the optical basicity properties of glasses.As the concentration of scandium rises, the optical basicity values increase, indicating a higher capacity for oxide ions to donate electrons to neighboring cations and it is as depicted in figure 3[12]

3 )Figure 2 :
Figure 2: Variation of Density and molar volume with mol% of Sc2O3The Clasius-Mosotti relation was used to determine the electronic polarizability[13], typically, polarization rises with the quantity of NBO atoms.As the concentration of scandium rises, the electronic polarizability for manufactured glass matrix rises which is depicted as in figure3.Technology-wise, the majority of the physical and chemical properties of glasses that are governed by electronic polarizability are equally important to that of optical and electronic materials.The charge overlap between adjacent ions is what Yamashita and Kuroswa's interaction parameter (A) (means physically) refers to[14].Table1provides a summary of the Ath values that were acquired.Figure3depicts the relationship between optical basicity Λth and the interaction parameter Ath for current glasses, demonstrating a strong linear association between them.

Figure 3 :
Figure 3: Variation of Optical basicity, electronic polarizability and interaction parameter with mol% of Sc2O3.Table 1. Physical parameters of BLA glasses.

Figure 5 :
Figure 5: Absorption spectrum of Sc 3+ doped BLA glass samples.3.4.1,Direct and indirect optical energy band gapsThe optical band gaps (Eg), which are calculated using Davis and Mott's equation.Tauc's plots of these glass matrices are depicted in Figures6 and 7. Using these plots, the optical energy gap for direct and indirect transitions was estimated.The evaluated energy gap values are shown in Table3.According to Figure10, a plot of Eg versus mol % of Sc2O3 content demonstrates that Eg declines as Sc2O3 content rises.The structural changes that are occurring in glasses is the main reason for Eg decrease as the Sc2O3 content rises.In general higher negative charge is present on non-bridging oxygen atoms.The non-bridging oxygen atoms with bigger magnitude of negative charge makes it simpler for their electrons to be excited to the conduction band of cations, which reduces the optical band gap[22,23].Additionally, we assume that adding Sc 3+ ions to the existing glass will cause them to occupy interstitial sites and function as modifiers, which will ultimately cause the breakdown of glass network.As a result, the band edge changes with increasing Sc2O3

Figure 8 :
Figure 8: Variation of Urbach energy gap with mol% of Sc2O3 3.4.3Index of refractionUsing the Dimitrov and Sakka relation, the refractive indices for the direct (nd) and indirect (nind) energy gaps of fabricated glass samples was found[27].With an increase in scandium concentration, the index of refraction for the current glass matrix rises.The results are as shown in table3.Figures 9 show how the amount of scandium affects the optical energy band gap (for direct and indirect light) and index of refraction (n) .
Figure 8: Variation of Urbach energy gap with mol% of Sc2O3 3.4.3Index of refractionUsing the Dimitrov and Sakka relation, the refractive indices for the direct (nd) and indirect (nind) energy gaps of fabricated glass samples was found[27].With an increase in scandium concentration, the index of refraction for the current glass matrix rises.The results are as shown in table3.Figures 9 show how the amount of scandium affects the optical energy band gap (for direct and indirect light) and index of refraction (n) .

Figure 9 :
Figure 9: Direct optical energy band gap and indices of refraction with concentration of Sc2O3 plot for glass matrix [12]

Table 1 .
Physical parameters of BLA glasses.

Table 2 .
FT-IR band assignments in Scandium doped BLA glasses

Urbach energy (eV) mol% of Sc 2 O 3
[26]25]bachenergy(Eu)Glassy materials exhibit a tailing of state densities in the direction of the band gap.The breadth of these band tails is referred to as the urbach energy[24,25].The urbach energy (Eu) has been calculated using the proper relation.The reciprocal of slope of linear portion obtained in the of logα verses energy of incident photon gives urbach energy.The urbach energy values obtained for fabricated glass matrix are tabulated in table 3 and variation of urbach energy with mol % of Sc2O3 is as in fig.8.It is observed that the Urbach energy values rise as the mol % of Sc2O3 increases.This behaviour is ascribed to the weak links turning into defects, which raises the level of disorder in the glass structure and increases the number of localised states in the band gap[26].