Effect of the Addition of WO₃ on the Structure and Luminescent Properties of ZnO-B₂O₃:Eu³⁺ Glass

Abstract

Glasses with the compositions in mol % of 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xWO₃, x = 0, 1, 3, 5 and 10 were obtained by applying the melt-quenching method and investigated by Raman spectroscopy, DSC analysis and photoluminescence (PL) spectroscopy. Raman spectra revealed that tungstate ions incorporate into the base zinc borate glass as tetrahedral [WO₄]²⁻ groups, and octahedral [WØ₄O₂]²⁻ species with four bridging and two non-bridging oxygen atoms. There are also metaborate, [BØ₂O]⁻ and pyroborate units, [B₂O₅]⁴⁻, in the glass networks. The glasses are characterized by good transmission in the visible region, at about 80%. Photoluminescence (PL) spectra evidenced that WO₃ is an appropriate constituent for the modification of zinc borate glass structure and for enhancing the Eu³⁺ luminescent intensity. The most intense luminescence peak observed, at 612 nm, suggests that the glasses are potential materials for red emission.

1. Introduction

Distinct visible emission of glasses doped with rare earth (RE) ions has attracted researchers to the development of glasses for various optical devices such as lasers, upconverters, stimulated phosphors, white light-emitting diodes (WLEDs) and optical amplifiers [1]. The luminescent and absorption properties of RE ions in glasses greatly depend on the chemical composition, structure and nature of the bonds of the host glass [2].

B₂O₃ is a good glass former and can form glass alone with good transparency, high chemical durability, thermal stability and good rare earth ion solubility [3]. Borate-based glasses are good hosts for RE ions, having flexibility for both the size and composition of the materials [4]. In the glass network, ZnO acts as a glass former as well as modifier, which depends on its mole concentration [5]. Its presence in the glass composition shortens the time taken for the solidification of glasses during the quenching process. Glasses containing ZnO have high chemical stability and less thermal expansion. Their wide band gap and intrinsic emitting property make them promising candidates for the development of optoelectronic devices, solar energy concentrators, ultraviolet emitting lasers, and gas sensors [6]. ZnO is thermally stable and appreciably covalent in character [7]. WO₃ is a semi-glass former with various structural units, like tetrahedral and octahedral units (WO₄ and WO₃) of W⁶⁺ and W⁵⁺ ions in the glass network [8]. Particularly, the addition of WO₃ to glasses enables special features such as the enhancement of the devitrification resistance and chemical durability of the glasses and increases in the solubility of rare earth ions in the host glass network [9]. Tungsten ions are well known for their unusual influence on the optical and electrochemical properties of glasses [10,11]. Due its high polarizability, WO₃ could contribute to a great extent to the obtaining of high-refractive-index glasses, the enhancement of non-linear optical properties and improvements in the luminescent properties of RE ions [10].

In our recent articles, we have reported about the synthesis of tungsten-modified zinc borate glasses and glass crystalline materials of compositions 50ZnO:(50 − x)B₂O₃:xWO₃, x = 0, 10, 15, 20 mol % doped with different amounts of Eu₂O₃ (0.1; 0.5; 1; 2; 5 and 10 mol %) [12,13]. It was found that the addition of 10 mol % WO₃, at the expense of B₂O₃, increases glass density, providing clear and homogeneous bulk glass samples and exhibiting a better emission performance compared to the base binary zinc borate glass. Partially crystallized specimens comprising ZnWO₄ as the crystalline phase were obtained from the compositions having 15 and 20 mol % WO₃.

As a continuation of our previous studies, mentioned above, in this work, we investigate the effect of the addition of smaller amounts of WO₃ (up to 5 mol %) on the structure and luminescent properties of ZnO-B₂O₃ glass doped with 0.5 mol % Eu₂O₃ by using Raman spectroscopy, differential scanning calorimetry and photoluminescent spectroscopy. The aim is to find the optimal glass composition, ensuring the most appropriate structure for accommodating the active rare earth ion, that will improve its luminescence properties.

2. Results

2.1. Thermal Analysis

Bulk transparent glasses were obtained from the nominal compositions in mol % of 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xWO₃, x = 0, 1, 3, 5. Glass samples having 10 mol % WO₃ (50ZnO:40:B₂O₃:0.5Eu₂O₃:10WO₃), previously reported by us in ref. [13], were prepared and investigated again in the present work to compare their structure and luminescence behavior with those of glasses having lower WO₃ content. Having in mind the high vapor pressure of WO₃ oxide, which is the doping component, we accept that its actual concentration does not change significantly from the nominal one after the melting of the compositions. The X-ray diffractograms of the investigated glasses, along with the photographic images of the samples, are shown in references [13,14]. All investigated samples were X-ray amorphous. The prepared WO₃--free glass was colorless, while WO₃--containing glasses were brownish, due to the presence of some amount of Eu²⁺ ions, established by EPR measurements in ref. [13].

50ZnO:(50 − x)B₂O₃:xWO₃:0.5Eu₂O₃, (x = 0, 1, 3, 5 and 10 mol %) glasses have been also investigated by DSC analysis in order to obtain information for some thermal parameters and for structural changes that take place due to the compositional changes [15]. The glass transition temperature, T_g, has been determined, since it is connected with both the strength of inter-atomic bonds and glass network connectivity. A higher T_g corresponds to a more rigid structure, whereas the glasses having a loose-packed structure have a lower T_g [16,17,18]. Figure 1 compares the DSC curves of the glasses investigated in this work. Two humps, corresponding to the two glass transition temperatures T_g1 and T_g2, are observed. Their values are listed in

Table 1. Values of glass transition temperatures (T_g) and average single-bond enthalpy (E_B) of the investigated glasses.

Sample ID	Tg1/°C	Tg2/°C	EB/kJ mol−1
x = 0	570 (843 K)	-	548
x = 1	568 (841 K)	688 (961 K)	546
x = 3	565 (838 K)	680 (953 K)	543
x = 5	543 (816 K)	653 (926 K)	540
x = 10	532 (805 K)	626 (899 K)	532

. The presence of two glass transition effects is connected with the formation of two amorphous phases with different compositions in the investigated glasses.

As one can see, the addition of WO₃ into zinc borate glass causes a slight decrease in the glass transition temperature values. Having in mind our previous IR data and as well as the established values of structurally sensitive physical parameters [14], we explain the observed reduction in T_g1 and T_g2 as a result of increasing non-bridging atoms with the addition of WO₃.

The values of T_g1 and T_g2 were correlated with average single-bond enthalpy, E_B, of glasses using the following relationship proposed in ref. [19]:

$$E_B=\frac{E_{50Zn\text{-}O}+E_{(50-x)B\text{-}O}+E_{xW\text{-}O}+E_{0.5Eu\text{-}O}}{100}$$

where E_W-O; E_B-O, E_Eu-O and E_Zn-O are the bond dissociation energies for the single bonds: W-O; B-O, Eu-O and Zn-O, respectively [20]. E_B values gradually decrease with WO₃ loading because of the gradual replacement of stronger B-O bonds with higher bond dissociation energy (808 kJ mol⁻¹) by weaker W-O bonds with smaller bond dissociation energy (653 kJ mol⁻¹) [20].

2.2. Structural Analysis

In our previous studies, we have investigated the structure of these glasses by applying IR spectroscopy [13,14]. The analysis revealed the presence of several borate structural units in the glass network as [BØ₂O]⁻ and [BØ₄]⁻ metaborate groups, and pyroborate [B₂O₅]⁴⁻ entities. WO₃ modifies the borate network, resulting in a decrease in [BØ₄]⁻ units and the favoring of the formation of pyroborate dimers, [B₂O₅]⁴⁻. Tungstate ions incorporate into base zinc borate glass as tetrahedral [WO₄]²⁻ groups, and octahedral [WØ₄O₂]²⁻ species with four bridging and two non-bridging oxygen atoms. The densities of these glasses have been measured, and on this basis, several structurally sensitive parameters such as molar volume (V_m), oxygen volume (V_o) and oxygen packing density (OPD) have been also established and are reported in refs. [13,14]. Their values also showed the depolymerization of the glass structure of the base zinc borate glass (increasing NBOs) when WO₃ is added.

In the present study, we have used, additionally, Raman spectroscopy in order to obtain more complete structural information about the investigated glasses. The Raman spectra obtained are shown in Figure 2. The Raman spectrum of the base zinc borate glass, without WO₃ (Figure 2; x = 0), is in good agreement with what has been reported by other authors for similar compositions [21,22,23]. It contains a strong and broad band centered at about 870 cm⁻¹, bands at 800 cm⁻¹ and 775 cm⁻¹, a broad shoulder at about 705 cm⁻¹, features at about 260 cm⁻¹ and 315 cm⁻¹ and a high-frequency envelope from about 1200 to 1550 cm⁻¹.

The most prominent band, at 870 cm⁻¹, observed only in the Raman spectrum of glass x = 0, is due to the symmetric stretching of B-O-B bridges in pyroborate dimers, [B₂O₅]⁴⁻ [21,22,23]. The band at 800 cm⁻¹ is connected with the ring breathing of the boroxol rings, while the not-well-resolved band at 775 cm⁻¹ signals the presence of six-membered borate rings consisting of two [BO⁰] triangles and one tetrahedral unit [BO₄]⁻ (i.e., triborate rings) [21,22,23]. The broad shoulder at about 705 cm⁻¹ contains contributions of at least four borate arrangements: deformation modes of metaborate chains, [BØ₂O]⁻ (Ø = bridging oxygen, O⁻ = nonbridging oxygen), in-plane and out-of-plane bending modes of both polymerized (BØ⁰) species and isolated orthoborate units (BO₃)³⁻, and bending of the B-O-B connection in the pyroborate dimers, [B₂O₅]⁴⁻) [21,22,23]. The higher-frequency activity in the 1200 to 1550 cm⁻¹ range is connected with the stretching vibrations of B-O⁻ bonds, involving non-bridging oxygen (NBO) in the pyroborate dimers (1240 cm⁻¹), and in metaborate triangular units, BØ₂O⁻ (1380 cm⁻¹) [21,22,23]. The lower-frequency features at 260 and 315 cm⁻¹ are related to the Zn-O vibrations and Eu-O vibrations, respectively [13]. The addition of WO₃ influences the Raman spectrum of the glass x = 0 considerably. A new and strong band at 970 cm⁻¹ appears, due to the ν₁ symmetric stretching mode of isolated [WO₄]²⁻ tetrahedra, charge balanced by Zn²⁺ and Eu³⁺ ions [13]. Another band, characteristic of the [WO₄]²⁻ tetrahedral units, is the band at 800 cm⁻¹, which is due to the asymmetric stretching ν₃ mode of tetrahedral [WO₄]²⁻ groups. This band significantly overlaps with the ring breathing mode of the boroxol rings. The band at 775 cm⁻¹, due to the six-membered borate rings with one [BO₄]^- unit present in the spectrum of WO₃-free zinc borate glass (Figure 2, x = 0), disappears in the spectra of glasses containing WO₃, indicating the destruction of these structural groups when WO₃ is added. Thus, depolymerization of the borate oxygen network took place, which is in agreement with the previously reported IR data [13,14]. The low-frequency band at 315 cm⁻¹, attributed to the Eu-O vibration in the glass x = 0, overlaps with the ν₂ [WO₄]²⁻ mode, which explains its increasing intensity upon WO₃ loading. Another new low-frequency band at 355 cm⁻¹ is attributed to the ν₄ bending mode of the [WO₄]²⁻ tetrahedra [13]. In addition to tungstate tetrahedral groups, there are also tungstate octahedral [WØ₄O₂]²⁻ species with four bridging and two non-bridging oxygen atoms in the structure of the studied glasses, which by edge-sharing form [W₂O₈⁴⁻] polymeric anions [13]. These tungstate species are charge balanced by Zn²⁺ and Eu³⁺ cations [13]. The presence of several new bands in the spectra of WO₃-containing glasses at 840 cm⁻¹, 865 cm⁻¹, 663 cm⁻¹, 690 cm⁻¹ and 400 cm⁻¹ can be discussed in terms of the vibrations of the WO₂ terminal units and [W₂O₄]_n chains of the [W₂O₈^4-]_n anions, as was reported in the literature for the ZnWO₄ compound [13,24]. In particular, Raman bands at 840 and at 865 cm⁻¹ are connected with ν_as(WO₂) and ν_s(WO₂) vibrations of terminal WO₂ units. The bands at 663, 690 cm⁻¹ and 400 cm⁻¹ are related to ν_as[W₂O₄]_n and ν_s[W₂O₄]_n vibrations, which involve mainly two-oxygen bridges (W₂O₂) of the chain structure [W₂O₄]_n [13]. On the other hand, the band at 840 cm⁻¹ in the spectra of WO₃-containing glasses can be also connected with the symmetric B-O-B stretching of pyroborate dimmers, [B₂O₅]⁴⁻ [21,23]. Because of its complex character, the band growth with increasing WO₃ loads observed is not possible to explain in a straightforward manner. The Raman activity in the high-frequency region, 1200–1550 cm⁻¹, is connected with the vibration of B-O-containing borate arrangements. More particularly, the higher-frequency activity at 1240 cm⁻¹ reflects the symmetric stretch of boron-non-bridging oxygen bonds, ν(B-O⁻) of the pyroborate dimers, while the other two features at 1380 and 1400 cm⁻¹ are due to the B-O⁻ stretching in metaborate triangular units, [BØ₂O]⁻ [21,22,23]. There is a red shift in the frequency of the band at 1380 cm⁻¹ to 1320 cm⁻¹ in the spectra of glasses x = 5 and x = 10. As is reported in ref. [21], the frequency of the boron oxygen stretching in trigonal metaborate units is highly sensitive to the surrounding environment, i.e., the degree of the covalency of the non-bridging oxygen with the charge-balancing cations. The red shift observed indicates that in the glasses having higher WO₃ content (5 and 10 mol %), [BØ₂O]⁻ entities are charge balanced mainly by Zn²⁺ and Eu³⁺ ions. The proposed band assignments are summarized in

Table 2. Peak positions in the Raman spectra of glasses 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xWO₃, (x = 0, 1, 3, 5 and 10 mol %) and their assignments.

Peak Positions	Assignments	Ref.
260	ν (Zn-O)	[21,22]
315	ν (Eu-O) + ν2 [WO4]2	[13]
355	ν4 [WO4]2−	[13]
400	νs[W2O4]n	[13]
663	νas[W2O4]n	[13]
690	νas[W2O4]n	[13]
705	δ[BØ2O]− + in-plane and out-of-plane bending modes of both polymerized (BØ0) species and isolated orthoborate units (BO3)3-+ bending of the B-O-B connection in the pyroborate dimers, [B2O5]4−	[21,22,23]
800	ring breathing of the boroxol rings+ ν3[WO4]2−	[13,21,22,23]
840	νas(WO2) + B-O-B stretching of [B2O5]4−	[13]
865	νs(WO2)	[13]
870	B-O-B bridges in pyroborate dimers, [B2O5]4−	[21,22,23]
970	ν1[WO4]2−	[13]
1240	B-O− stretch in pyroborate units	[21,22,23]
1380–1320	B-O− stretch in metaborate units	[21,22,23]
1400	B-O− stretch in metaborate units	[21,22,23]

.

2.3. Optical Transmission Spectra

Figure 3a,b presents the optical transmission spectra and absorption coefficient data of the investigated glasses. As is seen from Figure 3a, glasses are characterized as having good transmission in the visible region of the electromagnetic spectrum at around 80%; this slightly decreases with the WO₃ loading. The changes in the optical transmission observed are due to the low coloration of the glasses containing WO₃. In addition, the absence of any absorption bands in the visible range indicates that there are no tungstate ions in the lower-than-W⁺⁶ valance state, since the reduced W⁺⁵ and W⁺⁴ ions produce very intensive absorption bands in the visible range due to d-d transition [25].

Two weak absorption bands at 392 and 463 nm are observed, corresponding to the f-f transitions of Eu³⁺ ions between the ground and the excited states. We have calculated the absorption coefficient (α) using the following equation:

$$\alpha =\left\{\mathit{ln}\left(\frac{100}{T}\right)\right\}/d$$

where T is the percentage transmission and d is the thickness of the glass. The absorption coefficients versus wavelength spectra are presented in Figure 3b. The maximum absorption values of the glasses increase with the increase in WO₃ content and vary between 280 and 311 nm.

2.4. Luminescent Properties

The optical properties of the obtained glasses were further studied. The excitation spectra of 50ZnO:(50 − x)B₂O₃:xWO₃:0.5Eu₂O₃ (x = 0, 1, 3, 5 and 10) glasses, shown in Figure 4, are recorded at room temperature, monitoring the most prominent Eu³⁺ emission at 612 nm (⁵D₀ → ⁷F₂ transition). The spectra consist of two distinct parts. The first part is composed of a broad band in the spectral range from 250 nm to 350 nm due to the ligand-to-metal charge transfer transitions (LMCT) of O²⁻ → W⁶⁺ [26] and O²⁻ → Zn²⁺ [27] inside the WO_n (WO_n = WO₄ and WO₆) and ZnO_n (ZnO_n = ZnO₄) host absorbing groups, respectively, and from the oxygen 2p orbital to the empty 4f orbital of europium (O²⁻→ Eu³⁺) [28,29,30,31]. Additionally, several sharp intra-configurational 4f → 4f transitions of Eu³⁺ can be observed in the spectra at 317 nm, 375 nm, 380 nm, 392 nm, 413 nm, 463 nm, 524 nm, 531 nm and 576 nm due to ⁷F₀ → ⁵H₃, ⁷F₀ → ⁵D₄, ⁷F₀ → ⁵G₂, ⁷F₁ → ⁵L₇, ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₃, ⁷F₀ → ⁵D₂, ⁷F₀ → ⁵D₁, ⁷F₁ → ⁵D₁, ⁷F₀ → ⁵D₀, respectively [32]. The ⁷F₀ → ⁵H₃ excitation peak at 317 nm is superposed over the broad charge transfer band. The strongest excitation lines are observed at 392 nm and 463 nm, assigned to the ⁷F₀ → ⁵L₆ and ⁷F₀ → ⁵D₂ transitions in the near-UV and visible blue regions. These wavelengths are suitable for excitation with commercial near-ultraviolet light-emitting diodes (LEDs) (250–400 nm) and blue LED chips (430–470 nm).

As can be seen from Figure 4, with increasing concentration of WO₃ in the glass composition, the intensity of Eu³⁺ f-f transitions increases for up to 5 mol % WO₃. A further increase in WO₃ leads to a decrease in the excitation intensity. Accordingly, the addition of WO₃ into Eu³⁺-doped 50ZnO:50B₂O₃ host glass up to 5 mol % is beneficial to achieve good excitation, because as a rule, Eu³⁺ excitation peaks are characterized by low intensity due to the parity-forbidden law. The appearance of absorption of the host matrix, when monitoring the Eu³⁺ emission at 612 nm, has been shown to play an important role in the enhancement of the rare earth emission intensity through the occurrence of non-radiative energy transfer, in particular from WO_n and ZnO_n structural polyhedra to the Eu³⁺ ion [13,14,26,33,34,35,36,37,38]. This process is known as “host sensitized” energy transfer. Additionally, according to data from the literature, the 350 nm–700 nm region is registered to be the broad emission band (red line, Figure 5) of WO₃ [39] and ZnO [27]. In the same spectral region are also located the excitation bands of Eu³⁺ (black line) [32]. The other requirement for energy transfer is the overlap of the host group emission and the excitation levels of the active ion. As can be seen from Figure 5, in our case, this condition is satisfied.

In Figure 6, it can be seen the emission spectra of pure and Eu³⁺-doped 50ZnO:40B₂O₃:10WO₃ glass matrix were acquired upon excitation at the maximum value of the host charge transfer band at 248 nm (WO_n and ZnO_n absorbing groups). The observed decrease in the emission of the host absorbing groups in Eu³⁺-doped glass (red line), as compared to pure host emission (black line) [27,39], evidences that energy transfer has occurred. In other words, the energy absorbed by tungstate and zincate groups is further transferred non-radiatively to the active Eu³⁺ ion. The results obtained above also imply that Eu³⁺ and the WO_n and ZnO_n groups are closely coordinated in the structure [33]. It is known that the probability of energy transfer increases when the host absorbing groups, in our case WO_n or ZnO_n and the Eu³⁺ active ion, are nearest neighbors in the structure [40]. The emission spectra of Eu³⁺-doped glasses are also obtained by monitoring the ⁷F₀ → ⁵L₆ transition at 392 nm (Figure 7).

The characteristic intra-configurational transition of Eu^{3+ 5}D₀→⁷F_J, where J = 0, 1, 2, 3, 4 at 578 nm, 591 nm, 642 nm, 652 nm and 700 nm, appeared in the spectra [32]. The addition of WO₃ into the glass composition leads to an increase in the emission intensity for up to 5 mol %. At 10 mol % WO₃, a luminous quenching is observed in the spectra. Among the Eu³⁺ sharp emission bands, the most intense one, located at 612 nm, was allowed by the electric dipole and was sensitive to the changes in the surrounding ⁵D₀→⁷F₂ transition (red color of emission), followed by the magnetic dipole, which allowed the ⁵D₀→⁷F₁ transition (orange emitting color), which is insensitive to the coordination environment around Eu³⁺ ions. The ratio between these emissions, known as the asymmetric ratio R, can reveal the degree of asymmetry in the local environment around the Eu³⁺ and the strength of the Eu–O covalence for various Eu³⁺-doped compounds. The lower the value of the asymmetry parameter, the higher the symmetry around the active ion, and the lower the Eu–O covalency and emission intensity. The increase in R value is due to the increase in asymmetry and covalency between the Eu³⁺ ion and the ligands [41]. The R values of the synthesized glasses are listed in

Table 3. Values of the relative intensity ratio (R) for 50ZnO:(50 − x)B₂O₃:xWO₃:0.5Eu₂O₃ (x = 0, 1, 3, 5 and 10) glasses.

Glass Composition	Relative Luminescent Intensity Ratio, R	Reference
50ZnO:50B2O3:0.5Eu2O3	4.34	Current work
50ZnO:49B2O3:1WO3:0.5Eu2O3	5.67	Current work
50ZnO:47B2O3:3WO3:0.5Eu2O3	5.71	Current work
50ZnO:45B2O3:5WO3:0.5Eu2O3	5.82	Current work
50ZnO:40B2O3:10WO3:0.5Eu2O3	5.57	Current work + [13]
50ZnO:(50 − x)B2O3: xNb2O5:0.5Eu2O3:, x = 0, 1, 3 and 5 mol %	4.31–5.16	[42]
50ZnO:40B2O3:10WO3:xEu2O3 (0 ≤ x ≤ 10)	4.54÷5.77	[13]
50ZnO:40B2O3:5WO3:5Nb2O5:xEu2O3 (0 ≤ x ≤ 10)	5.09÷5.76	[34]
4ZnO:3B2O3 0.5–2.5 mol % Eu2O3	2.74–3.94	[43]
60TeO2:39ZnO:1Eu2O3	3.25	[44]
60TeO2:20ZnO:19LiF:1Eu2O3	3.70	[44]
60TeO2:19ZnO:10Na2O:10Li2O:1Eu2O3	3.73	[44]
60ZnO:20B2O3:(20 − x)SiO2−xEu2O3 (x = 0 and 1)	3.166	[6]
[{(TeO2)0.7:(B2O3)0.3}0.7:(ZnO)0.3](1−y)(Eu2O3)y with 0.01 = y ≤ 0.05	2.15–3.08	[45]
59.5Li2O:39.5B2O3:1Eu2O3	4.18	[46]
19.5Na2O:20MgO:59.5SiO2:1Eu2O3	4.43	[47]
(15 − x)WO3–5Al2O3–80TeO2–xEu2O3; x = 0.1÷5 mol %	5.4÷6	[48]

along with other data reported in the literature about Eu³⁺-doped oxide glasses [6,13,34,42,43,44,45,46,47,48]. The relatively higher values compared to other reported Eu³⁺-doped oxide glasses indicate that Eu³⁺ occupies crystallographic sites with low symmetry and also provide evidence of the high Eu³⁺-O²⁻ covalency. The highest obtained R value of 5.82, attributed to 50ZnO:45B₂O₃:5WO₃:0.5Eu₂O₃, is an indication of the highest emission intensity. Additionally, the Eu^{3+ 5}D₀→⁷F₁ transition splits into three emission peaks, centered at 586 nm, 591 nm and 596 nm. This effect probably arises from crystal field splitting, which causes a single transition to produce multiple emission peaks [49]. Furthermore, the appearance of the ⁵D₀→⁷F₀ transition, which is sensitive to the crystal field and is forbidden based on the standard Judd–Ofelt theory, indicates that the Eu³⁺ ion occupies non-centrosymmetric sites with C_2ν, C_n or C_s symmetry [50].

CIE Color Coordinates and CCT (K) Values

To better understand the luminescent behavior and the actual color of emissions, the standard Commission Internationale de l’Eclairage (CIE) 1931 chromaticity diagram was used [51]. The color chromaticity coordinates of the synthesized glasses are calculated from the PL spectra (Figure 7), using SpectraChroma software, Version 1.0.1 (CIE coordinate calculator) [52] and are shown at Figure 8. The obtained values of glasses with different WO₃ content, enlisted at

Table 4. CIE chromaticity coordinates and correlated color temperatures (CCTs, K) of 50ZnO:(50 − x)B₂O₃:xWO₃:0.5Eu₂O₃ (x = 0, 1, 3, 5 and 10).

Glass Composition	Chromaticity Coordinates (x, y)	CCT (K)
50ZnO:50B2O3:0.5Eu2O3	0.645, 0.346	2301.26
50ZnO:49B2O3:1WO3:0.5Eu2O3	0.651, 0.348	2324.59
50ZnO:47B2O3:3WO3:0.5Eu2O3	0.650, 0.350	2270.30
50ZnO:45B2O3:5WO3:0.5Eu2O3	0.652, 0.347	2359.79
50ZnO:40B2O3:10WO3:0.5Eu2O3	0.654, 0.345	2439.74
NTSC standard for red phosphors	0.670, 0.330
Y2O2S:Eu3+	0.658, 0.340

, are almost similar and are very close to the CIE coordinate of standard red light (0.67, 0.33) and to the color coordinates of the commercial red phosphor Y₂O₂S:Eu³⁺ (0.658; 0.340) [53].

The values of correlated color temperature (CCT) were calculated by the McCamy empirical expression [54]:

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

$$n=\frac{(x-x_e)}{(y-y_e)}$$

Here, x_e = 0.332, y_e = 0.186 are the epicenter coordinates and x and y are the calculated color chromaticity coordinates. In our case, the calculated CCT values shown in Table 4 are in the range of 2270.30 K ÷ 2439.74 K, and the Eu³⁺-doped glasses can be referred to as warm light-emitting materials.

3. Discussion

Raman analysis revealed that tungstate ions incorporate into the base zinc borate glass as tetrahedral [WO₄]²⁻ groups, and octahedral [WØ₄O₂]²⁻ species with four bridging and two non-bridging oxygen atoms. There are also metaborate, [BØ₂O]⁻ and pyroborate units, [B₂O₅]⁴⁻ in the glass networks. Tungstate, metaborate and pyroborate units are charge balanced by Zn²⁺ and Eu³⁺ ions via Zn-O-W, Zn-O-B, Eu-O-W and Eu-O-B bonding. As it was proved in our previous paper, W-O-B bonding is not possible [25]. Thus, the investigated glasses are characterized by a heterogeneous structure, because in the main zinc borate glass network, regions rich in tungsten are formed. Structural heterogeneity in these glasses increases with increasing WO₃ content, as the number of tungstate structural units, as well as Zn-O-W bonds, increase. This observation has been also confirmed by DCS analysis, wherein two glass transition effects have been noted in the DSC curve of glasses, due to the presence of two amorphous phases with different compositions. On the other hand, the addition of WO₃ into the base zinc borate glass causes depolymerization of the borate oxygen network, i.e., increasing the number of non-bridging oxygen atoms. In this way, with increasing WO₃ concentrations, the structural disorder in the amorphous network increases, and also, a more loosened glass structure is formed. The established structural features of the investigated glasses determine their good emission properties. All glasses are characterized by a high-intensity red luminescence band at 612 nm corresponding to the forced electric dipole transition (ED) ⁵D₀ → ⁷F₂, which is stronger for the WO₃-containing glasses as compared with the WO₃-free glass. The addition of WO₃ to the base zinc borate glass improves the Eu³⁺’s luminescence properties, which can be explained by the presence of both borate and tungstate units in the active ions’ surroundings, which increases the Eu³⁺’s site asymmetry and hence the Eu³⁺’s emission intensity. Also, the presence of tungstate groups around Eu³⁺ ions ensures an occurrence of non-radiative energy transfer from tungstate units to the active ions, which additionally improves the Eu³⁺’s luminescence behavior. The observed maximum of Eu³⁺ emission for glass x = 5 shows that this glass composition ensures the most appropriate glass structure for accommodating the active ion. With a further increase in WO₃ content of 10 mol %, glass network heterogeneity is too high to promote the good distribution of the rare earth ions in the matrix, and, thus, the emission properties of europium are reduced. On the other hand, with the incorporation of a higher amount of WO₃, the average distance between tungstate groups would be decreased, which would make the energy transfer between them more sufficient and, thus less energy would be expected to be transferred to the Eu³⁺ ions [55].

4. Materials and Methods

Glasses with nominal compositions 50ZnO:(50 − x)B₂O₃:xWO₃:0.5Eu₂O₃, (x = 0, 1, 3, 5 and 10 mol %) were obtained by applying the conventional melt-quenching method, using commercial powders of reagent-grade WO₃ (Merck KGaA, Darmstadt, Germany), ZnO (Merck KGaA, Amsterdam, The Netherlands), H₃BO₃ (SIGMA-ALDRICH, St. Louis, MO, USA), and Eu₂O₃ (SIGMA-ALDRICH, St. Louis, MO, USA) as starting materials. The details of glass synthesis were given in refs. [13,14]. The glass transition (T_g) temperatures of the glasses were determined by differential scanning calorimetry (DSC) using a Netzsch 404 Pegasus instrument, 2021 Selb, Germany, at a heating rate of 10 K/min in an Ar flow of 10 mL/s, using corundum crucibles with lids. Optical transmission spectra at room temperature for the glasses were measured by a spectrometer (Ocean Optics, HR 4000, Duiven, 2010, The Netherlands) using a UV LED light source at 385 nm. Photoluminescence (PL) excitation and emission spectra at room temperature for all glasses were measured with a Spectrofluorometer FluoroLog3-22, 2014 (Horiba JobinYvon, Longjumeau, France). Raman spectra were recorded with a Raman spectrometer (Delta NU, Advantage NIR 785 nm, 2010, Midland, ON, Canada).

5. Conclusions

The present investigation demonstrates the relationship between the host glass structure and the optical properties of Eu³⁺-doped glasses 50ZnO:(50 − x)B₂O₃:xWO₃, x = 0 1, 3, 5 and 10 mol %. The result indicates that the addition of WO₃ of up to 5 mol % to the base zinc borate glass improves the luminescence intensity of doped rare earth ions, which is attributed to the formation of a more disordered glass network where Eu³⁺ ions are surrounded by both borate and tungstate units, which ensures a highly asymmetric local structure around Eu³⁺ ions sites, accordingly yielding a strong red emission of the active ions. A further increase in the tungsten content leads to luminescent quenching. All findings obtained here are favorable for the elaboration of novel red-emitting glass materials.