Network Structure and Luminescent Properties of ZnO–B₂O₃–Bi₂O₃–WO₃:Eu³⁺ Glasses

Abstract

In this study, we investigated the influence of Bi₂O₃ and WO₃ on both structure and optical properties of 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃; x = 1, 5, 10 glasses doped with 0.5 mol% Eu₂O₃. IR spectroscopy revealed the presence of trigonal BØ₃ units connecting superstructural groups, [BØ₂O]⁻ metaborate groups, tetrahedral BØ₄⁻ units in superstructural groupings (Ø = bridging oxygen atom), borate triangles with nonbridging oxygen atoms, [WO₄]²⁻ tetrahedral, and octahedral WO₆ species. Neutron diffraction experimental data were simulated by reverse Monte Carlo modeling. The atomic distances and coordination numbers were established, confirming the short-range order found by IR spectra. The synthesized glasses were characterized by red emission at 612 nm. All findings suggest that Eu³⁺ doped zinc borate glasses containing both WO₃ and Bi₂O₃ have the potential to serve as a substitute for red phosphor with high color purity.

1. Introduction

Currently, white-light-emitting diodes are being investigated extensively as the next-generation solid-state light source owing to the advantages they bring to the table, including safety, an environmentally friendly nature, high stability, low power consumption, and long operational lifetime [1]. One of the ways in which white light can be obtained is by combining tri-color phosphors, such as ZnS:Cu⁺, Al³⁺ (green) [2], BaMgAl₁₀O₁₇:Eu²⁺ (blue) [3], and Y₂O₂S: Eu³⁺ (red) [4], coated on InGaN-based LED chip, emitting around 400 nm (near-UV). However, the red-emitting phosphor shows lower efficiency (eight times lower) compared to the blue and green phosphors, as well as exhibiting chemical instability under UV radiation which may cause environmental pollution due to the release of sulfide gas. The other commercially applied red phosphors, such as Y₂O₃:Eu³⁺ and YVO₄:Eu³⁺, also cannot achieve high emission efficiency [5]. Therefore, red-emitting phosphor with chemical and thermal stability and high efficiency upon near-UV excitation remains to be found.

Europium (III) ion is being considered as a suitable activator for red emission, resulting from its ⁵D₀ → ⁷F_j (j = 0–4) transitions in the visible range [6]. Unfortunately, Eu³⁺-doped materials cannot be efficiently excited by the present LED chips, because its excitation peaks are weak in nature due to parity-forbidden f–f transitions. Searching for host materials that can overcome the weak Eu³⁺ absorptions is important for achieving high excitation and emission efficiency of the red luminescence. A possibility is introducing sensitizers into the host composition, such as Ce³⁺, Bi³⁺, Tb³⁺, etc. [7,8]. It is well known that the luminescent properties of Eu³⁺-doped materials can be modified by changing the host structure and composition. Glass materials are suitable matrixes for doping with lanthanide ions due to their chemical stability, high optical homogeneity, absence of absorbing particles, and low nonlinear refractive indices. Among many potential glass materials for luminescence applications, binary ZnO–B₂O₃ glasses have been attracting continuous scientific interest. Homogeneous binary zinc–borate glasses are formed in a very narrow range of compositions because of the existence of a very large region of immiscibility of two liquids in a ZnO–B₂O₃ system [9]. However, these glasses are characterized by good chemical and thermal stability, high mechanical strength, low dispersion, and low glass transition temperature. They possess high transparency (up to 90%) from the visible to mid-infrared region of the spectrum [10]. Eu³⁺-doped zinc borate glasses yield very strong orange/red photoluminescence by UV excitation, especially for low europium concentrations (<10¹⁹ cm⁻³) [11]. It has been reported that the addition of WO₃ and/or Bi₂O₃ to ZnO–B₂O₃ glasses induces the expansion of the glass-forming region and also lowers the phonon energy [12,13]. In our recent works, we reported, for the first time, the preparation of tungsten-containing ZnO–B₂O₃ glasses doped with Eu³⁺ active ion and their luminescent properties [14,15]. The obtained results from glass structure, physical, thermal, and optical properties indicate the suitability of the 50ZnO:40B₂O₃:10WO₃ glass network for the luminescence performance of Eu³⁺ ions. The positive effect of the addition of WO₃ on the luminescence intensity is proven by the stronger Eu³⁺ emission of the zinc–borate glass containing WO₃ compared to the WO₃-free zinc–borate glass, a phenomenon engendered mainly by the energy transfer from tungstate groups to the Eu³⁺ ions (sensitizing effect). The most intense luminescence peak observed at 612 nm and the high-integrated emission intensity ratio (R) of the ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ transitions at 612 nm and 590 nm of 5.77 suggest that the glasses have the potential for red emission materials.

Another desirable component for luminescent glass hosts is Bi₂O₃ oxide, commonly used as an activator, emitting in the spectral region of 380–700 nm due to ³P₁ → ¹S₀ transition upon NUV excitation. Among many studies, the Bi³⁺ ion is also recognized as a favored sensitizer, which can greatly enhance the luminescence of the rare-earth ions (Eu³⁺, Sm³⁺, Tb³⁺) through resonant energy transfer [16,17]. High bright red emission in Eu³⁺ containing zinc–borate glasses codoped with Bi³⁺ was observed, enhanced by 346 nm excitation (¹S₀-³P₁ of Bi³⁺ ions) due to the sensitization effect of Bi³⁺ codopant [18]. Zinc bismuth borate glasses doped with different Eu³⁺ concentrations (1, 3, 5, 7, and 9 mol%) were prepared, and the systematic analysis of the results suggested that the glass doped with a Eu³⁺ concentration of 5 mol% is suitable for LED and display device applications [19].

More recently, we prepared zinc–borate glasses modified with Bi₂O₃. Bulk, transparent, dark brownish glasses with composition 50ZnO:(40 − x)B₂O₃:10Bi₂O₃:0.5Eu₂O₃:xWO₃, x = 0 and 0.5, were synthesized. The obtained structural and optical data indicate that a zinc–borate glass network containing Bi₂O₃ provides highly asymmetric sites of Eu³⁺ ions, leading to high emission intensity. Moreover, the presence of WO₃ also leads to the increase in emission intensity of the rare-earth Eu³⁺ ion, as a result of the nonradiative energy transfer from the glass host to the active ion [20]. These data above show that the ZnO–B₂O₃ glass system containing both bismuth and tungstate oxides is a particularly interesting host for the europium ions in red phosphors applications.

Here, we continued our investigations by preparing such a glass composition with increasing WO₃ content and with the addition of low Bi₂O₃ concentrations (1 mol%) in order to meet the requirement of colorless glasses for optical application. The aim was to obtain bulk, colorless glasses with compositions 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%, and to establish the influence of Bi₂O₃ and WO₃ on glass formation, structure, and optical properties.

2. Materials and Methods

2.1. Sample Preparation

Glasses of the compositions in mol% 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃; x = 1, 5, 10, doped with 0.5 mol% Eu₂O₃ were obtained by applying the melt quenching method, using reagent-grade ZnO (Merck KGaA, Amsterdam, The Netherlands), WO₃ (Merck KGaA, Darmstadt, Germany), B₂O₃ (SIGMA-ALDRICH, St. Louis, MO, USA), Bi₂O₃ (Alfa Aesar, Karlsruhe, Germany), and Eu₂O₃ (SIGMA-ALDRICH, St. Louis, MO, USA) as raw materials. B₂O₃ enriched with ¹¹B isotope (99.6%) was used in order to avoid the high neutron absorption cross-section of the ¹⁰B isotope. Further in the text, samples’ names are abbreviated to ZBBW1:Eu, ZBBW5:Eu, and ZBBW10:Eu, where the number refers to the WO₃ content in the compositions. The homogenized batches were melted at 1200 °C for 20 min in a platinum crucible in air. The melts were cast into graphite molds to obtain bulk glass samples. Glasses obtained by us earlier with the same compositions (50ZnO:(50 − x)B₂O₃:xWO₃; x = 1, 5, 10) doped with 0.5 mol% Eu₂O₃ without Bi₂O₃, and denoted as ZBW1:Eu, ZBW5:Eu, and ZBW10:Eu, were also included here in order to establish the influence of bismuth addition on the structure and luminescence properties of the new glass compositions. The glasses without Bi₂O₃ containing 1 and 5 mol% WO₃ were reported in the 6^th International Conference on Optics, Photonics, and Lasers (OPAL’ 2023), while the glass with the highest WO₃ amount of 10 mol% was represented in ref. [15].

2.2. Characterization Techniques

The phase formation of the samples was established by X-ray phase analysis with a Bruker D8 advance diffractometer, Karlsruhe, Germany, using Cu K_α radiation in the 10 < 2θ < 60 range. The thermal stability of the obtained glasses was examined by differential scanning calorimetry (DSC) with a Netzsch 404 F3 Pegasus instrument, Selb, Germany, in the temperature range 25–750 °C at a heating rate of 10 K/min in an argon atmosphere. The density of the obtained glasses at room temperature was measured by the Archimedes principle using toluene (ρ = 0.867 g/cm³) as an immersion liquid on a Mettler Toledo electronic balance of sensitivity 10⁻⁴ g. The IR spectra of the glasses were measured using the KBr pellet technique on a Nicolet-320 FTIR spectrometer, Madison, WI, USA, with a resolution of ±4 cm⁻¹, by collecting 64 scans in the range 1600–400 cm⁻¹. A random error in the center of the IR bands was found to be ±3 cm⁻¹. The EPR analyses were carried out in the temperature range 120–295 K in X band at frequency 9.4 GHz on a spectrometer Bruker EMX Premium, Karlsruhe, Germany. Optical absorption spectra (UV–VIS–NIR) in the range 190–1500 nm were obtained with an error ˂ 1% using a commercial double-beam spectrometer (UV-3102PC, Shimadzu, Kyoto, Japan). Photoluminescence (PL) excitation and emission spectra at room temperature for all glasses were measured with a Spectrofluorometer FluoroLog3–22, Horiba JobinYvon, Longjumeau, France. Neutron diffraction measurements were carried out in the momentum transfer range, Q = 0.45–9.8 Å⁻¹, for 24 h using neutrons of de Broglie wavelength, λ = 1.069 Å, at the 2-axis PSD diffractometer of Budapest Neutron Centre. The powder glass samples were mounted in a thin-walled cylindrical vanadium can with a diameter of 8 mm for the neutron diffraction experiments. The neutron diffraction data were corrected for detector efficiency, background scattering, and absorption effects, and normalized with vanadium [21]. The total structure factor, S(Q), was calculated using local software packages.

2.3. The Reverse Monte Carlo Simulation

Reverse Monte Carlo (RMC) simulations were performed on neutron diffraction datasets of the experimental total structure factor, S(Q), to determine the short-range structural properties of glasses by using RMC++ software (https://www.szfki.hu/~nphys/rmc++/downloads.html, accessed on 15 October 2023) [22]. The RMC technique minimizes the squared difference between the experimental S(Q) and the simulated one from a 3-dimensional atomic configuration by using the following equations:

$$S(Q)=\sum _{i,j}^k{w}_{ij}{S}_{ij}(Q)$$

(1)

$$S_{ij}(Q)=1+\frac{4\pi {\rho }_0}{Q}{\int }_0^{r_{max}}r\left[g_{ij}(r)-1\right]\mathit{sin}Qr\hspace{0.33em}dr$$

(2)

$$w_{ij}=\frac{c_i{c}_j{b}_i{b}_j}{{\left[{\sum }_{i,j}^k{c}_i{b}_j\right]}^2}$$

(3)

where c_i and b_i are the molar fraction and coherent neutron scattering length for atoms of type i, the S_ij(Q) denotes the partial structure factors, and w_ij are the neutron scattering weight factors for the 21 atomic pairs for the ZBBW:Eu series (explanation: k = 5, thus k(k + 1)/2 = 15 different atomic pairs are present). RMC simulations were used to generate partial atomic pair correlation function, g_ij(r), and coordination number distributions. The simulation was started with an initial random configuration by building a box that contained 10.000 atoms of Zn, B, Bi, W, Eu, and O, with the atomic density, ρ_o, values of 0.0947 Å⁻³, 0.0919 Å⁻³, and 0.0888 Å⁻³ for the samples ZBBW1:Eu, ZBBW5:Eu, and ZBBW10:Eu, respectively. The RMC model box lengths for the three samples were 23.63 Å, 23.87 Å, and 24.14 Å for ZBBW1:Eu, ZBBW5:Eu, and ZBBW10:Eu samples, respectively.

In the RMC simulation procedure, constraints were used for the minimum interatomic distances between atom pairs (cut-off distances) to avoid unreasonable atom contacts. For each sample, about fifty RMC configurations were obtained with more than 2,600,000 accepted configurations of atoms.

3. Results

3.1. XRD Analysis and DSC Studies

Bulk, transparent, slightly colored glasses (insets, Figure 1a) of 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%, were obtained in this study. The measured X-ray diffraction patterns are shown in Figure 1a, and confirm the amorphous nature of the prepared materials. Glasses without Bi₂O₃ (50ZnO:(50 − x)B₂O₃:xWO₃:0.5Eu, x = 1, 5, and 10 mol%) were obtained earlier. The XRD patterns of the glass samples having 1 and 5 mol% WO₃ were present in the 6th International Conference on Optics, Photonics, and Lasers (OPAL’ 2023). The photograph of the glass with the highest WO₃ amount of 10 mol% was represented in ref. [15].

DSC curves of the glass samples 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%, obtained are presented in Figure 2. Glasses are characterized with two humps corresponding to the two glass transition temperatures, T_g1 and T_g2. The two glass transition effects observed are connected with the presence of two amorphous phases with different compositions in the investigated glasses. In the DSC curve of the glass having the highest WO₃ concentration of 10 mol%, an exothermic peak due to the glass crystallization at temperature Tc = 683 °C was observed. For the glasses containing lower WO₃ concentration of 5 and 1 mol%, glass crystallization effects did not appear, evidencing that these glasses possess higher thermal stability, which decreases with increasing WO₃ content. The DSC analysis shows that the thermal parameters of glasses present here do not differ significantly from those obtained for the glasses with the similar compositions without Bi₂O₃ reported in Ref. [15]. However, the thermal stability of glasses containing 1 mol% Bi₂O₃ was slightly lowered, most probably because of the increased structural heterogeneity and, hence, higher crystallization ability of the composition.

3.2. IR Spectral Analysis

Structural information of the studied glasses was obtained by comparative analysis of IR spectra of glasses with the same compositions without Bi₂O₃ and containing 1 mol% Bi₂O₃, which are shown in Figure 3.

The IR spectra of Bi₂O₃-free glasses with the lower WO₃ concentration of 5 and 1 mol% were previously reported by us in the 6th International Conference on Optics, Photonics, and Lasers (OPAL’ 2023), while the glass with the highest WO₃ amount of 10 mol% was discussed earlier in ref. [15]. The IR spectra of the glasses without Bi₂O₃ contain bands characteristic to the metaborate groups BØ₂O⁻ (shoulder at 1460 cm⁻¹; band at 625–640 cm⁻¹), pyroborate dimmers, B₂O₅⁴⁻ (weak band at 1110 cm⁻¹; band at 625–640 cm⁻¹), and superstructural groupings with BØ₃ and BØ₄⁻ species (bands at 1250 cm⁻¹ and at 1040–1050 cm⁻¹, band at 680 cm⁻¹) [14,23,24,25]. There are also WO₆ (bands at 870 cm⁻¹ and at 680 cm⁻¹) and [WO₄]²⁻ tetrahedra (band at 480 cm⁻¹) [23]. The addition of Bi₂O₃ to the glass compositions causes the BO₃ →BO₄ transformation, resulting in the increase in the number of superstructural units (increased intensity of the bands at 1240 and 1045–1035 cm⁻¹ and band at 680 cm⁻¹) [23]. For both the glass series, a partial [WO₄]²⁻ → WO₆ transformation with increasing WO₃ concentration occurs, manifested by the disappearance of the bands at 940 cm⁻¹ and 880 cm⁻¹ (ν₃[WO₄]²⁻), and formation of an intense and well-formed band at 860–870 cm⁻¹ (νWO₆). The tungstate octahedral species sharing common corners (W–O–W) and edges (W₂O₂) are supposed, having in mind the structural and IR data of crystalline Bi₂WO₆, Bi₂W₂O₉, and ZnWO₄ phases [26,27,28,29,30,31]. These compounds consist of corner- or edge-shared WO₆, whose IR spectra contain strong bands situated in the same spectral regions, from 870–800 cm⁻¹ and 700–600 cm⁻¹. Tungstate and borate species are charge-balanced by Bi³⁺, Zn²⁺, and Eu³⁺ ions via Bi–O–W, B–O–Bi, Zn–O–W, Zn–O–B, Eu–O–W, and Eu–O–B bonding. Additionally, the new high frequency band at 1350 cm⁻¹ observed in glasses having higher WO₃ of 5 and 10 mol% (Figure 2, ZBBW5:Eu and ZBBW10:Eu) is attributed to stretching of B–O⁻ bonds in BØ₂O⁻ triangles, and its presence suggests stronger interaction between Bi³⁺ ions and nonbridging oxygens [32]. Bi³⁺ ions are incorporated in the structure of investigated glasses as BiO₆ octahedra (bands at 640 and at 480 cm⁻¹). Thus, the addition of Bi₂O₃ to glasses 50ZnO:(49 − x)B₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%, leads to the formation of more stable and reticulated glass structure, compared with the glasses with the same composition without Bi₂O₃. The Zn²⁺ ions generally participate in the borate glasses as ZnO₄ tetrahedra with characteristic Zn²⁺ motion at 225 cm⁻¹ [33]. The frequency of the Eu–O vibration, ν(Eu–O), has been measured at about 280 cm⁻¹ for glasses (1 − 2x)Eu₂O₃ − x(SrO − B₂O₃) [34].

The detailed assignments of the bands observed in the IR spectra of the present glasses are summarized in

Table 1. Infrared bands (in cm⁻¹) and their assignments for glasses 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%.

Infrared Bands Position (cm−1)	Assignment	Ref.
475	ν4[WO4]2− + Bi–O vibrations in the BiO6 groups	[32,35]
680	Bending vibrations of B–O–B bonds in superstructural	[32]
640–625	Bending vibrations of B–O–B bonds in meta- and pyroborates + Bi–O vibrations in the BiO6 groups	[32]
860–870	νWO6	[14,23]
940; 880	ν3[WO4]2− in distorted tetrahedra	[23]
1050–1035	νasBØ4− involved in superstructural units	[14,23]
1100	νas(B–O–B); B–O–B bridge in pyroborate units, B2O54−	[24,35]
1245	νas(B–O–B); B–O–B bridges connect BO3 units + BO3 stretch in meta-, pyro-, orthoborate units	[15,36]
1350	ν(B–O−) stretch in BØ2O− units charge balanced by Bi3+	[32]
1460	ν(B–O−) stretch in BØ2O− units	[23]

.

3.3. Density, Molar Volume, Oxygen Packing Density, and Oxygen Molar Volume

A structural information of two series of glasses was also gained by density (ρ_g) measurement, on which basis the values of several physical parameters listed in

Table 2. Values of physical parameters of glasses 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%: density (ρ_g), molar volume (V_m), oxygen molar volume (V_o), oxygen packing density (OPD).

Sample ID	ρg (g/cm3)	Vm (cm3/mol)	Vo (cm3/mol)	OPD (g atom/L)
ZBW1:Eu	3.475± 0.002	22.70	11.29	88.55
ZBW5:Eu	3.689± 0.002	23.14	11.51	86.86
ZBW10:Eu *	3.910 ± 0.002 *	23.91 *	11.73 *	84.27 *
ZBBW1:Eu	3.679 ± 0.001	22.57	11.20	89.28
ZBBW5:Eu	3.889 ± 0.005	23.01	11.42	87.57
ZBW10:Eu	4.175 ± 0.001	23.38	11.60	86.18

* The physical parameters of glass ZBW10:Eu were previously reported in Ref. [15].

(molar volume (V_m), oxygen molar volume (V_o), and oxygen packing density (OPD)) are evaluated, using the conventional formulae [37]. Bi₂O₃ containing glasses are characterized with the higher density as compared with the respective Bi₂O₃-free glasses because of the replacement of lighter B₂O₃ (molecular weight 69.62 g/mol) with heavier Bi₂O₃ (molecular weight 465.96 g/mol). The V_m and V_o values of glasses having 1 mol% Bi₂O₃ are lower, while their OPD values are higher as compared with the values of the same parameters established for the glasses without Bi₂O₃, evidencing better packing and bonding in the Bi₂O₃-containing glass network and lower number of nonbridging oxygens (NBOs) [38].

3.4. RMC Modeling and Results

The RMC technique provided an excellent fit of the simulated structure factors (S(Q)-1) with the experimental one for 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol% (Figure 4).

From the RMC simulation, partial atomic pair-correlation functions, g_ij(r), and average coordination number distributions, CN_ij, were revealed, with good stability and statistics. The Zn–O distribution functions show symmetrical peaks centered in the range of 1.95 ± 0.01 Å (

Table 3. Interatomic X–O distances, g_ij(r), obtained from RMC simulation. The errors are estimated from the reproducibility of various RMC runs.

Title 1	Zn–O gij(r) (Å)	B–O gij(r) (Å)	Bi–O gij(r) (Å)	W–O gij(r) (Å)	Eu–O gij(r) (Å)	O–O gij(r) (Å)
ZBBW1:Eu	1.95 ± 0.01	1.40/1.80 ± 0.05	2.00 ± 0.05	1.75 ± 0.05	2.20 ± 0.05	2.35 ± 0.03
ZBBW5:Eu	1.95 ± 0.01	1.40/1.80 ± 0.05	2.00 ± 0.05	1.75 ± 0.05	2.20 ± 0.05	2.35 ± 0.03
ZBB10:Eu	1.95 ± 0.01	1.40/1.80 ± 0.05	2.00 ± 0.05	1.75 ± 0.05	2.20 ± 0.05	2.35 ± 0.03

).

In function of g_ij(r), we specify a range in r over which atoms are counted as neighbors. This can be understood as defining coordination shells. Introducing a min point (positions of minimum values on the lower) and max point (the upper side of the corresponding peak), these are presented in

Table 4. Average coordination numbers, CN_ij, calculated from RMC simulation. In brackets, the interval is indicated, where the actual coordination number was calculated.

Sample	Zn–O CNij	B–O CNij	W–O CNij	O–O CNij
ZBBW1:Eu	4.01 ± 0.05 (min: 1.80–max: 2.20)	3.90 ± 0.05 (min: 1.20–max: 1.65)	6.20 ± 0.1 (min: 1.65–max: 2.23)	5.63 ± 0.1 (min: 2.20–max: 2.60)
ZBBW5:Eu	3.99 ± 0.05 (min: 1.80–max: 2.20)	3.52 ± 0.05 (min: 1.20–max: 1.65)	6.42 ± 0.1 (min: 1.60–max: 2.25)	5.32 ± 0.1 (min: 2.20–max: 2.60)
ZBBW10:Eu	3.97 ± 0.05 (min: 1.80–max: 2.20)	3.48 ± 0.05 (min: 1.20–max: 1.65)	6.73 ± 0.1 (min: 1.60–max: 2.25)	5.54 ± 0.1 (min: 2.20–max: 2.60)

, where we present the average coordination numbers (summarized in Table 4).

The average coordination number of the Zn–O was obtained from the RMC analysis, and it was found that Zn⁴⁺ was tetrahedrally coordinated with oxygens in the glassy network for all studied samples (Table 4). The B–O distribution function showed a relatively broad first neighbor distance at 1.40 ± 0.05 Å, and a slight shoulder at 1.80 ± 0.1 Å appeared in function of concentration in the 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, x = 1, 5, and 10 mol%, glass samples. The B–O coordination was in the range of 3.48 ± 0.05 to 4.00 ± 0.05, and obtained the changes in BO₃/BO₄ ratio. The boron atoms were coordinated mostly by three and four oxygen atoms, forming trigonal BO₃ and tetrahedral BO₄ units, in agreement with coordination numbers in SiO₂–Na₂O–B₂O₃ glasses [39], in MoO₃–ZnO–B₂O₃ glasses [40], and in ZnO–B₂O₃–Li₂O–Al₂O₃ glasses [41]. The nearest W–O distances showed characteristic peaks at 1.75 ± 0.05 (Table 3) for both series. The average coordination number of W–O was in a wide range, from 6.20 ± 0.1 to 6.73 ± 0.1 (see Table 3) within the limits of experimental uncertainty. Based on the coordination numbers, we can predict that the W–O network consists of WO₄ and WO₆ units. The WO₄/WO₆ ratio changed with the WO₃ concentration, and samples with highest WO₃ concentration (10 mol%) had mostly WO₆ units with fewer WO₄ units. In the case of Bi–O and Eu–O, thanks to the very low Bi₂O₃ and Eu₂O₃ concentration, it was not relevant to obtain reliable numbers for the coordination.

3.5. EPR Spectroscopy

The EPR analyses of ZBBW5:Eu were carried out at 295 K and 120, and in Figure 5 the obtained spectra are shown. As seen, the spectra contain multiple signals with different intensities and g-factors. The most prominent features are assigned to impurities of isolated Fe³⁺ ions (the signal with g = 4.25) and isolated Mn²⁺ ions (six hyperfine structure lines marked with *, inset).

Eu²⁺ ions possess electron spin S = 7/2, and in their EPR spectrum seven signals were observed, corresponding to seven allowed transitions occurring between four Kramers’ doublets (ms = ±1/2, ms = ±3/2, ms= ±5/2, and ms = ±7/2) according to the selection rule [42]. Depending on the zero-field splitting parameters (D, E) and crystal field symmetry, the EPR spectra of Eu²⁺ usually include a part of these signals. In the spectra of ZBBW5:Eu are discernable a set of not-well-resolved and low-intensive signals with g factors about 6.0, 4.7, 3.4, and 2.8 located in the range 0–300 mT. These signals could be assigned to Eu²⁺ ions [42,43] localized in a low-symmetry crystal field with large zero-field splitting (D > hν).

In addition, the central region of the spectrum shows a signal with g = 2.00. The assignment of this signal is somewhat difficult, as it could derive both from Fe³⁺ ion impurities existing in the sample and Gd³⁺ ions in a highly symmetric environment. That is why its attribution remains unclear.

To summarize, the EPR spectra recorded for sample ZBBW5:Eu confirm the presence of Eu²⁺ ions, with their concentration being assessed as extremely low based on the comparison between the background spectrum and the analyzed spectrum.

3.6. Luminescent Properties

Figure 6 shows the photoluminescent excitation spectra of Eu³⁺-doped 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃, x = 1, 5, and 10 mol%, glasses monitoring the ⁵D₀ → ⁷F₂ red emission of Eu³⁺ at 612 nm [6].

The low-intensive broad band below 350 nm is ascribed to the characteristic absorption of Bi³⁺ (¹S₀ → ³P₁) [17], the charge transfer bands (CTB), resulting from energy transitions from O²⁻ to W⁶⁺ in WO₄ and WO₆ groups and O²⁻ to Eu³⁺, as well as the ground 4f state of Eu³⁺ to W⁶⁺ [6,44,45,46,47,48]. The existence of the excitation band of host lattice absorption at Eu³⁺ emission (612 nm) implies the existence of nonradiative energy transfer from Bi³⁺ and WO_n groups to the active rare-earth ion [48,49]. The sharp lines in 350–600 nm range correspond to the f → f intraconfigurational forbidden transitions of Eu³⁺ from the ground state (⁷F₀) and from the first excited state (⁷F₁): ⁷F₀ → ⁵D₄ (360 nm), ⁷F₀ → ⁵G₂ (375 nm), ⁷F₁ → ⁵L₇ (381 nm), ⁷F₀ → ⁵L₆ (392 nm), ⁷F₀ → ⁵D₃ (412 nm), ⁷F₀ → ⁵D₂ (463 nm), ⁷F₀ → ⁵D₁ (524 nm), ⁷F₁ → ⁵D₁ (531 nm), and ⁷F₀ → ⁵D₀ (576 nm) [6]. Among them, the electronic transition at 392 nm is the strongest one and was used as an excitation wavelength. Compared to the CTB, the intensity of the narrow f–f lines is stronger. This is favorable for Eu³⁺-doped luminescent materials, since, in general, the intensity of these Eu³⁺ transitions is weak due to the parity-forbidden law. Thus, the obtained glasses can be effectively excited by near-UV and blue light, which is compatible with the present LED chips.

The emission spectra of Eu³⁺-doped 50ZnO:(49 − x)B₂O₃: 1Bi₂O₃:xWO₃, x = 1, 5, and 10 mol%, glasses under the excitation of λ_ex = 392 nm consist of five emission peaks centered at 578, 592, 612, 651, and 700 nm, originating from ⁵D₀ →⁷F_J (J = 0, 1, 2, 3, 4) intraconfigurational transitions of Eu³⁺ (Figure 7) [6].

The characteristic single broad band emission of Bi^3+, originating from ³P₁ → ¹S₀ transition is located at 380–700 nm [50]. In the same spectral region, we also registered the broad emission band of the WO_n group [51]. The general requirement for energy transfer from both WO₃ and Bi₂O₃ to the rare-earth ion is satisfied, i.e., there exists a spectral overlap between the excitation peaks of Eu³⁺ (Figure 6) and the emission band of Bi³⁺ and WO₃ (Figure 7). As a result, both oxides can act as sensitizers, transferring the emission energy nonradiatively to the activator Eu³⁺ by quenching their luminescence. Moreover, an indication of energy transfer is the absence of characteristics of WO₃ and Bi₂O₃ emission bands [15,20,48]. As can be seen, with the increase of WO₃ up to 10 mol% (Figure 7) and with the introduction of Bi₂O₃ in the 50ZnO:40B₂O₃:10WO₃:0.5Eu₂O₃ glass composition (Figure 8), a significant enhancement of the emission intensities was achieved.

The most intensive emission peak, observed at 612 nm, corresponds to the hypersensitive to the site symmetry electric dipole (ED) transition ⁵D₀ → ⁷F₂, while the second-most intensive magnetic dipole (MD) ⁵D₀ → ⁷F₁ one is insensitive to the site symmetry and is considered almost constant [6,44,52,53]. The integrated emission intensity ratio (R) of these two transitions ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ is used to estimate the degree of symmetry around Eu³⁺ ions and the strength of covalence of the europium–oxygen bond. The higher R values indicate more site asymmetry of the rare-earth ion, a high covalency between Eu³⁺ and O²⁻ ions, and an enhanced emission intensity [6,54,55]. The intensity ratios, R, of the present glasses (R = 4.7–5.7) (

Table 5. Comparison of the luminescence intensity ratio (R) of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transition of Eu³⁺-doped oxide glasses.

Glass Composition	R Values	Ref.
50ZnO:48B2O3:1Bi2O3:1WO3:0.5Eu2O3	4.61	Present work
50ZnO:44B2O3:1Bi2O3:5WO3:0.5Eu2O3	5.04	Present work
50ZnO: 40B2O3:10WO3:0.5Eu2O3	5.57	[12]
50ZnO:39B2O3:1Bi2O3:10WO3:0.5Eu2O3	5.73	Present work
50ZnO:40B2O3:10WO3:xEu2O3 (0 ≤ x ≤ 10)	4.54–5.77	[15]
50ZnO:40B2O3:5WO3:5Nb2O5:xEu2O3 (x = 0, 0·1, 0·5, 1, 2, 5 and 10)	5.09–5.76	[36]
50ZnO:(40 − x)B2O3:10Bi2O3:0.5Eu2O3:xWO3, x = 0 and 0.5	3.58; 3.79	[20]
20ZnO:8Al2O3:(12 − x)Bi2O3:60B2O3:xEu2O3	1.951–2.78	[56]
39.5Li2O:59.5SiO2:1Eu2O3	3.20	[57]
4ZnO:3B2O3 0.5 ÷ 2.5 mol% Eu3+	3.94–2.74	[58]
Eu3+: 45B2O3-5ZnO-49PbO	3.03	[59]
15PbF2:25WO3:(60 − x)TeO2:xEu2O3 x = 0.1, 0.5, 1.0 and 2.0 mol%	2.37–2.78	[60]
40ZnO:(30 − x) B2O3:30P2O5:xEu2O3 (0.1 ≤ x ≤ 0.9)	2.96–3.65	[61]
60ZnO:(40x)B2O3:0.2Eu2O3:xBi2O3 (x = 0, 0.1, 0.2, 0.5, 1.0)	2.98	[18]
(100 − x):(0.2Bi2O3–0.8GeO2):xEu2O3 (x = 0.5, 1, 1.5, 2 mol%)	3.94–4.21	[62]
Eu3+:Y2O3	3.8–5.2	[63]
Eu3+ doped Y2O2S	6.45–6.62	[64]

) are higher than most of the other reported Eu³⁺-doped glasses and have close values to Eu³⁺:Y₂O₃ and Eu³⁺:Y₂O₂S, indicating that the synthesized glasses are characterized by a more distorted environment of the Eu³⁺ ion and a high covalent bonding between Eu³⁺ and the surrounding ligands, thus achieving an enhanced Eu³⁺ emission intensity [15,18,20,36,56,57,58,59,60,61,62,63,64].

The R values were found to increase from 4.61 to 5.73 (Table 5) as the WO₃ concentration raised from 1 to 10 mol%. The incorporation of small amounts of Bi₂O₃ (1 mol%) into the glass structure also led to an increase in the asymmetric ratio from 5.57 for the glass 50ZnO:40B₂O₃:10WO₃:0.5Eu₂O₃ to 5.73 for the glass 50ZnO:39B₂O₃:1Bi₂O₃:10WO₃:0.5Eu₂O₃. These R values were much higher as compared to the R value for glass with high Bi₂O₃ content (10 mol%) (50ZnO:(40 - x)B₂O₃:10Bi₂O₃:0.5Eu₂O₃:xWO₃, x = 0 and 0.5) (R = 3.79) [20]. This result shows that we have found an appropriate glass composition for hosting an active rare-earth ion that provide a high Eu³⁺ emission intensity.

4. Discussion

In this study, by analyzing the IR spectra of zinc–borate glasses containing 1, 5, and 10 mol% WO₃, with and without Bi₂O₃, it was found that 1 mol% Bi₂O₃ leads to an increase in the number of the BO₄⁻ involved in superstructural groupings and formation of Bi–O–B, Bi–O–W, Zn–O–B, Zn–O–W, Eu–O–W, and Eu–O–B cross-links in the glass structure. With WO₃ loading in the Bi³⁺-containing glasses, a partial [WO₄]²⁻ → WO₆ transformation took place. The tungstate octahedra shared common corners and/or edges, forming W–O–W and W₂O₂ bonds. There were also B–O–B bonds with different numbers with the WO₃ content. The RMC modeling also revealed the presence of both WO₆ and WO₄ units and trigonal BO₃ and tetrahedral BO₄ units varying in amount with the composition. Thus, the bismuth-containing glasses were characterized by a more reticulated and rigid network that ensures low symmetry sites of Eu³⁺ ions, which is favorable for the luminescence emission of the active Eu³⁺ ion. The structural features revealed by IR analysis agreed well with the measured density and calculated physical parameters. Bi₂O₃-containing glasses were characterized with the lower molar and oxygen molar volume and higher oxygen packing density, as compared with Bi₂O₃-free glasses with the same compositions, as a result of the formation of highly cross-linked structure and the presence of new mixed bonding with participation of Bi³⁺ ions. A significant enhancement of the Eu³⁺ emission was established in the glasses 50ZnO:(49 − x)B₂O₃:1Bi₂O₃:xWO₃:0.5Eu₂O₃, (a) x = 1 mol%, (b) x = 5 mol%, (c) x = 10 mol%, in the presence of low Bi₂O₃ content (1 mol%) and with the increase of WO₃ content (up to 10 mol%). The photoluminescent spectra of the new glasses showed an intensive red luminescence at 612 nm as well as a very large value of the luminesce ratio R (over 5), both evidencing that Eu³⁺ ions occupied distorted sites in the created glass network. Particularly, the glass with 10 mol% WO₃ showed the strongest emission of the active ions as a result of the structural features established and also because of an energy transfer from tungstate and bismuthate groups to the active ion.

5. Conclusions

The results of this investigation show that the zinc borate glass matrix with the simultaneous presence of both Bi₂O₃ and WO₃ is very suitable for implementing the active Eu³⁺ as it possesses a reticulated and rigid glass structure, ensuring a more asymmetrical local structure around Eu³⁺ sites, accordingly yielding a higher luminescence of the incorporated Eu³⁺ ions. On the other hand, both bismuth and tungsten oxides have a synthesizer effect by transferring the emission energy nonradiatively to the activator Eu³⁺, which additionally improves its luminesce properties. This suggests that the obtained glasses are potential candidates for red-light-emitting phosphors.