Impact of Gd 2 O 3 Incorporation in Structural, Optical, Thermal, Mechanical, and Radiation Blocking Nature in HMO Boro-Tellurite Glasses †

: The glass system of B 2 O 3 -SiO 2 -TeO 2 -Bi 2 O 3 -ZnO-BaO doped with Gd 2 O 3 (x = 0, 1, 2, 3, and 4 mol%) (BiTeGd-x) was prepared by using the melt-quench technique. The density of glasses increased from 5.323–5.579 g cm − 3 for 0–4 mol% with an increase in Gd 2 O 3 concentration. The simulation results obtained using Photon Shielding and Dosimetry (PSD) software (Phy-X version) produced the maximum mass attenuation coefﬁcient (MAC) and minimum half-value layer (HVL) in the entire photon energy spectrum 0.015–15 MeV, suggesting the highest potential of BiTeGd-4 glass to act as a shield against low and high-energy radiation photons.


Introduction
The usefulness of glass materials for protection against radiation has been extensively explored by researchers.Simple fabrication techniques, non-toxicity, transparency, chemical durability, and chemical flexibility are the main contributing factors of glasses to enhance radiation shielding proficiency and achieve high density [1].Certain glasses designed have transcended conventional shielding materials such as concretes and bricks with appropriate attenuation coefficients and half-value layers (HVL).Tellurite glasses show promising results for thermal and chemical stability with a low melting temperature and density [2].
Rare-earth oxide such as Gd 2 O 3 has been reported to improve the physical, optical, and mechanical properties of tellurite glasses by increasing their density, refractive index, and hardness values [3].Kaewjaeng et al. [4] suggested that Gd 3+ doping reduced HVT significantly, allowing the glass to perform better than commercial X-ray windows, concrete, and bricks [5].There are limited investigations on the effect of Gd 3+ ions and the overall improvement of the stability of glasses and radiation shielding properties.The present study was carried out to explore the potential of this glass system in radiation field application by analyzing the physical, optical, and radiation-blocking development of B 2 O 3 -SiO 2 -Gd 2 O 3 -TeO 2 -Bi 2 O 3 -ZnO-BaO glass system (BiTeGd-x).

Materials and Methods
Melt quenching was conducted to produce the BiTeGd-x system where Gd 2 O 3 mol% varied as 0, 1, 2, 3, and 4. A furnace temperature of 1100-1120 • C was used for its synthesis [6].The density of prepared glasses was determined using Archimedes theory and distilled water.Carl Zeiss FESEM recorder was used to study its surface morphology through EDAX measurement.Theoretical values of terms to evaluate the gamma-ray shielding property such as mass attenuation coefficient (MAC), and HVL of the synthesized glasses were obtained by using Photon Shielding and Dosimetry (PSD)(Phy-X version) software in an energy region of 0.015-15 MeV [7].

Physical Properties
The synthesized Gd 3+ tellurite glasses are shown in Figure 1, where Gd 2 O 3 incorporation improved the transparency and changed the color of the glass from reddish-orange to light yellow.Archimedes' principle was used to determine the density of the sample (Table 1).Physical parameters such as molecular weight and molar volume were calculated for the fabricated glasses with other parameters using the following relations.

Physical Properties
The synthesized Gd 3+ tellurite glasses are shown in Figure 1, where Gd2O3 incorporation improved the transparency and changed the color of the glass from reddish-orange to light yellow.Archimedes' principle was used to determine the density of the sample (Table 1).Physical parameters such as molecular weight and molar volume were calculated for the fabricated glasses with other parameters using the following relations.
For the number density of Gd 3+ ions, The interionic separation between Gd 3+ ions was obtained with All calculated parameters are summarized in Table 1.Gd 3+ increased the density of the glass from 5.323 to 5.5793 gcm −3 from 0 to 4 mol% of Gd2O3.The high molecular weight of Gd2O3 compared to TeO2 caused a density increase in the glass.The tendency of Gd 3+ ions to form a closed-packed network by filling the interstitial spaces has also increased the density.Moreover, the Gd 3+ ions have an ionic radius of 1.19 Å which is greater than that of Te 4+ and Bi 3+ ions (0.99 and 1.03 Å), which also increased the density.Molar volume (Vm) initially decreased but increased depending on the concentration of Gd2O3.BiTeGd-1 glass with the minimum molar volume confirmed polymerization in this glass network [8].The number density of Gd 3+ ions increased with the increase in Gd2O3 concentration.The interionic radius (ri) decreased with successive addition of Gd2O3 molecules, indicating the shrinkage of ionic clouds due to Gd 3+ ions.The XRD images of the Gd 3+ glasses are shown in Figure 2a.Sharp crystalline peaks were absent in the XRD images, assuring the amorphous nature of the current glasses.In  For the number density of Gd 3+ ions, The interionic separation between Gd 3+ ions was obtained with All calculated parameters are summarized in Table 1.Gd 3+ increased the density of the glass from 5.323 to 5.5793 gcm −3 from 0 to 4 mol% of Gd 2 O 3 .The high molecular weight of Gd 2 O 3 compared to TeO 2 caused a density increase in the glass.The tendency of Gd 3+ ions to form a closed-packed network by filling the interstitial spaces has also increased the density.Moreover, the Gd 3+ ions have an ionic radius of 1.19 Å which is greater than that of Te 4+ and Bi 3+ ions (0.99 and 1.03 Å), which also increased the density.Molar volume (V m ) initially decreased but increased depending on the concentration of Gd 2 O 3 .BiTeGd-1 glass with the minimum molar volume confirmed polymerization in this glass network [8].The number density of Gd 3+ ions increased with the increase in Gd 2 O 3 concentration.The interionic radius (r i ) decreased with successive addition of Gd 2 O 3 molecules, indicating the shrinkage of ionic clouds due to Gd 3+ ions.
The XRD images of the Gd 3+ glasses are shown in Figure 2a.Sharp crystalline peaks were absent in the XRD images, assuring the amorphous nature of the current glasses.In addition, the broad hump observed in Bragg's angle of 20-30 • also reflected the non-Eng.Proc.2023, 55, 97 3 of 6 crystallinity of the glasses.The surface morphology of Gd 3+ -doped tellurium borosilicate glass BiTeGd-2 was examined by using SEM images (Figure 2b).
Eng. Proc.2023, 55, x FOR PEER REVIEW 3 of 6 addition, the broad hump observed in Bragg's angle of 20-30 0 also reflected the non-crystallinity of the glasses.The surface morphology of Gd 3+ -doped tellurium borosilicate glass BiTeGd-2 was examined by using SEM images (Figure 2b).The occurrence of smooth and homogenous texture in the SEM images without any cluster of unresolved particles proved the amorphous character of the synthesized glasses.The compositional analysis of the BiTeEu-2 glass was performed using EDAX measurement.The EDAX spectrum shows properly distributed elements such as boron (B), oxygen (O), silicon (Si), europium (Eu), tellurium (Te), bismuth (Bi), barium (Ba), and zinc (Zn) (Figure 2c).Aluminium (Al) was detected in the glass composition because of the alumina crucible used in the glass melting process.A bar chart representing the weights of all constituent elements (Figure 2d) exhibited the highest weight of Bismuth (Bi) as Bi has the heaviest atomic weight.

Optical Properties
The function of Gd 3+ ions in enhancing the optical properties of the glass was studied by calculating parameters such as refractive index (n), dielectric constant (ε), molar refractivity (Rm), reflectance loss (R in %), and molar electron polarizability (αm) [9] using the following set of equations.The occurrence of smooth and homogenous texture in the SEM images without any cluster of unresolved particles proved the amorphous character of the synthesized glasses.The compositional analysis of the BiTeEu-2 glass was performed using EDAX measurement.The EDAX spectrum shows properly distributed elements such as boron (B), oxygen (O), silicon (Si), europium (Eu), tellurium (Te), bismuth (Bi), barium (Ba), and zinc (Zn) (Figure 2c).Aluminium (Al) was detected in the glass composition because of the alumina crucible used in the glass melting process.A bar chart representing the weights of all constituent elements (Figure 2d) exhibited the highest weight of Bismuth (Bi) as Bi has the heaviest atomic weight.

Optical Properties
The function of Gd 3+ ions in enhancing the optical properties of the glass was studied by calculating parameters such as refractive index (n), dielectric constant (ε), molar refractivity (R m ), reflectance loss (R in %), and molar electron polarizability (α m ) [9] using the following set of equations.
The values are presented in Table 2.The refractive index continuously increased with doping Gd 2 O 3 .The refractive index was influenced by a larger atomic radius (1.79 Å) of Gd [10] which is greater than that of tellurium (1.6 Å) and boron (0.98 Å).The higher polarization ability of cations resulting from higher cation radius of Gd 3+ ions induces high n values, providing a platform for current Gd 3+ -doped glasses in the non-linear optical application.An enhanced refractive index was also associated with a high dielectric constant, molar refractivity, reflectance loss, and molar electron values as shown in Table 2.The absorption spectra recorded in the UV-visible region for the Gd-doped glasses are shown in Figure 3a.The synthesized glasses including the undoped glass showed maximum absorption in the UV region (300-400 nm).In addition, all the glasses showed a broad low intense absorption peak around 500 nm which corresponds to the absorption of Bi 3+ ions.Higher transmittance observed in the visible of all samples was proof of improved transparency of the Gd-doped glass.
Eng. Proc.2023, 55, x FOR PEER REVIEW 4 of 6 The values are presented in Table 2.The refractive index continuously increased with doping Gd2O3.The refractive index was influenced by a larger atomic radius (1.79 Å) of Gd [10] which is greater than that of tellurium (1.6 Å) and boron (0.98 Å).The higher polarization ability of cations resulting from higher cation radius of Gd 3+ ions induces high n values, providing a platform for current Gd 3+ -doped glasses in the non-linear optical application.An enhanced refractive index was also associated with a high dielectric constant, molar refractivity, reflectance loss, and molar electron values as shown in Table 2.
The absorption spectra recorded in the UV-visible region for the Gd-doped glasses are shown in Figure 3a.The synthesized glasses including the undoped glass showed maximum absorption in the UV region (300-400 nm).In addition, all the glasses showed a broad low intense absorption peak around 500 nm which corresponds to the absorption of Bi 3+ ions.Higher transmittance observed in the visible of all samples was proof of improved transparency of the Gd-doped glass.The relationship between absorbance and optical band gap Eg was provided by Tauc's relation as follows.
where α is the absorption coefficient, also given by α = 2.303 A/t, A is the absorbance, t is the thickness of the sample material, B is the band tailing parameter, and the exponent γ depends on the kind of electronic transition mechanism.Because glasses are amorphous, we took γ = 2 as the transitions are indirect in nature.The resulting Tauc plot is represented in Figure 3b, where the extrapolation of the linear part of the curve is taken as Eg (Table 2).The Eg values showed that the band gap of Gd 3+ -doped glass was less than that of the undoped glass except for the BiTeGd-3 sample.The relationship between absorbance and optical band gap E g was provided by Tauc's relation as follows.
where α is the absorption coefficient, also given by α = 2.303 A/t, A is the absorbance, t is the thickness of the sample material, B is the band tailing parameter, and the exponent γ depends on the kind of electronic transition mechanism.Because glasses are amorphous, we took γ = 2 as the transitions are indirect in nature.The resulting Tauc plot is represented in Figure 3b, where the extrapolation of the linear part of the curve is taken as E g (Table 2).
Eng. Proc.2023, 55, 97 5 of 6 The E g values showed that the band gap of Gd 3+ -doped glass was less than that of the undoped glass except for the BiTeGd-3 sample.

Radiation Shielding Parameters
MAC and HVL data simulated by Phy-X/PSD software in the 0.015-15 MeV photon energy spectrum are represented in Figure 4 [7,10].The influence of energy on the MAC values of the glass was evident from the rapidly falling trend in the lower energy range, constancy in the intermediate range, and an increase at the higher end of the spectrum.This showed the occurrence of photoelectric absorption, Compton scattering, and electronpositron pair formation in the three energy regions.The two sharp peaks at 0.035 and 0.1 MeV were the K-edges associated with Te and Bi.Furthermore, the effect of varying the Gd 2 O 3 content in MAC graphs on the continuous increase in MAC with the Gd 3+ content was studied.Such an effect was observed because of the increasing density values of Gd 2 O 3 from 5.323 to 5.579 gcm −3 from 0 to 4 mol% in concentration.Therefore, BiTeGd-4 glass exhibited the maximum attenuation compared to other Gd glasses.

Radiation Shielding Parameters
MAC and HVL data simulated by Phy-X/PSD software in the 0.015-15 MeV photon energy spectrum are represented in Figure 4 [7,10].The influence of energy on the MAC values of the glass was evident from the rapidly falling trend in the lower energy range, constancy in the intermediate range, and an increase at the higher end of the spectrum.This showed the occurrence of photoelectric absorption, Compton scattering, and electron-positron pair formation in the three energy regions.The two sharp peaks at 0.035 and 0.1 MeV were the K-edges associated with Te and Bi.Furthermore, the effect of varying the Gd2O3 content in MAC graphs on the continuous increase in MAC with the Gd 3+ content was studied.Such an effect was observed because of the increasing density values of Gd2O3 from 5.323 to 5.579 gcm −3 from 0 to 4 mol% in concentration.Therefore, BiTeGd-4 glass exhibited the maximum attenuation compared to other Gd glasses.
In Figure 4b, low-energy photons generated low HVL values due to the high photon absorption mechanism.HVL increased continuously in the energy range of 0.1-5 MeV, showing the maximum value at 5 MeV and a decrease to the energy of 15 MeV.This implied that when the energy increased, the radiation penetrated deeper into the glass, and therefore it was essential to increase the thickness of the material to shield it from the high radiation photons.The maximum HVL was found at 5 MeV for BiTeGd-4 glass as 3.4619 cm, which was thick enough to protect the glass from high-energy radiation.Additionally, the HVL decreased with successive doping of Gd2O3 in the range of 0.015-15 MeV suggesting that BiTeGd-4 glass had the lowest HVL.

Conclusions
Improved transparency with enhanced density values was proved with tellurite glasses doped with Gd2O3.Non-crystalline nature and smooth glass surface morphology were verified by XRD and FESEM results.Optical parameters including the refractive index continuously increased with an increase in Gd2O3.MAC and HVL parameters In Figure 4b, low-energy photons generated low HVL values due to the high photon absorption mechanism.HVL increased continuously in the energy range of 0.1-5 MeV, showing the maximum value at 5 MeV and a decrease to the energy of 15 MeV.This implied that when the energy increased, the radiation penetrated deeper into the glass, and therefore it was essential to increase the thickness of the material to shield it from the high radiation photons.The maximum HVL was found at 5 MeV for BiTeGd-4 glass as 3.4619 cm, which was thick enough to protect the glass from high-energy radiation.Additionally, the HVL decreased with successive doping of Gd 2 O 3 in the range of 0.015-15 MeV suggesting that BiTeGd-4 glass had the lowest HVL.

Conclusions
Improved transparency with enhanced density values was proved with tellurite glasses doped with Gd 2 O 3 .Non-crystalline nature and smooth glass surface morphology were verified by XRD and FESEM results.Optical parameters including the refractive index continuously increased with an increase in Gd 2 O 3 .MAC and HVL parameters computed by PSD software in the energy range of 0.015-15 MeV showed that BiTeGd-4 was the optimum glass for gamma radiation shielding.

Figure 4 .
Figure 4. (a) MAC and (b) HVL of BiTeGd glasses simulated for energies in 0.015-15 MeV range using Phy-X/PSD software.

Figure 4 .
Figure 4. (a) MAC and (b) HVL of BiTeGd glasses simulated for energies in 0.015-15 MeV range using Phy-X/PSD software.