Impact of Dy2O3 Substitution on the Physical, Structural and Optical Properties of Lithium–Aluminium–Borate Glass System

In this study, a series of Li2O-Al2O3-B2O3 glasses doped with various concentrations of Dy2O3 (where x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%) were prepared by using a conventional melt-quenching technique. The structural, physical and optical properties of the glasses were examined by utilising a variety of techniques instance, X-ray diffraction (XRD), UV–Vis-NIR spectrometer, Fourier transform infrared (FTIR) and photoluminescence (PL). The XRD spectra demonstrate the amorphous phase of all glasses. Furthermore, the UV-vis-NIR spectrometers have registered optical absorption spectra a numbers of peaks which exist at 1703, 1271, 1095, 902, 841, 802, 669, 458, 393 and 352 nm congruous to the transitions from the ground of state (6H15/2) to different excited states, 6H11/2, 6F11/2 + 6H9/2, 6F9/2 + 6H7/2, 6F7/2, 6F5/2, 6F3/2, 4F9/2, 4I15/2, 4F7/2 and 6P7/2, respectively. The spectra of emission exhibit two strong emanation bands at 481 nm and 575 nm in the visible region, which correspond to the transitions 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2. All prepared glass samples doped with Dy2O3 show an increase in the emission intensity with an increase in the concentration of Dy3+. Based on the obtained results, the aforementioned glass samples may have possible applications, such as optical sensor and laser applications.


Introduction
Borate glass has been acknowledge as a good host for various rare-earth (RE) oxides among the traditional glass formers due to their strong glass formulation when compared with other conventional systems such as phosphates, germanates, vanadates and tellurite glass [1]. In addition, the glassy system is available easily, inexpensive, simple to prepare and a good host for a variety of elements [2]. The structural, physical, and optical characteristics of the glasses are greatly influenced by the composition and synthesis conditions. Therefore, to accomplish high emission efficiency, most of the glass system is activated using suitable transitional metals and/or rare-earth elements. m refers to the volume of boron atoms per mole and is given by where X B is the mole fraction and V m is Molar volume for the glass samples [15]. The ion concentration can be acquired using the next expression where X% is the mole percent of dopant, N T is the Avogadro number, and M T is the molecular weight. Based on the ion concentrations, it is also possible to compute three essential physical parameters, such as Polaron radius (r p ), inter-nuclear distance (r i ) and field strength where Z m is the atomic mass of the dopant [16,17]. The non-crystalline phases for the selected samples were observed using X-ray diffraction (XRD) system PANalytical X'pert PRO (PW3040/60 MPD, Philips, EA, The Netherland). The system was combined with the software of diffraction analysis based on the 2θ range from 10 • to 80 • with the steps 0.02 • . Perkin Elmer Spectrum 100 (Waltham, MA, USA) instrument was utilized to obtain the Appl. Sci. 2020, 10, 8183 4 of 17 FTIR spectrum absorption for studied glasses with the size <63 µm in Attenuated-Total Reflectance (ATR) mode within wavenumber range of 400-4000 cm −1 .

UV-Vis-NIR Absorption Spectra
UV-Vis spectrophotometer for reflective spectroscopy (RSA) (Lambda 35 Perkin Elmer, MA, USA) was used to estimate the absorption spectra of the glass samples within wavelength ranges of 200-2600 nm. Using absorption, the energy gap (E g ) can be obtained by applying the Mott and Davis relation (αhν) = A hν − E g n /(hν) (9) where α is coefficient of optical absorption, A is a constant, (hν) is the incident photon energy, and E g is the indirect permitted optical band gap energy. The direct E g value is obtained from the plot (αhν) 1/2 , and hν by extrapolating the linear-compatible regions to the value (αhν) 1/2 = 0 [16]. Urbach energy (E u ) offers knowledge about glass disorder. This E u can be measured by using the relation [8] α(ν) = c. exp( hν /E u ) (10) where c is constant and E u is Urbach energy. The oscillator strength f exp of glasses can be determined using the next equation where ε(ν) is the coefficient of molar the absorption of the each band at an energy of ν(cm −1 ) [17][18][19].
The refractive index (n) for the electronic polarization of ions and the local field of materials is among the most important optically dependent material parameters. The next relation can be used to define the refractive index (n) Reflective loss on the surface of the glass is determined by using the refractive index of the Fresnel formula Using the Volf and Lorentz-Lorenz formula, the molar refraction (R m ) for all samples was measured [20,21] Molar refractivity (R M ) can be acquired using the following equation [22] R m = n 2 − 1 The following relation can be used to estimate molar polarizability (α m ) [23] Metallisation criterion is the prediction of the metal or isolating behaviour of the condensed matter and is determined using the following relation [24] The materials are considered metallic when R m /V m > 1, and they are considered insulating when R m /V m < 1. Further, the polarizability of electrons (α o ) and optical basicity (Λ) linked electronegativity (χ) can be obtained by [25] where (E g ) is optical band gap. The electronic polarizability is given by The relation between the electronic oxide polarizability and optical basicity is described by [26].
The dielectric constant (ε) and optical dielectric constant can be calculated using the following formulae [8]. The dielectric constant has calculated using refractive index of the glass ε = n 2 (21) where (n) is the refractive index. The optical dielectric constant of the glass calculated by the following relation where ε is the dielectric constant.

Photoluminescence (PL) Spectrum
The LS55 Luminescence Spectrophotometer (Perkin Elmer, MA, USA) was used to determine photoluminescence between the wavelengths of 200 and 1300 nm. The luminescence signal was analysed based on excitation and emission methods using a Monk-Gillieson monochromator.

Results and Discussion
The pattern of the XRD for all the glass samples did not show any sharp diffraction or peaks, as shown in Figure 1, which confirms the amorphous nature for all the studied glass samples. Figure 2 shows the FTIR spectrum for lithium-aluminium-borate (LAB) glass doped with various Dy 3+ ion concentrations. All infrared spectrums revealed several absorption bands, as listed in Table 2.
Density (ρ) is a key physical parameter for analysing the physical features of glass samples, as it indicates the relation between the masses and the volume within the glass system. Likewise, the molar volume (V m ) also correlates directly to the oxygen distribution in the glass structure. Figure 3 shows the relation between the density and molar volume of the glass upon adding the different concentrations of Dy 2 O 3 .
Appl. Sci. 2020, 10, 8183 6 of 17 The pattern of the XRD for all the glass samples did not show any sharp diffraction or peaks, as shown in Figure 1, which confirms the amorphous nature for all the studied glass samples. Figure 2 shows the FTIR spectrum for lithium-aluminium-borate (LAB) glass doped with various Dy 3+ ion concentrations. All infrared spectrums revealed several absorption bands, as listed in Table 2.        [28]. [29].
Density ( ρ ) is a key physical parameter for analysing the physical features of glass samples, as it indicates the relation between the masses and the volume within the glass system. Likewise, the molar volume (Vm) also correlates directly to the oxygen distribution in the glass structure. Figure 3 shows the relation between the density and molar volume of the glass upon adding the different concentrations of Dy2O3. As illustrated in Table 3, the Vm of these glasses increases slightly with increasing Dy2O3 concentration up to 0.4 mol%, but the Vm values decrease gradually from 29.28 up to 29.17 cm 3 with the addition of Dy2O3 up to 1 mol%. This enhancement in Vm value can be related to the decrement in glass compactness. Upon further addition of Dy2O3, the Vm values reduce gradually as a result of the  Table 3, the V m of these glasses increases slightly with increasing Dy 2 O 3 concentration up to 0.4 mol%, but the V m values decrease gradually from 29.28 up to 29.17 cm 3 with the addition of Dy 2 O 3 up to 1 mol%. This enhancement in V m value can be related to the decrement in glass compactness. Upon further addition of Dy 2 O 3 , the V m values reduce gradually as a result of the increasing compactness of the glass system [27][28][29]. The replacement with Dy 2 O 3 instead of B 2 O 3 changes the ratio of boron to oxygen, creating BO 4 units that contribute to the compactness of the glass structure, thereby increasing the glass density. Furthermore, the molecular weight of Dy 2 O 3 is higher than B 2 O 3 , meaning a significant increase in glass density [30]. The calculated r p , r i and F values are listed in Table 3, and Figure 4 displays the behaviour of these parameters. The decrease in r p and r i with increased Dy 2 O 3 is related to the decrease in the Dy-O distance, as a result of which the strength of the Dy-O bond increases, producing stronger field around Dy 3+ ions [8]. Besides, the addition of Dy 2 O 3 to the glass network led to overcrowding that decreased the average distance between the RE-oxygen. The significant increment in field strength values is, therefore, due to the appearance of strong linkages in the glass matrix between Dy 3+ and B ions. [10]. It is noted from this table that the boron-boron separation ‹d B-B › decreases with an increase in the Dy 2 O 3 concentration due to the stretching force of the binds in the glass network. The calculated rp, ri and F values are listed in Table 3, and Figure 4 displays the behaviour of these parameters. The decrease in rp and ri with increased Dy2O3 is related to the decrease in the Dy-O distance, as a result of which the strength of the Dy-O bond increases, producing stronger field around Dy 3+ ions [8]. Besides, the addition of Dy2O3 to the glass network led to overcrowding that decreased the average distance between the RE-oxygen. The significant increment in field strength values is, therefore, due to the appearance of strong linkages in the glass matrix between Dy 3+ and B ions. [10]. It is noted from this table that the boron-boron separation ‹dB-B› decreases with an increase in the Dy2O3 concentration due to the stretching force of the binds in the glass network.   current samples showed a hypersensitive transition at 1270 nm ( H15/2 → H11/2) with high intensity and were subjected to the rule of selection |ΔS| = 0, |ΔL| ≤ 2, and |ΔJ| ≤ 2, where these transitions are more sensitive than others [33]. The variation between transition levels and their respective oscillator of strength for the glass samples are tabulated in Table 4. Noting that, the influence of Dy 3+ ion on difference absorption bands led to their appropriate wavelengths, energies and oscillator strengths [34].    The spectra show ten inhomogeneous of absorption bands existing at the wavelengths 352, 393, 458, 669, 802, 841, 904, 1095, 1271 and 1703 nm due to the transitions of Dy 3+ at the ground state ( 6 H 15/2 ) into different excited states ( 6 P 7/2 ), ( 4 F 7/2 ), ( 4 I 15/2 ), ( 4 F 9/2 ), ( 6 F 3/2 ), ( 6 F 5/2 ), ( 6 F 7/2 ), ( 6 F 9/2 + 6 H 7/2 ), ( 6 F 11/2 + 6 H 9/2 ) and ( 6 H 11/2 ), respectively [17]. Due to the strong absorption of LAB host glass, some of the absorption bands have disappeared in ultraviolet (UV) regions and are very sparse at 352 nm ( 6 H 15/2 → 6 P 7/2 ), 393 nm ( 6 H 15/2 → 4 F 7/2 ) and 456 nm ( 6 H 15/2 → 4 I 15/2 ), and also they have very low intensity [32]. Besides, the current samples showed a hypersensitive transition at 1270 nm ( 6 H 15/2 → 6 H 11/2 ) with high intensity and were subjected to the rule of selection |∆S| = 0, |∆L| ≤ 2, and |∆J| ≤ 2, where these transitions are more sensitive than others [33]. The variation between transition levels and their respective oscillator of strength for the glass samples are tabulated in Table 4. Noting that, the influence of Dy 3+ ion on difference absorption bands led to their appropriate wavelengths, energies and oscillator strengths [34].  Figure 5 illustrate the optical energy band gaps for direct and indirect based on the curves of the UV-absorption spectrum. Figure 6a and 6b show the indirect and direct bandgap, respectively. The energy band gap value can be obtained by using Equation (9) to plot (αhν) n against photon energy (hv). Then, the linear extrapolating region of the curves extending to the X-axis gives the energy bandgap (E g ) reading. In the recent glass system, the values of the direct E g exhibit from 3.650 to 3.706 eV, and the values of indirect (E g ) show a decrease from 3.189 to 2.556 eV with increasing dopant contents, as listed in Table 5. The declines in E g values may be due to structural changes due to the addition of Dy 3+ ions. The addition of Dy 2 O 3 can contribute to an increase in electron localisation that increases donor centres in the glass matrix. This increment causes a decrease in E g values [35]. This is also because a new extrinsic band is formed by Dy 3+ on the grid between the boron and oxygen ions. As a consequence, there is an amount of possible reduction in (B-O-B) [36].

As illustrated in
also because a new extrinsic band is formed by Dy on the grid between the boron and oxygen ions. As a consequence, there is an amount of possible reduction in (B-O-B) [36].   Figure 7 indicates that the energy of Urbach decreases with changes in the Dy2O3 concentration. The reduction in the energy of Urbach is attributed to the creation of fewer defects, as reported [37]. Figure 8 and Table 6 show a slight gradual increase in refractive index values from 2.29 to 2.35 with increasing Dy2O3 concentration that can be attributed to the increase in electronic polarizability from 2.67 to 2.75 [17]. The sample has a higher refractive index, as it has a smaller bandgap value that reflects the compactness of the glass network structure. Meanwhile, the increment in molar refractivity and electronic polarisation values with increasing Dy2O3 concentration is indicated to form more non-bridging oxygen (NBOs) in the glass matrix [12]].   Figure 7 indicates that the energy of Urbach decreases with changes in the Dy 2 O 3 concentration. The reduction in the energy of Urbach is attributed to the creation of fewer defects, as reported [37]. Figure 8 and Table 6 show a slight gradual increase in refractive index values from 2.29 to 2.35 with increasing Dy 2 O 3 concentration that can be attributed to the increase in electronic polarizability from 2.67 to 2.75 [17]. The sample has a higher refractive index, as it has a smaller bandgap value that reflects the compactness of the glass network structure. Meanwhile, the increment in molar refractivity and electronic polarisation values with increasing Dy 2 O 3 concentration is indicated to form more non-bridging oxygen (NBOs) in the glass matrix [12].
(b) Figure 6. Bang gap energy: (a) Tauc's plot for allowed direct transitions (n = 2); (b) Tauc's plot for allowed indirect transitions (n = 1/2). Figure 7 indicates that the energy of Urbach decreases with changes in the Dy2O3 concentration. The reduction in the energy of Urbach is attributed to the creation of fewer defects, as reported [37]. Figure 8 and Table 6 show a slight gradual increase in refractive index values from 2.29 to 2.35 with increasing Dy2O3 concentration that can be attributed to the increase in electronic polarizability from 2.67 to 2.75 [17]. The sample has a higher refractive index, as it has a smaller bandgap value that reflects the compactness of the glass network structure. Meanwhile, the increment in molar refractivity and electronic polarisation values with increasing Dy2O3 concentration is indicated to form more non-bridging oxygen (NBOs) in the glass matrix [12]].  Table 6 presented the optical properties of the glass samples. It is observed that the molar refractivity (RM), reflection loss (RL) and refractive index (n) has a significant influence on the polarizability (αm) which demonstrates that the refractive index of the glasses does not solely depend on the density. It is known that the samples containing NBOs have great polarizability compared      Table 6 presented the optical properties of the glass samples. It is observed that the molar refractivity (R M ), reflection loss (R L ) and refractive index (n) has a significant influence on the polarizability (α m ) which demonstrates that the refractive index of the glasses does not solely depend on the density. It is known that the samples containing NBOs have great polarizability compared with samples containing bridging oxygen (BOs). The results presented here are in agreement with another work [38].
The theoretical optical basicity (Λ) is a calculation of oxygen's capacity to contribute a negative charge load in glasses. To classify the covalent/ionic ratios of the glass, the theoretical optical basicity may be used, because the increment in (Λ) values indicate the declining covalence. Table 6 indicates that the values of Λ are within the range 1.240-1.285 and found to increase with increasing Dy 2 O 3 concentrations. Here, the increment in optical basicity values means the ability of oxide ions to transfer electrons in the cations surrounding them [7]. Figure 9 represents the decline in the metallization criterion with an increase in the Dy 2 O 3 concentration. The obtained values confirm the non-metallic nature of the current glass samples [8]. From Table 3, the results show that the refractivity R M , α M , ε and optical dielectric constant increase with increasing Dy 2 O 3 , meaning an increase in NBOs inside the glass matrix. The PL spectra for the current samples of various Dy 3+ -doped compositions of (LAB) glass registered at room temperature in the wavelength region 420-720 nm below the excitation of the wavelength 375 nm are exhibited in Figure 10. The PL spectra for the current samples of various Dy 3+ -doped compositions of (LAB) glass registered at room temperature in the wavelength region 420-720 nm below the excitation of the wavelength 375 nm are exhibited in Figure 10. The PL spectra for the current samples of various Dy 3+ -doped compositions of (LAB) glass registered at room temperature in the wavelength region 420-720 nm below the excitation of the wavelength 375 nm are exhibited in Figure 10. It was noticed that the emission peaks' intensity increased gradually as Dy 3+ concentrations increased from 0.2 mol% to 1 mol%. The bands obtained in this present work are in agreement with the previous investigations [39]. Five emission peaks were spotted, containing two comparatively intense emission bands at nearly 481 and 575 nm, respectively, for the transitions 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2, and three considerably feeble bands at almost 458, 661 and 689 nm corresponding to the transitions 4 I15/2 → 6 H15/2, 4 F9/2 → 6 H11/2 and 4 F9/2 → 6 H9/2, respectively [40]. These transitions are similar to other study, where the transition 4 I15/2 level excites the Dy 3+ ions at band 458 nm [12]. The excited It was noticed that the emission peaks' intensity increased gradually as Dy 3+ concentrations increased from 0.2 mol% to 1 mol%. The bands obtained in this present work are in agreement with the previous investigations [39]. Five emission peaks were spotted, containing two comparatively intense emission bands at nearly 481 and 575 nm, respectively, for the transitions 4 F 9/2 → 6 H 15/2 and 4 F 9/2 → 6 H 13/2 , and three considerably feeble bands at almost 458, 661 and 689 nm corresponding to the transitions 4 I 15/2 → 6 H 15/2 , 4 F 9/2 → 6 H 11/2 and 4 F 9/2 → 6 H 9/2 , respectively [40]. These transitions are similar to other study, where the transition 4 I1 5/2 level excites the Dy 3+ ions at band 458 nm [12]. The excited Dy 3+ ions populate the 4 F 9/2 meta-stable state during rapid of non-radiative decay process due to the small energy gap between 4I 15/2 and 4 F 9/2 states [41]. The main higher band at 575 nm ( 4 F 9/2 → 6 H 13/2 ) in the yellow range of the visible spectrum is a supersensitive transition following the selection rules (∆J = ±2 and ∆L = ±2) [42].

Conclusions
In this study, new glass samples of the 23Li2O-7.5Al2O3-(69.5 − x) B2O3: xDy2O3 system were prepared using the melt-quenching technique. The amorphous nature of glass samples was confirmed by the XRD analysis. Notably, the density and optical basicity increased with the presence

Conclusions
In this study, new glass samples of the 23Li 2 O-7.5Al 2 O 3 -(69.5 − x) B 2 O 3 : xDy 2 O 3 system were prepared using the melt-quenching technique. The amorphous nature of glass samples was confirmed by the XRD analysis. Notably, the density and optical basicity increased with the presence of BO 4 tetrahedral units and, due to the structural changes, led to the decline in the direct and indirect energy gaps. From PL results, five emission bands were observed around at 458, 481, 575,661 and 689 nm, which are attributable to Dy 3+ transitions of 4 I 15/2 → 6 H 15/2 , 4 F 9/2 → 6 H 15/2 , 4 F 9/2 → 6 H 13/2 , 4 F 9/2 → 6 H 11/2 and 4 F 9/2 → 6 H 9/2 , respectively. The 576 nm ( 4 F 9/2 → 6 H 13/2 ) band is the largest. Hence, optical properties and other physical parameters, such as refractive index, density, molar volume, molar refractive, electrical polarisation and optical basicity, show a strong connexion with the speciation of dysprosium ions.