Luminescence Features of Eu₂O₃-Doped Antimony Borate Glasses with High Quantum Efficiency

Abstract

Boro-antimonite glasses doped with Eu³⁺ and having the general composition (90-x) Sb₂O₃–xB₂O₃–10Li₂O-0.5Eu₂O₃ (x = 0 to 60 in 10 mol. % increment) were prepared using the melt quenching method. The influence of B₂O₃/Sb₂O₃ substitution on the spectroscopy and photoluminescence of Eu³⁺ ions was analyzed by studying the measured and calculated properties of these glasses. The relative value of a given property was shown to increase or decrease by up to 26% with the addition of up to 60 mol. % B₂O₃, while the number of Eu³⁺ ions per unit volume increased by approximately 32%. Strong emissions were obtained in association with the transitions of Eu³⁺ (⁵D₀→⁷F_j, j = 1–4). A weak, broad emission centered at 450 nm was also detected. This emission is clearly linked to the glass composition. It originates from a potential presence of Eu²⁺ ions. This enhances ⁵D₀ level emission via charge transfer. The radiative and experimental lifetimes of the ⁵D₀ level increase linearly with B₂O₃ content. This results in high quantum efficiency (η) ranging from 74 to nearly 84%. Tunable chromaticity, as defined by the CIE 1931 standard, was achieved, resulting in a warm orange-red color with high brightness. These new glasses have a variety of potential laser-related applications.

1. Introduction

Luminescent materials such as Rare Earth (RE)-doped phosphors or glasses are of great interest due to their applications in various fields such as telecommunications, photovoltaics, display devices, modern lighting technology, optical fibers, mid-infrared lasers, and solid-state lasers [1,2,3,4,5]. The optical properties of RE ions in glasses are strongly influenced by the chemical composition, the structure, and the nature of the bonds in the host matrix [6,7]. Among rare earth ions, europium oxide is one of the most used ions due to its strong red emission originating from the ⁵D₀→⁷F₂ transition [8,9,10]. In addition, the ⁵D₀→⁷F_j (j = 1–5) line emissions can be adjusted to produce a combination of green, orange and red light, which has long been used in glasses and phosphors [11,12]. In addition, blue emission can usually be produced by Eu²⁺ from the reduction of Eu³⁺, resulting in a broad emission band extending from ultraviolet to blue, which corresponds to 4f⁶5d→4f⁷ transition [13,14,15], enabling a tunable white emission in LED display devices.

Due to its unique characteristics, Eu³⁺ exhibits intense emission and excitation lines, as well as hypersensitivity of the ⁵D₀→⁷F₂ electric dipole. This provides more information about the local symmetry of the site than the absorption spectrum. Furthermore, the ⁵D₀→⁷F₁ magnetic dipole transition is independent of the Eu³⁺ ion environment and is used to calculate the asymmetric ratio (R) in order to interpret the covalence and local symmetry of the ligand surrounding Eu³⁺ ions [16,17,18,19].

Among traditional oxide glasses, borate glass is a suitable host for RE elements, particularly Eu³⁺, due to its high transparency in the UV-Vis region, low melting point, high thermal stability, and good solubility of RE ions, but it has high phonon energy [20,21].

On the other hand, Sb₂O₃, an unconventional glass former [22] with low phonon energy (600 cm⁻¹) [23], as evidenced by its wide transmission window (approximately 0.35–7.5 µm) [24], is an emerging glass of great interest due to its high density, high refractive index [25], low melting and glass transition temperature, high linear thermal expansion, and high nonlinear refractive index [26,27]. Several studies have considered Sb₂O₃ as a promising host for RE elements [6,28,29,30].

This paper presents the results of a systematic study of mixed antimony borate glasses modified with lithium oxide and doped with europium. Combining the two glass formers yields different phonon energies and a bonding environment that significantly influences the radiative and non-radiative processes of the europium ions. This study focuses on the optical properties of (90-x) Sb₂O₃-xB₂O₃-10Li₂O doped with 0.5 (mol. %) of Eu³⁺ ions. Specifically, the study examines how B₂O₃ substitution influences optical absorption, excitation, emission, fluorescence decay lifetime, and Eu³⁺ quantum efficiency. The results are promising for a versatile, tunable laser light emitter.

2. Materials and Methods

Ternary oxide glasses in the chemical composition (90-x) Sb₂O₃–xB₂O₃–10Li₂O doped with 0.5 (mol. %) of Eu³⁺ ions were prepared by the melt-quenching technique, where x = 0, 10 to 60 by a step of 10 (mol. %). The seven glasses are referred to as SLB0, SLB1 …, as mentioned in

Table 1. Glass ID, characteristic temperatures T_g, T_x, T_p and glass stability ΔT of SLB glasses.

Glass ID	Composition (mol%)				Tg	Tx	Tp	ΔT
	Sb2O3	B2O3	Li2O	Eu2O3	(°C)
SLB0	89.5	0	10	0.5	268	414	467	146
SLB1	79.5	10	10	0.5	275	434	473	159
SLB2	69.5	20	10	0.5	279	439	484	160
SLB3	59.5	30	10	0.5	293	-	-	-
SLB4	49.5	40	10	0.5	302	-	-	-
SLB5	39.5	50	10	0.5	316	-	-	-
SLB6	29.5	60	10	0.5	327	-	-	-

. The starting materials used in the preparation of these glasses are analytical purity grades: Sb₂O₃ (supplied by Aldrich (Aldrich Chimie, St. Quentin Fallavier, France), 99.9%), B₂O₃, Li₂CO₃ (supplied by Biochem (Biochem chemopharma, Cosne Sur Loire, France), 99%), and Eu₂O₃ (provided by Sigma-Aldrich, Darmstadt, Germany, 99.5%). Moreover, 8 g of mixed powders were thoroughly mixed in an agate mortar and melted in borosilicate glass tubes with a diameter of 15 mm. The tubes were then flame-heated between 750 and 1000 °C for 10 to 15 min until a clear homogeneous liquid formed. The melted liquid is poured into a preheated brass mold below its glass transition temperature (T_g) at T_g-20 °C and then placed in an electric oven for 5 h to release the thermal stresses introduced by the quenching process. The glass is then gradually cooled to room temperature. Glasses containing concentrations greater than 60% B₂O₃ have high viscosities and could not be produced in this way. Finally, the samples were polished to a thickness of about 1 mm suitable for optical measurements. XRD scans of the produced glasses were collected using a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (wavelength 1.5418 Å). The characteristic temperatures of these glasses are, namely, the temperature of glass transition (T_g), the temperature of onset of crystallization (T_x), and the temperature of the peak of crystallization (T_p). They were determined using alumina pans from room temperature to 500 °C using differential scanning calorimetry (DSC) (SETARAM, LABSYS evo, Sophia Antipolis, France). The absorption spectra of the glasses were measured using a Perkin Elmer Lambda 35 UV-Vis spectrophotometer (PerkinElmer, Waltham, MA, USA) with 1 nm resolution in the 200–1100 nm range. The Perkin Elmer Spectrum Two (PerkinElmer, Shelton, CT, USA) was used to measure both the absorption spectra with 1 nm resolution in the 1.8–10 µm near infrared red (NIR) range as well as the ATR/FTIR spectra for the structural analysis within the range [4000–400 cm⁻¹]. The density (ρ) of the glass was measured using Archimedes’ principle with an Ohaus Adventurer AX type microbalance (OHAUS Europe GmbH, Nänikon, Switzerland). (ρ) is measured with an accuracy of ±0.002 g/cm⁻³. The refractive index was measured using a METRICON Model 2010/M (Metricon Corporation, Pennington, NJ, USA) at 632.8 nm wavelength. Photoluminescence spectra were collected using a Horiba FLUROMAX-4 spectrofluorometer (Horiba, Piscataway, NJ, USA). A 150 W, Ozone-free xenon arc lamp is used as an excitation source within the 250–850 nm range. Emission is detected within the 350–850 nm range. Excitation and emission spectra were acquired using a 2 nm slit width and a1 nm interval for both signals. The fluorescence decay curves were measured by monitoring the emission at 612 nm with an excitation wavelength of 361 nm. The chromaticity coordinates and color parameters of the emitted light were determined using the 1931 Commission Internationale de l’éclairage (CIE 1931) standard with freely available software “LED Color Calculator” version 7.23 edited by OSRAM, Premstaetten, Austria.

3. Results and Discussion

3.1. Thermal Properties

The prepared glasses are pure vitreous. Figure 1a shows the XRD scan of SLB0 and SLB1 glass, for example. These scans reveal an absence of crystalline peaks and show only a halo at 2θ = 30°, characteristic of a vitreous structure. The DSC scans of SLB glasses are illustrated in Figure 1b. The characteristic temperatures T_g, T_x and T_p are measured with accuracies of ± 2, ± 2, and ±1 °C, respectively. They are listed in Table 1. Depending on the glass composition, T_g and the thermal glass stability factor ΔT of SLB glasses increase linearly with increasing B₂O₃. Replacing Sb₂O₃ with B₂O₃ gradually increases the stability factor of the glass. This factor increases from 146 °C for borate-free glass to more than 160 °C for glasses with a higher B₂O₃ content. This is evident in the broadening and subsequent disappearance of the crystallization peak in glasses containing more than 20 (mol. %) B₂O₃. This indicates that crystallization does not occur, meaning the glass is thermally stable and can be used for drawing optical fibers [24].

3.2. Physical Properties

3.2.1. Optical Properties

In addition to the measured physical properties of density (ρ) and molar volume (V_m), some other important physical parameters of these glasses are also calculated using well-known appropriate expressions [23]. The number of Eu³⁺ ions per unit volume (ions.cm⁻³) is calculated using the following equation:

$$N=N_a\times \rho \times x\times E{u}^{3+}/M$$

where N_a is Avogadro’s number, ρ is the density of the glass, M is its average molecular weight, and x is the Eu³⁺ concentration. N was used to calculate the polaron radius (r_p), using the relation

$$r_p=1/2{(\pi /6N)}^{1/3}.$$

The average separation of Eu³⁺ ions (r_i) is derived from the expression

$$r_i=N^{-1/3},$$

And the field strength F is calculated using the relation

$$F=Z/r^2$$

where Z is the charge of the ion and r is the Eu³ radius.

Table 2. Physical properties of SLB glasses and their relative ratio Δ = SLB6/SLB0.

Physical Properties	SLB0	SLB1	SLB2	SLB3	SLB4	SLB5	SLB6	Δ (%)
d (g/cm−3)	4.976	4.833	4.626	4.383	4.138	3.822	3.816	−23.31
Vm (cm3 mol−1)	53.388	51.419	49.744	48.220	46.549	45.573	40.394	−24.33
N × 1020 (ions cm−3)	1.128	1.171	1.211	1.249	1.294	1.322	1.491	32.18
rp (Å)	8.339	8.236	8.144	8.061	7.966	7.910	7.599	−8.87
ri (Å)	20.697	20.440	20.212	20.005	19.771	19.630	18.859	−8.88
F × 1014 (cm−2)	4.314	4.423	4.523	4.617	4.728	4.795	5.195	20.42

summarizes all the calculated values for these parameters.

An increase in the content of boron oxide results in a reduction in both density and molar volume. The density is governed by the addition of either light or heavy elements; the density of B₂O₃ is 1.797 (g·cm⁻³) is lower than that of B₂O₃ 5.054 (g·cm⁻³) [27]. More boron oxide decreases density. The decrease in molar volume can similarly be related to the decrease in the ionic radius between Sb and B. Furthermore, a decrease in the molar volume implies proportional reductions in bond length, the interatomic distance between atoms, the polaron radius, and the interionic distance. Consequently, replacing antimony oxide with boron oxide should increase compactness as boron oxide is a strong glass-former that creates strong atomic bonds. It also generates more bridging oxygen (BO) which increase the glass rigidity and the glass transition temperature T_g [27]. As a result, a considerable increase in the glass viscosity is observed during the melting of boron-rich samples.

According to the glass composition, the molar concentration of europium oxide is 0.5 (mol. %) for all glasses. Increasing the amount of boron oxide instead of antimony oxide reduces the molar volume of the glass, allowing more Eu³⁺ ions to be incorporated into the glass per unit volume. Consequently, the number of Eu³⁺ ions increases from 1.128 to 1.491 (ions/cm³). Furthermore, as rare earth ions generally occupy interstitial sites in the glass structure, the decrease in average distance between rare earth and oxygen corresponds to an increase in Eu-O bond strength and leads to an increase in the field strength around Eu³⁺ ions [31]. In summary, the change involved in the gradual replacement of Sb₂O₃ by up to 60 (mol. %) of B₂O₃ led to a decrease in the relative value of all properties up to nearly 24%, while the number per unit volume of Eu³⁺ ions increased by about 32% and by 20% for the field strength F.

3.2.2. Structural Analysis

Figure 2a shows the ATR-FTIR transmittance spectra of Eu³⁺-doped Sb₂O₃-B₂O₃-Li₂O glasses within the 4000–400 cm⁻¹ range. Boron oxide-based glasses consist of three-coordinated boron atoms that form triangular units [BO₃] as well as four-fold coordinated [BO₄] tetragonal units. Three [BO₃] bonded by bridging O (BO) form boroxol rings. However, in the negatively charged [BO₄] unit, the fourth oxygen atom (non-bonded oxygen or NBO) is shared with a modifier element, such as an alkaline element, to fully balance its charge. The spectra of SLB glasses can be divided into three regions [32]. The first region between 1500 cm⁻¹ and 1150 cm⁻¹ is due to the B–O vibrational modes of [BO₃] units. The second region between 1150 cm⁻¹ and 800 cm⁻¹ is attributed to the B–O vibrations of [BO₄] units. The third region between 800 and 400 cm⁻¹ relates to the vibrational modes of Sb₂O₃ according to Terashima et al. [27]. The vibrational spectra of vitreous antimony oxide exhibit four absorption bands that correspond to the vibration mode of the [SbEO₃] trigonal pyramid in C_s, or C₁ symmetry. Here, E denotes the lone pair of electrons of Sb₂O₃. This configuration is characteristic of the valentinite form of Sb₂O₃. The four bands are positioned at: 590 cm⁻¹ ν₃ (asymmetric stretching), 470 cm⁻¹ ν₄ (asymmetric bending), 690 cm⁻¹ ν₁ (symmetric stretching), and 540 cm⁻¹ ν₂ (symmetric bending) [27]. In SLB glasses, the first two observed vibrational bands at 456 and 560 cm⁻¹, and the two shoulders at 600 (not clearly observed) and 700 cm⁻¹ correspond well to the vibration modes of the [SbEO₃] unit.

As the B₂O₃ content increases, the bands related to Sb₂O₃ decrease progressively while B₂O₃-new related bands emerge in the first and second regions. The short band within 763 to 770 cm⁻¹ is due to the asymmetric bending vibrations of BO₃-O–BO₄ [32]. This band appears when the B₂O₃ content exceeds 20 (mol. %). In region two, the band at 902 cm⁻¹, results from the asymmetric stretching vibration of B-O-B in [BO₄] [32]. The band at 1079 cm⁻¹ is assigned to the vibration of pyroborate and the B–O bond in [BO₄]. The large band around 1208 cm⁻¹ is related to the B-O stretching vibrations of [BO₃] units in boroxol rings, and the band at 1314 cm⁻¹ is related to the B-O stretching vibrations of [BO₃] units in metaborate, pyroborate, and ortoborate groups. Finally, the bands at 2930 cm⁻¹ and 3350 cm⁻¹ are related to the CO₂ of the atmosphere and the OH vibration of the raw materials, respectively [32]. Analysis using ATR-FTIR reveals that the Eu³⁺-doped SLB glass structure consists of [SbEO₃], [BO₃], and [BO₄] units.

The following equations are often used to quantify the number of [BO₄] and [BO₃] structural units in Eu³⁺-doped SLB glasses [12]: N₄ = A₄/(A₄ + A₃) and N₃ = A₄/(A₄ + A₃). A₃ represents the fraction of the relative area of the vibrational bands of [BO₃] within the 1150–1500 cm⁻¹ region, and A₄ represents the fraction of the relative area of the vibrational bands assigned to [BO₄] within the 1150–800 cm⁻¹ region. The results of this analysis are illustrated in Figure 2b. There are always more [BO₃] units than [BO₄] units. As the B₂O₃ content in the glasses increases, the number of [BO₃] units rises linearly from 0.53 to 0.77. Conversely, the number of [BO₄] unit decreases from 0.47 to 0.23.

3.2.3. Transmission Spectra

The transmission spectra of SLB glasses doped with 0.5 (mol. %) Eu₂O₃ are measured in the UV-VIS region and are shown in Figure 3a. It is clear that the addition of boron oxide, a good traditional glass former, significantly increases the transparency of the antimony-based glass [8]. Thus, the cut-off value shifts from 384 nm for SLB0 glass to 325 nm in the UV region for SLB6 glass. Furthermore, the intensity of transmitted light of all glasses increases to 85% for higher B₂O₃ content across the entire range [400–1100 nm]. This is mainly due to a decrease in the reflection losses of the glasses caused by the decrease in the refractive index. The spectra reveal a few weak absorption bands in the visible range at 394, 465, 526, and 579 nm, related to the band transitions of Eu³⁺ ions well dissolved in these glasses. As shown in Figure 3b, the transmission cut-off, also known as multiphonon cut-off, moves from around 7.3 µm for SLB0 glass to around 2.4 µm for SLB6 glass in the infrared spectrum. The OH vibration band occurs at 3 µm for SLB0 glass. However, when boron oxide is added, it overlaps with B-O vibration bands. This overlap is clearly observed in SLB6 glass. The B-O vibrational band at 3960 and 4570 nm is caused by the borosilicate crucible that was used to make SLB0 glass. They are attributed to the overtone of [BO₃] and [BO₄] structural units, which occur at 1292 and 1094 (cm⁻¹) respectively [32]. However, the band at around 5500 nm is related to the second overtone of the Si-O vibration (910 cm⁻¹) of the dissolved silica (SiO₂) in the glass from the crucible [22].

3.2.4. Absorption Spectra

Important optical parameters are quantified by measuring the absorption spectrum in the UV-Vis-NIR range at room temperature. Figure 4a–c, for example, show the absorption spectra of SLB6 glass in different optical regions. Several absorption bands that are associated with the electronic transitions of Eu³⁺ can be seen [33]. They are clearly very narrow and appear with very low intensities, particularly at low B₂O₃ content. The absorption bands observed are related to the transitions from the ground state ⁷F₀ and the first thermally populated excited state and ⁷F₁ to the following higher excited states: ⁷F₀ to ⁵D₄ (361 nm), ⁵G₂ (375 nm), ⁵L₆ (395 nm), ⁵D₂ (465 nm), ⁵D₁ (526 nm) and ⁵D₀ (579 nm) and from ⁷F₁ to ⁵D₁ (533 nm) and ⁵D₀ (588 nm). Additionally, SLB6 glass exhibits a very weak band at 330 nm due to its high transparency in the UV range. This is attributed to ⁷F₁→⁵H₃ transition. All these transitions are spin-forbidden and therefore very weak. In the NIR regions, two other bands are clearly observed and are assigned to ⁷F₀→⁷F₆ and ⁷F₁→⁷F₆ transitions. They are spin-allowed and much stronger, as seen in Figure 4c. When the concentration of B₂O₃ is less than 40 (mol. %), the optical absorption edge of the host glass increases rapidly, and all absorption bands below 380 nm are masked. The appearance of all these bands confirms an effective absorption of Eu³⁺ ions in these glasses [33,34]. All bands in the spectrum show absolutely no apparent shift of less than 1 nm in the positions of the absorption peaks, but a clear increase in their intensity directly related to the increase in the number of Eu³⁺ ions per unit volume in each glass as the B₂O₃ content increases.

3.2.5. Optical Parameters

At high absorption levels (greater than 10⁴ cm⁻¹), the optical band gap (E_g) is often estimated from UV-Vis absorption spectra using the Tauc relation [8]. This relation plots (αhυ) against energy (hυ) in a linear manner:

$$(\alpha h\upsilon )=A{(h\upsilon -E_g)}^r/h\upsilon$$

where α is the absorption coefficient, hυ is the photon energy, A is an energy-independent constant, and E_g^ind is the optical band gap. r = 1/2 for an allowed direct transition, generally used for semiconductor materials. Indirect transitions in amorphous and glassy materials require momentum conservation, necessitating the presence of both a photon and a phonon. This results in a more gradual absorption front. Therefore, r = 2 is used and E_g = E_g^ind. Figure 5a plots the indirect energy band gap (α$h\nu$) versus energy hυ. The presence of a linear shape indicates an indirect optical transition. Then, linear extrapolation is used to determine E_g^ind by fitting the linear part to α = 0 with a 0.01 fitting accuracy. The fundamental absorption edge generally follows the Urbach rule at absorption levels below 10⁴ cm⁻¹. This can be estimated by the following equation [8]:

$$A(\upsilon )=B\exp (h\upsilon /\Delta E).$$

where ∆E is the Urbach energy and B is the band tailing factor, which is considered constant. ∆E is calculated using $\log \left(\mathrm{A}\right(\upsilon )$. As illustrated in Figure 5b, the inverse of the linear slope extrapolated to $\upsilon =0$ g divided by ln(e) gives the Urbach energy. An average of 0.008 eV was used to fit a linear extrapolation.

According to Brik et al. [35] and to Chialanza et al. [36], the optimal range for extracting data from each spectrum’s absorption edge was selected to improve accuracy. The E_g^ind values presented in

Table 3. Optical properties of SLB glasses and their relative ratio Δ = SLB6/SLB0.

Optical Properties	SLB0	SLB1	SLB2	SLB3	SLB4	SLB5	SLB6	Δ (%)
Cut off ±1	384	375	363	358	349	334	325	−15.36
Energy band gap $E_g^{ind}$ (eV) ± 0.01	2.73	2.96	3.19	3.19	3.31	3.40	3.44	25.93
Urbach energy (eV) ± 0.008	0.177	0.156	0.151	0.146	0.143	0.140	0.138	−22.00
Refraction index (n) ± 0.04	2.10	2.05	2.01	2.00	1.97	1.96	1.95	−6.94
Reflection losses R (%)	12.30	11.83	11.31	11.15	10.78	10.56	10.44	15.12
Dielectric constant ε	4.414	4.198	4.056	4.012	3.913	3.853	3.822	−13.41
Optical electronegativity (∆χ*)	1.199	1.263	1.308	1.322	1.356	1.376	1.387	15.67
Optical basicity (Λth)	1.101	1.069	1.046	1.039	1.022	1.012	1.007	−8.53

and illustrated in Figure 5c were found to increase from 2.73 to 3.44 eV. This is consistent with the values observed for borate glasses [12,35]. An increase in E_g^ind value as B₂O₃ replaces Sb₂O₃ indicates a change in the glass structure in the present ternary system. This results in greater separation between the valence and the conduction bands, indicating a more rigid, less polarizable glass structure. This change involves shifting from non-bridging oxygen (NBO) to bridging oxygen (BO). This shift is associated with an increase in [BO₃] units at the expense of [BO₄] units, thereby enhancing network connectivity and shifting the UV-Vis transmission cut-off toward lower wavelengths. This is in accordance with the previous optical and structural study. Conversely, as illustrated in Figure 5c, the ∆E decreases from 0.169 to 0.140 ev. This indicates variation of the band tail with B₂O₃ content. Replacing Sb₂O₃ by B₂O₃ in Eu³⁺-doped SLB glasses, decreases the Urbach energy, indicating a more ordered glass lattice with fewer localized states. This improvement in order results in better optical quality and thermal stability. Boron oxide forms rigid and more connected networks than antimony oxide. A regular increase in B₂O₃ forms more [BO₃] units than [BO₄] units. This reduces the non-bonded oxygen and increases polymerization. The result is a sharper absorption threshold and lower ∆E [8,35].

According to [37], the refraction index (n) of oxide compounds can be estimated based on E_g^ind values following the relation:

$${\displaystyle \frac{n^2-1}{n^2+2}}=\sqrt{{\displaystyle \frac{20}{E_g^{ind}}}}.$$

Several other parameters are also calculated, including the reflection losses percentage R (%), the dielectric constant ε, the optical electronegativity (∆χ*), and the optical basicity (Λth). These parameters had previously been described using standard formulas [31]. As shown in Table 3, the decrease in refractive index and optical basicity (Λth) with increasing B₂O₃ is due to a reduction in the number of large and highly polarizable Sb³⁺ ions, being replaced by less polarizable boron ions. Consequently, reflection losses and the dielectric constant, which depends on the refractive index, exhibit a similar trend.

The magnitude of optical electronegativity (∆χ*) is a direct indicator of the nature of chemical bonds in materials. Optical electronegativity is high in an ionic medium and decreases as the covalence of the glass increases [31]. Examining Table 3, the various optical parameters reveal that gradually replacing up to 60 (mol. %) of Sb₂O₃ with B₂O₃ increases E_g^ind by 26% and ∆χ* by 15.6%. Conversely, all other properties decreased between 7% and 15%, and the Urbach energy is reduced in SLB6 glass to 22%. ∆E from 0.177 to 0.138 ev.

As previously reported [26,27], the relatively high refractive index and polarisability of antimony borate glasses intensify the effect in nonlinear optics. These glasses show great promise for use in all-optical switching and nonlinear optical devices when combined with Eu³⁺ emissions.

3.3. Photoluminescence Study

3.3.1. Excitation Spectra

The excitation spectra of Eu³⁺-doped SLB glasses measured in the range [300–600 nm] by monitoring the emission signal at 612 nm are illustrated in Figure 6. All observed bands correspond well with the typical excitation lines of Eu³⁺ [33,34]. They are listed in the following order: 361, 381, 394, 414, 464, 525, 533, 578, and 588 nm and assigned respectively to the transitions ⁷F₀→⁵D₄, ⁷F₀→⁵G₂, ⁷F₁→⁵L₆, ⁷F₀→⁵D₃, ⁷F₀→⁵D₂, ⁷F₁→⁵D_1, ⁷F₀→⁵D₁, ⁷F₁→⁵D₀, and ⁷F₀→⁵D₀. As seen in the inset of Figure 6, SLB glasses (4–6) exhibit additional excited state levels in the form of a broad band containing two visible but very weak bands at 318 and 326 nm corresponding to the ⁷F₀→⁵H₄ and ⁷F₁→⁵H₃ transitions, respectively. Further interpretations will be discussed in the following sections. Clearly, the position of the different bands remains practically unchanged as B₂O₃ concentration increases; no shift is observed. Conversely, a gradual increase in the intensity of all peaks is evident. It is suggested that the addition of B₂O₃ alters the luminescent properties of the Eu³⁺ ions, potentially by modifying their environment or emission-level population rather than their electronic structure [38].

The most intense bands are observed at 394 and 464 nm due to the relatively strong electric dipolar nature, which is permitted at asymmetric sites. These wavelengths are generally considered to be the most efficient for exciting Eu³⁺ ions and are frequently used in photoluminescence studies of europium-doped materials [16]. As we will see later, the excitation spectra clearly show very low intensity levels alongside higher excited state levels of Eu³⁺ (⁵L₆ and ⁵D_1,2,3). They are uncommon and are typically only found in low phonon glasses, such as fluoride and germanate glasses [33,34]. Their presence at room temperature confirms the radiative efficiency of Eu³⁺ ions in these glasses.

To acquire emission spectra across the entire visible range [400–780 nm], a pumping wavelength of 361 nm in the NUV region was chosen. This avoided the need for chromatic filter accessories, which would have limited the measured wavelength range. From an experimental point of view, it is important to note the following: as shown in

Table 4. Relative intensity ratios SLBx/SLB0 (%) at different excitation wavelengths.

λExc (nm)	SLB1/SLB0	SLB2/SLB0	SLB3/SLB0	SLB4/SLB0	SLB5/SLB0	SLB6/SLB0
361	2	7	40	89	130	227
394	5	9	20	26	30	38
465	2	3	4	5	5	6

, the 361 nm wavelength has the highest relative intensity (A) of the entire wavelength listed. (A) is given by the ratio A = I_SLBx/I_SLB0, where x represents the samples (1 to 6). Surprisingly, this ratio increases drastically when x is greater than 2. The SLB (3, 4, 5, and 6) glasses become highly excited. This indicates that an increase in the B₂O₃ content in the glass leads to a considerable rise in the population of Eu³⁺ levels at wavelengths below 465 nm in response to excitation at 361. This effect is less pronounced at 394 and 465 nm.

3.3.2. Emission Spectra

Figure 7a shows the emission spectra of SLB glasses when excited at a wavelength of 361 nm within the range of 400–730 nm. Five clear peaks are visible at 577 (yellow), 590 (orange), 612 (red), 648 (dark red), and 697 (dark red) nm. These peaks are generated by intra-configurational transitions from the excited ⁵D₀ state to the ⁷F_j (j = 0 to 4) ground states. This is a characteristic feature of Eu³⁺ ions in low symmetry [16,39]. Additionally, two weak bands in the green region are observable at 533 and 553 nm, which are associated with⁵D₁→ (⁷F₁ and ⁷F₂) transitions, respectively. Furthermore, emissions from the ⁵D₀→⁷F₅ transitions at 740 nm and the ⁵D₀→⁷F₆ transition at 810 nm produce very faint emissions. This feature is uncommon, but it has been observed in low-phonon fluoride, germinate, and the present glasses [33,34,39]. Finally, a very weak, broad emission was observed in the blue region (420–520 nm) as shown in the inset of Figure 7a. The position of all observed emission bands does not change with composition.

As seen in Figure 7b, the normalized emission spectra are similar for all glasses except for the blue emission, which decreases as the concentration of B₂O₃ increases. However, as seen in Figure 7a, the intensity of the Eu³⁺ emission lines increases considerably with B₂O₃ concentration. The total intensity of Eu³⁺ emissions in SLBx glasses increases when the B₂O₃ content is higher than x = 30 (mol. %). Figure 8 shows that the integrated intensity of the ED (612 nm) and MD (590 nm) emissions increased simultaneously by a factor of two, three, and six for SLB (1–3) samples and 30, 50, and 70 times for SLB (4–6) compared to the SLB0 sample. As the B₂O₃ concentration increased, the intensity (I_blue) of the blue emission increased more slowly than that of the f-f lines emission of Eu³⁺. Conversely, the integrated emission I_blue/I₆₁₂ ratio was found to decrease progressively: 0.1, 0.08, 0.04, and 0.015 for SLB (0–3) glasses and 0.008, 0.006, and 0.004 for SLB (4–6) glasses. Figure 8 shows the ratio of this.

As reported previously [16,39], the ⁵D₀→⁷F_J emission lines provide insight into the asymmetry, local environment, and field strength surrounding Eu³⁺ ions in various hosts. The low intensity ⁵D₀→⁷F₀ and ⁵D₀→⁷F₃ lines are forbidden electric dipole transitions that occur via J mixing and only appear in a strong local field. The ⁵D₀→7F₀ line occurs only in weak C_n, C_nv, or C_s symmetry, meaning it cannot be split by the crystal field into more than one peak. The ⁵D₀→⁷F₁ line is a magnetic dipole transition that is largely independent of the surrounding Eu³⁺ ions; however, it becomes dominant in a high-symmetry environment. The splitting of the ⁵D₀→⁷F₁ emission lines into three lines suggests that Eu³⁺ ions occupy a single site. Conversely, the hypersensitive electric dipole transition ⁵D₀→⁷F₂ dominates the ⁵D₀→⁷F₁ line when Eu³⁺ occupies an inversion symmetric site under high field strength. The ⁵D₀→⁷F₄ emission, which occurs at 697 nm, is environment-dependent, but not hypersensitive.

3.3.3. Asymmetric Ratio

The asymmetric ratio defined as R = ⁵D₀→⁷F₂/⁵D₀→⁷F₁ can provide valuable information about the covalence and local symmetry of the ligands surrounding Eu³⁺ ions [16]. While it can typically be calculated using the respective emission intensity ratios. The integrated intensity line ratio is preferable for greater accuracy as it considers the additional intensity contributions arising from the splitting of the two emission lines in an asymmetric environment [16].

R values were found in the range 3.224–3.024. This corresponds to a scale of 1 to 7 in an asymmetric oxide glass environment [17,39]. These are summarized in Table 5.

This indicates that the environment of Eu³⁺ in these glasses is asymmetrical [33]. The increase of R with respect to B₂O₃ content (from 0 to 10 (mol.%) is most likely due to environmental changes resulting from [BO₄] and [BO₃] structural units being incorporated into the glass alongside existing Li⁺ and Sb³⁺ ions. Any Further increase in B₂O₃ content alters the [BO₃]/[BO₄] ratio. This occurs through a decrease in [BO₄] units and an increase in [BO₃] units. This increases the rigidity of the glass by replacing weaker SbO₃ units with stronger [BO₃] units. Consequently, above 10 (mol. %) of B₂O₃, the R ratio slightly decreases to 3.024. This non-monotonic behavior indicates the stabilization of an asymmetric local environment characterized by strong dominant covalent bonds as well as a structural crossover within the borate-antimonate lattice.

3.3.4. Origin of the Blue Emission

The high optical basicity of these air-synthesized glasses coupled with the strong oxidizing property of Sb₂O₃ and its low temperature melting (below 1000 °C) should stabilize the third trivalent state of europium, Eu³⁺, rather than allowing it to be reduced to the divalent state, Eu²⁺. This typically manifests as a broad excitation band located in the region [250–350 nm]. Figure 9a,b shows the excitation spectra of SLB0 and SLB6 glasses, respectively. Spectra were monitored at 460 and 490 nm within the blue emission range. Then, these spectra were compared to those monitored at 612 nm. Monitoring the emission at 460 nm for SLB0 glass reveals a broad, weak band extending from 250 to over 430 nm in the excitation spectrum. Two weak red peaks were present at 281 and 341 nm. These peaks are not attributed to Eu³⁺ but rather to the 4f⁶5d→4f⁷ transition of Eu²⁺ [15]. While monitoring the emission spectrum at 460 nm and 490 nm in SLB6 glass, we observed a weak but large band extending from 250 to over 430 nm. This large band has also been observed in Eu-doped CABAL glasses containing 50 (mol. %) of B₂O₃ that were synthesized in a reduced atmosphere [15]. This band has been associated with Eu²⁺. However, no evident peaks are observed. Conversely, a large intense band is observed from 300 to 350 nm, which is clearly seen when the emission spectra is monitored at 612 nm. Two peaks related to Eu³⁺ are observed within this large band. These large bands can also be attributed to the 4f⁶5d→4f⁷ transition of Eu²⁺. It can be concluded that the blue emission band is primarily caused by the low concentration of reduced Eu²⁺. As shown in Figure 9c, the emission spectra confirm that blue and ⁵D₀ emission lines can be obtained when the glass is excited at 341 nm. However, when excited at 282 nm, the ⁵D₀ emission lines decrease significantly, whereas the intensity of the blue emission remains unchanged. The results suggest that energy transfer may occur from Eu²⁺ to Eu³⁺. Additionally, blue emission can be produced in SLB0 glass with a tunable excitation wavelength between 280 and 350 nm. Figure 9d shows that blue emission from SLP0 glass occurs within the 280–350 nm excitation wavelength range. This is evident in the 3D image. Intense emissions are observed at the following wavelengths: 281, 310, and 341 nm.

3.3.5. Fluorescence Decay Analysis

The fluorescence decay profiles of the ⁵D₀ level were obtained by analyzing the emission line of the ⁵D₀→⁷F₂ transition. This transition corresponds to the most intense emission, which occurs at 612 nm when the glass is excited at 361 nm. Figure 10 (inset) shows the decay profiles of SLB glasses. Examining these profiles on a logarithmic scale reveals that they all align with a single exponential law as depicted in Figure 10. The experimental lifetime (τ_Exp) is determined by fitting an exponential profile to the curves. In contrast, the radiative lifetime (τ_rad) of the ⁵D₀ level of Eu³⁺; ions can be easily calculated from emission spectra using the Judd-Ofelt (JO) method [18,19]. This is based on the following formula [40].

$$(1/{\tau }_{rad})=14.65\times n^3\times I({}^{5}D_0\to {}^{7}F_j)/I({}^{5}D_0\to {}^{7}F_1).$$

where n is the refractive index. I(⁵D₀→⁷F_J) represents the total integrated emission from the ⁵D₀→⁷F_J manifold (J = 0–4), and I(⁵D₀→⁷F₁) represents the integrated intensity of the transition ⁵D₀→⁷F₁. The luminescence quantum efficiency $(\eta )$ is defined as the ratio of experimental lifetime to the radiative lifetime [33]. Also known as the internal quantum efficiency, it is the measure of the number of photons emitted by an excited ion. It expresses the ratio of non-radiative losses to radiative decay processes. This can be expressed using the following equation [40]:

$$\eta (\%)={\displaystyle \frac{{\tau }_{Exp}}{{\tau }_{rad}}}\times 100\%$$

Table 5 summarizes the results of τ_rad, τ_Exp and η. The results are summarized in Table 5. Τhe experimental lifetime of the ⁵D₀ level of Eu³⁺-doped SLB glasses increases linearly when boron oxide is added, ranging from 1.01 to 1.87 ms. The increase in the lifetime in parallel with the increase in 612 nm emission is related to the following points: Reduced non-radiative de-excitation, greater symmetry surrounding Eu³⁺ ions, and more efficient energy transfer. These factors result in more intense and longer-lasting red emissions, which often indicate suitability for use in red phosphors in LED or displays. Similarly, as the boron oxide content increased from 0 to 60 mol. %, the radiative lifetime increased linearly from 1.17 to 1.65 ms; this resulted in values that were almost higher than those of τ_Exp for all the prepared glasses. The calculated values of η were found to be already very high for SLB (0 to 2) glasses at around 80%. These values increased rapidly for boron oxide-rich glasses, reaching 92, 98, and 113% for SLB (3 to 6) glasses, respectively.

The obtained quantum efficiency values are high, with SLB6 glass exceeding 100%. This is related to the high refraction index values calculated using the relation based on the band gap energy. To address this anomaly, we measured the refractive index using METRICON Model 2010/M at a 632.8 wavelength. The results are displayed in Table 5. The measured refractive index (n₂) values are lower than the calculated values (n₁). As the B₂O₃ content increases, the deviation in refractive values also increases, reaching a maximum deviation of 0.2. Consequently, the deviation in radiative lifetime relative to the measured and calculated refractive indices also follows this trend. The quantum efficiencies obtained for the SLB0, SLB1, and SLB2 glasses are 78.44, 76.02, and 74.42, respectively. For the SLB3, SLB4, SLB5, and SLB6 glasses, the increase is from 77.96 to 83.96. These values are very high compared to other glasses [41,42,43,44,45].

Table 5. Refractive indice n₁ and n₂, asymmetric ratio R, radiative lifetime τ_rad(n1) and τ_rad(n2), experimental lifetime τ_Exp and quantum efficiency η_(n1) and η_(n2) of Eu³⁺-doped SLB glasses.

Glass ID	n1	n2	R	τrad(n1)	τrad(n2)	τExp	η(n1)	η(n2)
	(±0.04)	(±0.001)		(ms)	(ms)	(ms)	(%)	(%)
SLB0	2.10	2.042	3.22	1.17	1.28	1.01	80.15	78.44
SLB 1	2.05	1.988	3.36	1.38	1.49	1.13	79.16	76.02
SLB 2	2.01	1.953	3.21	1.46	1.63	1.22	79.98	74.42
SLB 3	2.00	1.903	3.16	1.51	1.75	1.37	89.42	77.96
SLB 4	1.97	1.863	3.18	1.57	1.88	1.50	91.58	79.68
SLB 5	1.96	1.819	3.07	1.61	1.94	1.61	98.29	82.59
SLB 6	1.95	1.774	3.02	1.65	2.22	1.87	113.38	83.96
PKAlCaFEu1 [41]				2.45		2.41	98
LLiFB [42]				1.75		1.34	77
PKSAEu [43]				2.87		2.50	63
ZnF2-WO3-TeO2-Eu [44]				2.90		1.27	44
1EBT [45]				2.87		2.50	39

Table 5. Refractive indice n₁ and n₂, asymmetric ratio R, radiative lifetime τ_rad(n1) and τ_rad(n2), experimental lifetime τ_Exp and quantum efficiency η_(n1) and η_(n2) of Eu³⁺-doped SLB glasses.

Glass ID	n1	n2	R	τrad(n1)	τrad(n2)	τExp	η(n1)	η(n2)
	(±0.04)	(±0.001)		(ms)	(ms)	(ms)	(%)	(%)
SLB0	2.10	2.042	3.22	1.17	1.28	1.01	80.15	78.44
SLB 1	2.05	1.988	3.36	1.38	1.49	1.13	79.16	76.02
SLB 2	2.01	1.953	3.21	1.46	1.63	1.22	79.98	74.42
SLB 3	2.00	1.903	3.16	1.51	1.75	1.37	89.42	77.96
SLB 4	1.97	1.863	3.18	1.57	1.88	1.50	91.58	79.68
SLB 5	1.96	1.819	3.07	1.61	1.94	1.61	98.29	82.59
SLB 6	1.95	1.774	3.02	1.65	2.22	1.87	113.38	83.96
PKAlCaFEu1 [41]				2.45		2.41	98
LLiFB [42]				1.75		1.34	77
PKSAEu [43]				2.87		2.50	63
ZnF2-WO3-TeO2-Eu [44]				2.90		1.27	44
1EBT [45]				2.87		2.50	39

As previously indicated, there has been significant improvement in the radiative behavior of Eu³⁺ ions in borate-rich regions. Increasing the effective population of emission levels boosted the total intensity of Eu³⁺ emission. This resulted in notable increases in the radiative and experimental lifetime. The higher quantum efficiency values, by adding B₂O₃, can have several origins, which we will review here. First, it could be related to a decrease in non-radiative relaxations. However, this theory is not really supported here because the absorption increases in the Infrared Region with B₂O₃ content, as can be seen here. Second, it could be related to a suppression of the quenching effect between Eu³⁺ ion pairs. Indeed, a decrease in the asymmetry ratio R is observed with the addition of B₂O₃. The possible charge transfer (CT) related to a small amount of Eu²⁺ that appears in the excitation spectra of Figure 4, as well as the CT involving defects, could be the third origin. This will be discussed in the next section.

3.3.6. Phonon Side Bands and Non-Radiative Transitions

Upon inspecting Figure 11a more closely, discontinuous dips in the form of absorption bands on the broad blue emission can be clearly seen (highlighted by vertical blue dashed lines). These absorption bands have been observed in Eu²⁺-doped glasses and phosphors. However, their origin remains unclear, with no attempt having been made to explain them [46,47]. As illustrated in the inset of Figure 11b, this study confirms that these absorption bands (dips) correspond precisely to the f–f transitions line (highlighted by vertical blue dashed lines) observed in the excitation spectra of Eu³⁺ located at 409 (⁵D₃→⁷F₀), 414 (⁵D₃→⁷F₁), 427 (⁵L₆→⁷F₃), 437 (⁵L₆→⁷F₃), 445 (⁵L₆→⁷F₃), 454 (⁵D₂→⁷F₀), 464 (⁵L₆→⁷F₄), 470 (⁵D₂→⁷F₁) and 486 nm (⁵L₆→⁷F₅). The bands exhibit higher intensities at the 464 and 394 nm wavelengths. These are the most efficient excitation levels of Eu³⁺. When NUV excitation at 361 nm is applied and the concentration of B₂O₃ rises, the produced broad blue emission is partially quenched by the high level f–f of Eu³⁺ ions (absorption dips) and ensures an energy transfer to ⁵L₆ and ⁵D(_{2, 3}). These levels then de-excite rapidly to the ⁵D₀ state, which increases the intensity of all ⁵D₀→⁷F_j emission lines (where j = 0, 1, 2, 3, and 4), thereby contributing to the formation of other emission pathways from the ⁷F_j levels. Figure 12 shows the partial energy level diagram of Eu³⁺-doped SLB glasses as well as the possible charge transfer mechanism from the blue emission of Eu²⁺ to the ⁵D₀ levels of Eu³⁺.

The non-radiative transitions caused by electron-phonon coupling are governed by the phonon side band (PSB). PSBs are produced when electrons are excited or emitted at the same time as phonons are created or annihilated. They are undesirable due to the fact that they generate additional non-radiative relaxation channels. However, they provide information on the effective phonon energy and the strength of electron-phonon coupling near the Eu³⁺ sites. As seen in the inset of Figure 11b, the zero phonon line (PZL) of Eu³⁺ ions is identified at a wavelength of 464 nm (highlighted by a vertical blue dashed line). The PSB line spacing is estimated based on the difference in wavenumber between the PZL and PSB (in cm⁻¹) [39]. The only bands at 438 and 442 nm (highlighted by vertical red lines) have not been identified as Eu³⁺ excitation lines; rather, they are thought to be possible phonon side bands (PSB). The corresponding energies were found to be 1279 and 1072 (cm⁻¹). These correspond to the stretching modes of the B-O bonds within the triangular units [BO₃] and tetragonal [BO₄] vibration units [48]. These energies are similar to those of the same units identified in the transmission spectrum of SLB6 glass within the NIR range.

The strength (g) of electron-phonon coupling can be used to quantify phonon-assisted non-radiative transition. This can be calculated from the intensity ratio of PSBs and PET. The g-values are 0.003 and 0.005 for PSBs at 438 and 442 nm, respectively. SLB6 glass was found to have the highest g-values of all the SLB glasses. However, these values are much lower than those reported in other papers, suggesting that phonon coupling alone cannot be the cause of non-radiative quenching in SLB glasses [33,47]. The presence of hydroxyl (OH) groups in these glasses is most likely a possible cause of non-radiative relaxation.

In summary, the integrated intensities of the five strong Eu³⁺ emission lines increase as the B₂O₃ content rises, which is possibly due to increased energy transfer from the possible Eu²⁺ fraction to the ⁵D₀ emitted levels of Eu³⁺ions. Consequently, these glasses are therefore ideal for use in red lasers due to the sharp red emission lines of ⁵D₀→⁷F₂ (from 605 to 630 nm) and of ⁵D₀→⁷F₄ (from 678 to 710 nm). Furthermore, their high transparency and wide transmission window make them suitable for use in solid-state laser applications, including photonics, luminescent materials, nonlinear optical devices, and multicolor display systems.

3.4. CIE Color Coordinates

The CIE 1931 (Commission Internationale d’Eclairage) diagram [16] is a universal method used by designers to create LED lighting solutions based on emission spectra. It uses the three primary colors -red, green, and blue (RGB) to represent the entire color spectrum. The color of a single light is designed per its coordinates (x, y) in a diagram showing the combination of red, green, and blue light that comprise the color. The standard coordinates (x = 0.33, y = 0.33) correspond to white light emission at the center of the diagram. This study provides information on two important light parameters: Correlated color temperature (CCT) and lumens.

CCT is the temperature of an ideal black body that emits light or a similar color to the being measured. Lumens or brightness refers to the intensity of light perceived by the human eye. It depends on color, distance, and the environment. It is expressed as a subjective visual sensation.

The CCT values can be calculated using the following relation [49]:

$$CCT=-449n^3+3525n^2-6823n+5520.33$$

where n is defined by; n = (x − x_e)/(y − y_e) and x_e = 0.3320, y_e = 0.1858. We used free software provided by OSRAM (color calculator version 7.23) to measure the CIE diagram and CCT of the SLB glasses. To evaluate the brightness expressed by Lumens efficacy (LE) of these glasses, the following equation is often used [50]:

$$LE(Lm/Wopt)=683(Lm/opt).{\displaystyle \frac{{\displaystyle \int I(\lambda ).V(\lambda )}.d\lambda }{{\displaystyle \int I(\lambda )}.d\lambda }}$$

where I(λ)dλ is the spectrum of the emission and V(λ) is the sensitivity curve of the human eye. The human eye is most sensitive to the visible spectrum at 555 nm; it achieves a maximum luminous efficacy of (683 Lm/Wopt).

Figure 13a shows the CIE diagram for the SLB glass’s chromatic light coordinates (x, y). The chromaticity coordinates (x, y) have a y-value close to 0.35, whereas the x-value ranges linearly from 0.494 to 0.614. Figure 13b,c show the emission spectrum corresponding to the CIE colors of the SLB0 and SLB6 glasses, respectively.

Table 6. CIE chromaticity parameters of Eu³⁺-doped SLB glasses.

SLBx Glasses	CIE ID	x	y	Lumens	CCT (K)
0	0	0.494	0.345	295	1836
10	1	0.513	0.35	299	1713
20	2	0.566	0.352	303	1399
30	3	0.604	0.352	307	1220
40	4	0.618	0.356	309	1178
50	5	0.617	0.354	287	1176
60	6	0.614	0.354	306	1187
Rb2Bi(PO4)(MoO4):1% Eu3+ λem =395 nm [50]	-	0.649	0.350	210	-
Li3Ba2Eu3(MoO4)8 λem = 615.5 nm [50]	-	-	-	312	-
Y2Mo4O15:75%Eu3+ λem = 613 nm [50]				242

summarizes the CIE color coordinates, CCT and lumen values for Eu-doped SLB glasses. The CCT values range from 1836 to 1187, corresponding to an orange-red color. Therefore, a low kelvin number indicates a warm, yellowish light, whereas a high kelvin number indicates a cool, bluish color. Warmer CCTs are often used for indoor lighting to create a calm natural atmosphere, whereas cooler CCTs are used for security lighting to improve visibility. It was found that an increase in B₂O₃ leads to a rapid increase in lumen values. Since lumen expresses brightness, this significant increase is fully justified by the rise in emission observed in the previous section. The lumen values obtained at the light-sensitive wavelength of 583 nm are higher than those of the phosphors shown in Table 6.

4. Conclusions

Glasses that are free of crystalline defects and have the specified composition are synthesized using the melt quenching method, in which Sb₂O₃ can be substituted with 0 to 60 (mol.%) of B₂O₃. These glasses were found to have very high thermal stability associated with a decrease in density and molar volume. Meanwhile, the transparency cut-off shifts from 385 to 344 nm. Consequently, the energy band gap increased, and the Urbach energy values decreased. The change involved in the gradual replacement of Sb₂O₃ by up to 60 (mol. %) of B₂O₃ led to a decrease in the relative value of the density, the molar volume, the polaron radius, the average separation of Eu³⁺ ion r_i, and the field strength F up to nearly 24%, while the concentrations per unit volume of Eu³⁺ ions increased by about 32% and by 20% for the field strength F. Additionally, E_g^ind increases by 26% and by 15.6%. Conversely, Urbach energy, refraction index, reflection losses, and optical basicity decreased by between 7% and 22%. Analysis using ATR-FTIR reveals that the Eu³⁺-doped SLB glass structure consists of [SbEO₃], [BO₃], and [BO₄] units. By monitoring the emission spectra at 612 nm, we found that as the B₂O₃ content in the glass increases, the population of Eu³⁺ levels increases considerably in response to excitation at 361 nm. This effect is less pronounced at 394 nm than at 465 nm. Consequently, this study has confirmed the presence of all possible excited Eu³⁺ levels in glasses with low phonon energy, and the total intensity of Eu³⁺ emissions in SLBx glasses increases considerably when the B₂O₃ content is higher than x = 30 (mol. %). Five distinct peaks were visible at the following wavelengths: 577 (yellow), 590 (orange), 612 (red), 648 (dark red), and 697 (dark red) nm. A very weak, broad emission was also observed in the blue region at 420–520 nm. Its origin is probably related to the possible presence of Eu²⁺. The enhancement of ⁵D₀→⁷F_j emission may result from charge transfer from the Eu²⁺ to the ⁵D₀ emitted levels of Eu³⁺ ions. Τhe experimental lifetime of the ⁵D₀ level of Eu³⁺-doped SLB glasses increases linearly when boron oxide is added. Similarly, the radiative lifetime also increases, leading to a higher quantum efficiency ranging from 74 to 84 in borate-rich glasses. This very significant increase in the luminescence and quantum efficiency can be explained by a reduction in non-radiative processes. The presence of OH hydroxyl groups in these glasses is the main cause of non-radiative relaxation. The chromaticity coordinates (x, y) have an x-value close to 0.35, whereas the y-value ranges linearly from 0.494 to 0.614. The CCT values range from 1836 to 1187, corresponding to a warm orange-red color. As lumen expresses brightness, the significant increase in lumen can be entirely explained by the rise in emission in these glasses.