Structure and Luminescent Properties of Niobium-Modified ZnO-B₂O₃:Eu³⁺ Glass

Abstract

The effect of the addition of Nb₂O₅ (up to 5 mol%) on the structure and luminescent properties of ZnO-B₂O₃ glass doped with 0.5 mol% (1.32 × 10²²) Eu₂O₃ was investigated by applying infrared (IR), Raman and photoluminescence (PL) spectroscopy. Through differential thermal analysis and density measurements, various physical properties such as molar volume, oxygen packing density and glass transition temperature were determined. IR and Raman spectra revealed that niobium ions enter into the base zinc borate glass structure as NbO₄ tetrahedra and NbO₆ octahedra. A strong red emission from the ⁵D₀ level of Eu³⁺ ions was registered under near UV (392 nm) excitation using the ⁷F₀ → ⁵L₆ transition of Eu³⁺. The integrated fluorescence intensity ratio R (⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁) was calculated to estimate the degree of asymmetry around the active ion, suggesting a location of Eu³⁺ in non-centrosymmetric sites. The higher Eu³⁺ luminescence emission observed in zinc borate glasses containing 1–5 mol% Nb₂O₅ compared to the Nb₂O₅-free zinc borate glass evidences that Nb₂O₅ is an appropriate component for modifying the host glass structure and improving the emission intensity.

1. Introduction

Glasses accommodating rare-earth ions have been studied for years as luminescent materials in solid-state lasers, photonics, and opto-electronic devices like optical amplifiers, multicolor displays and detectors. Among them, glasses containing trivalent europium ion have been the subject of a great deal of interest due to its intense red emission [1,2,3,4,5]. Currently, heavy emphasis has been given to the discovery of new glass compositions for exploitation as Eu³⁺-doped luminescent hosts, as the optical properties of the active rare-earth ions in glasses strongly depend on the chemical composition of the glass matrix [6]. Glasses containing Nb₂O₅ are suitable matrices for doping with active Eu³⁺ ions since Nb⁵⁺ ions can modify the environment around the rare-earth ions due to their higher polarizability [7]. Also, Nb₂O₅ possesses significant optical characteristics, such as low phonon energy, high refractive index (n = 2.4), NIR and visible transparency, that are directly related to the luminescence properties [8,9]. The optical properties and glass-forming ability of Nb₂O₅-containing glasses are strongly related with the structural features of glasses and more particularly with the coordination state of Nb⁵⁺ ions and their way of bonding in the glass network, making the structural role of Nb₂O₅ in various glass compositions also a subject of intensive research. IR and Raman spectroscopic studies indicate that the niobium present in the amorphous network in the form of octahedral NbO₆ units or NbO₄ tetrahedral groups with different degrees of distortions and types of bonding (by corners and by edges) [10,11,12].

In this work, we report on the preparation, structure and photoluminescence properties of glasses 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xNb₂O₅, (x = 0, 1, 3 and 5 mol%). The aim is to investigate the effect of the addition of Nb₂O₅ to the binary 50ZnO:50B₂O₃ glass, on the glass structure and photoluminescence properties of the active Eu³⁺ ions doped in this host glass matrix.

2. Materials and Methods

Glasses with the composition in mol% of 50ZnO:(50 − x)B₂O₃:xNb₂O₅:0.5Eu₂O₃, (x = 0, 1, 3 and 5 mol%) were prepared by the melt-quenching method using reagent-grade ZnO (Merck KGaA, Amsterdam, The Netherlands), WO₃ (Merck KGaA, Darmstadt, Germany), H₃BO₃ (SIGMA-ALDRICH, St. Louis, MO, USA) and Eu₂O₃ (SIGMA-ALDRICH, St. Louis, MO, USA) as starting compounds. The homogenized batches were melted at 1240 °C for 30 min in a platinum crucible in air. The melts were cast into pre-heated graphite molds to obtain bulk samples. Then, the glasses were transferred to a laboratory electric furnace, annealed at 540 °C (a temperature 10 °C below the glass transition temperature) and cooled down to room temperature at a very slow cooling rate of about 0.5 °C/min in order to remove the thermal stresses. The amorphous state of the samples was confirmed by X-ray diffraction analysis (XRD) with a Bruker D8 Advance diffractometer, Karlsruhe, Germany, using Cu Kα radiation in the 10 < 2θ < 60 range. The glass transition temperature (T_g) of the synthesized glasses was determined by differential thermal analysis (DTA) using a Setaram Labsys Evo 1600 apparatus (Setaram, Caluire-et-Cuire, France) at a heating rate of 10 K/min in air atmosphere. The density of the obtained glasses at room temperature was estimated by Archimedes’ principle using toluene (ρ = 0.867 g/cm³) as an immersion liquid on a Mettler Toledo electronic balance with sensitivity of 10⁻⁴ g. From the experimentally evaluated density values, the molar volume (V_m), the molar volume of oxygen (V_o) (volume of glass in which 1 mol of oxygen is contained) and the oxygen packing density (OPD) of glasses obtained were estimated using the following relations, respectively:

$$V_m=\frac{\sum x_i{M}_i}{{\rho }_g}$$

(1)

$$V_o=V_m\times \left(\frac{1}{\sum x_i{n}_i}\right)$$

(2)

$$OPD=1000\times C\times \left(\frac{{\rho }_g}{M}\right)$$

(3)

where x_i is the molar fraction of each component i, M_i the molecular weight, ρ_g is the glass density, n_i is the number of oxygen atoms in each oxide, C is the number of oxygens per formula units, and M is the total molecular weight of the glass compositions. 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 transmission spectra at room temperature for the glasses were measured by spectrometer (Ocean optics, HR 4000, Duiven, 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 (Horiba JobinYvon, Longjumeau, France). The IR spectra of the obtained samples 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 as ±3 cm⁻¹. Raman spectra were recorded with a Raman spectrometer (Delta NU, Advantage NIR 785 nm, Midland, ON, Canada).

3. Results

3.1. XRD Spectra and Thermal Analysis

The amorphous nature of the prepared materials was confirmed by X-ray diffraction analysis. The measured X-ray diffraction patterns are shown in Figure 1. The photographic images (insets, Figure 1) show that transparent bulk glass specimens were obtained. The Eu³⁺-doped Nb₂O₅-free base zinc borate glass was colorless, while the glass samples having Nb₂O₅ were light yellowish due to the presence of Nb⁵⁺ ions [13].

The DTA data of investigated glasses are presented on Figure 2. All curves contain exothermic peaks over 500 °C corresponding to the glass transition temperature, T_g. In the DTA lines, there is an absence of glass crystallization effects. However, the T_g values of Nb₂O₅-containing glasses were slightly lower as compared with the Eu³⁺-doped Nb₂O₅-free base zinc borate glass due to the formation of weaker Nb-O bonds (bond dissociation energy—753 kJ/mol) at the expense of stronger B-O bonds (bond dissociation energy—806 kJ/mol) [14].

3.2. Raman Analysis

The effect of Nb₂O₅ addition on the structure of glass 50ZnO:50B₂O₃:0.5Eu₂O₃ was studied by applying IR and Raman spectroscopy techniques. The Raman spectra of the 50ZnO:(50 − x)B₂O₃:xNb₂O₅:0.5Eu₂O₃, (x = 0, 1, 3 and 5 mol%) glasses are shown in Figure 3.

The spectrum of Nb₂O₅-free glass (Figure 3, spectrum x = 0) agreed well with what has been reported by other authors for similar compositions [15,16,17]. The most prominent band at 877 cm⁻¹ in the base binary glass x = 0 was assigned to the symmetric stretching of pyroborate dimers, [B₂O₅]⁴⁻ [15,16,17]. The two shoulders observed at 800 cm⁻¹ and 770 cm⁻¹ are due to the ring breathing of the boroxol rings and of the six-membered borate rings with one BO₄ tetrahedron (tri-, tetra- and pentaborate rings), respectively [15]. The broad shoulder at about 705 cm⁻¹ contains contributions of at least four borate arrangements: metaborate chains [BØ₂O⁻]_n (deformation modes; 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⁻ [15,16,17]. The weak lower-frequency features at 270, 300 and 430 cm⁻¹ are related to the Zn-O vibrations, Eu-O vibrations and borate network deformation modes, respectively [15,18]. The higher-frequency activity at 1235 cm⁻¹ reflects the stretching of boron-non-bridging oxygen bonds, ν(B-O⁻) of the pyroborate dimers, while the other two features at 1365 and 1420 cm⁻¹ are due to the B-O⁻ stretching in metaborate triangular units BØ₂O⁻ [15]. The addition of Nb₂O₅ to the 50ZnO:50B₂O₃:0.5Eu₂O₃ glass led to the increase in the intensity of the bands at 705, 800 and 877 cm⁻¹. Moreover, the shoulder at 800 cm⁻¹ observed in the x = 0 glass spectrum became a peak in the Raman spectrum of glass having 1 mol% Nb₂O₅ (Figure 3 spectrum x = 1). With future increase in Nb₂O₅ content (Figure 3 spectrum x = 3 and x = 5), the peak at 800 cm⁻¹ again turns into a shoulder. According to the Raman spectral data for the other niobium-containing glasses and crystalline compounds, the niobium can be present in the amorphous networks and in the crystalline structures in the form of NbO₄ tetrahedral and octahedral NbO₆ units with different degrees of polyhedral distortion and different kinds of connection (by corners or edges) [10,19]. Slightly and highly distorted octahedral units give rise to intensive bands in the regions 500–700 cm⁻¹ and 850–1000 cm⁻¹, respectively [10,19,20]. The vibration frequencies of NbO₄ tetrahedra, that have been observed only in a few niobate crystals (LnNbO₄, Ln = Y, Yb, La, Sm) and their melts containing NbO₄ ions, occurred in the range 790–830 cm⁻¹ [10,19,20,21]. In the 800–850 cm⁻¹ range, stretching vibrations of Nb-O-Nb bonding in chains of corner-shared NbO₆ are also reported [10,22]. On this basis, the increased intensity of the bands in the intermediate spectral range 600–1000 cm⁻¹ observed in the spectra of Nb₂O₅-containing glasses compared to the Nb₂O₅-free glass is because of the overlapping contribution of the vibrational modes of niobate and borate structural groups present in the glass networks. The band at 800 cm⁻¹ observed in the x = 1 glass is due to the coupled mode including the ring breathing of the boroxol rings, the symmetric stretching ν₁ mode of tetrahedral NbO₄ groups, and vibrations of Nb-O-Nb bonding [10,19]. Because of the complex character of this band, its transformation into a shoulder in the spectra of glasses x = 3 and x = 5 having higher Nb₂O₅ content is difficult to explain. However, the slight increase in the intensity of the low-frequency band at 430 cm⁻¹ due to the bending (δ) vibrations of the NbO₆ octahedra shows that with the increasing Nb₂O₅ concentration, NbO₄ → NbO₆ transformation takes place [23]. In addition, the reduced intensity of the band at 800 cm⁻¹ observed in the glasses x = 3 and x = 5 also suggests decreasing numbers of NbO₄ tetrahedra. This assumption is confirmed also by the variations in the physical parameters established, which will be discussed in the next paragraph of the paper. Stretching vibration ν₁ of terminal Nb-O (short or non-bridging) bonds from NbO₆ octahedra or short Nb-O bonds forming part of Nb-O-B bridges contribute to the band at 877 cm⁻¹ [11]. The broad Raman shoulder at 705 cm⁻¹ is attributed to the vibration of less-distorted NbO₆ octahedra with no non-bridging oxygens, which overlap with the out-of-plane bending of triangular borate groups [10,15,16,17,19,24]. The nature of borate units also changes with the addition of Nb₂O₅ into the base x = 0 glass, which is manifested by the disappearance of the shoulder at 770 cm⁻¹ due to the ring breathing of the six-membered borate rings with one BO₄ tetrahedron (tri-, tetra- and pentaborate rings) together with the increased intensity of the band over 1200 cm⁻¹ due to the vibration of trigonal borate units containing non-bridging oxygens. These spectral changes suggest that niobium oxygen polyhedra enter into the base zinc borate glass network by destruction of the superstructural borate units and favor formation of pyroborate [B₂O₅]⁴⁻ (band at 1235 cm⁻¹) and metaborate BØ₂O⁻ groups (bands at 1365 and at 1420 cm⁻¹), which are charge-balanced by niobium.

3.3. IR Analysis

Information for the structure of the present glasses was also obtained by using IR spectroscopy. The normalized IR spectra of the glasses 50ZnO:(50 − x)B₂O₃:xNb₂O₅:0.5Eu₂O₃, (x = 0, 1, 3 and 5 mol%) are depicted in Figure 4. All glass spectra are characterized by a stronger absorption in the 1600–1150 cm⁻¹ range, a wide spectral contour in the region 1150–750 cm⁻¹ and strong bands in the 750–500 cm⁻¹ range. IR spectra of Nb₂O₅-containing glasses (Figure 4, x = 1, x = 3, x = 5) exhibit also a band at 470 cm⁻¹, reaching the highest intensity in the x = 3 glass spectrum. The stronger absorption in the 1600–1150 cm⁻¹ range is connected with the stretching vibration of the B-O bonds in the trigonal borate units [25]. The IR activity in the spectral range 1150–750 cm⁻¹ arises from the vibrations of B-O bonds in [BØ₄]⁻ species, the vibrations of Nb-O-Nb bonding in chains of corner-shared NbO₆ groups, and Nb-O short bond vibrations in highly distorted NbO₆ octahedra and NbO₄ tetrahedra [10,15,23,26]. The strong bands in the 750–500 cm⁻¹ range are connected with the bending modes of trigonal borate entities that overlap with the ν₃ asymmetric stretching vibrations of corner-shared NbO₆ groups [10,23,26]. The low-frequency band at 470 cm⁻¹, visible in the spectra of glasses containing Nb₂O₅ (x = 1, x = 3 and x = 5), can be related to the NbO₆ stretching modes, having in mind the data in ref. [23] for Eu³⁺-doped crystalline rare-earth niobate Gd₃NbO₇. The structure of this compound consists of GdO₈ units forming infinite chains along the [001] direction alternately with the NbO₆ units and its IR spectrum containing the strong band at 483 cm⁻¹ due to the stretching (ν) vibrations of NbO₆ octahedra [23].

Analysis of the IR spectra obtained shows that various borate and niobate structural units co-exist in the structure of the investigated glasses and their vibrational modes are strongly overlapped. That is why a deconvolution process of the IR glass spectra was performed to make a more precise assignment of the peaks observed; the resulting spectra are shown in Figure 5.

The observed absorption bands in the deconvoluted spectra of the investigated glasses can be interpreted having in mind the band assignments proposed by Topper et al. in ref. [15] for xZnO-(1 − x)B₂O₃ glasses just above the metaborate stoichiometry, as well as taking into account our previous spectral investigation on 50ZnO:40B₂O₃:10WO₃:xEu₂O₃ (0 ≤ x ≤ 10) and (50 − x)WO₃:25La₂O₅:25B₂O₃:xNb₂O₅ (0 ≤ x ≤ 20) glasses reported in refs. [10,18]. Some other spectral data available in the literature for the similar glass and crystalline compounds were also taken into account [23,25,26,27]. The results are summarized in

Table 1. IR peak positions of the deconvoluted IR spectra of glasses 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xNb₂O₅, (x = 0, 1, 3 and 5 mol%) and their assignments.

Peak #	Peak Position, cm−1				Band Assignment	Ref.
	0% Nb2O5	1% Nb2O5	3% Nb2O5	5% Nb2O5
1	566	602	591	-	Bending modes of various trigonal borate units.	[15,18]
2	646	641	-	668	Bending modes of various trigonal borate units.	[15,18]
3	700	715	717	720	Bending modes of various trigonal borate units.	[15,18]
4	841	-	-	-	B-O stretching modes of [BØ]− from ring type superstructures containing one or two tetrahedral boron sites + νas of tetrahedral metaborate groups.	[15]
5	936	-	-	-	B-O stretching modes of [BØ]− from ring type superstructures containing one or two tetrahedral boron sites + νas of tetrahedral metaborate groups.	[15]
6	1025	1031	1034	1005	B-O stretching modes of [BØ]− from ring type superstructures containing one or two tetrahedral boron sites + νas of tetrahedral metaborate groups.	[15]
7	1105	1111	1111	1101	B-O stretching modes of [BØ]− from ring type superstructures containing one or two tetrahedral boron sites + νas of tetrahedral metaborate groups.	[15]
8	1236	1236	1228	-	Stretching vibrations of BØ3 triangles involved in various ring type superstructural borate groups (boroxol rings, tri-,tetra- and pentaborates).	[15,18]
9	1313	-	-	-	B-O− stretch in pyroborate units.	[25]
10	1387	1371	1398	1383	Stretching vibrations of non-bridging B-O− bonds in metaborate units, BØ2O−.	[10,15]
11	1472	1459	1462	1466	Stretching vibrations of non-bridging B-O− bonds in metaborate units, BØ2O−.	[10,15]
12	1535	1519	1508	1520	Stretching of B-Ø bonds in neutral BØ3 triangles.	[25]
13	-	480	481	479	Stretching vibrations of NbO6.	[23,27]
14	-	681	680	692	ν3 asymmetric stretching vibrations of NbO6.	[10]
15	-	825	825	827	Vibrations of Nb-O-Nb bonding in chains of corner shared NbO6 groups.	[10]
16	-	876	876	877	ν1 symmetric mode of short Nb-O bonds in distorted NbO6 and NbO4 units.	[10]
17	-	1197	1179	1180	B-O-B stretch in pyroborate units, BØO22−.	[25]
18	-	1276	1284	1258	ν3 asymmetric stretching mode of orthoborate groups, BO33−.	[25]
19	-	-	628	629	ν3 asymmetric stretching vibrations of NbO6.	[10,26]
20	-	1054	1054	1049	B-O stretching modes of [BØ]− from ring type superstructures containing one or two tetrahedral boron sites + νas of tetrahedral metaborate groups.	[15]

.

The IR data show that the addition of Nb₂O₅ into the 50ZnO:50B₂O₃ glass doped with 0.5 mol% (1.32 × 10²²) Eu₂O₃ produces some changes in the IR spectrum, reflecting structural changes taking place with the composition. The most obvious effects are the reduction in the number of [BØ₄]⁻ bands in the region 750–1150 cm⁻¹, together with the strong decrease in the relative area of the band number 8 at 1236 cm⁻¹ (stretching vibration of BØ₃ triangles involved in various ring type superstructural borate groups). At the same time, new bands 13; 14; 15; 16 and 19 related to the vibrations of niobate structural units NbO₆ and NbO₄ (see Table 1) and 17, 18 connected with the vibration of pyro- and orthoborate groups in the network of Nb₂O₅-containing glasses appeared. The decreased number of bands due to the [BØ₄]⁻ tetrahedra, and the strong reduction in band 8 at 1236 cm⁻¹ (B-O-B bridges connecting superstructural groups through three-fold coordinated boron centers) are in agreement with the conclusions of the Raman analysis above and correspond to the destruction of borate superstructural units containing tetrahedral groups and increasing numbers of BO₃-containing entities. On the other hand, the IR spectrum of Eu³⁺-doped crystalline Gd₃NbO₇ contains strong bands at 483 cm⁻¹ and at 627 cm⁻¹ (stretching vibration of NbO₆) such as new bands 13 and 19 present in the IR spectra of Nb₂O₅-containing glasses. Since the spectral similarity supposes structural similarity, we suggest that the structure of investigated glasses is similar to the structure of the crystalline Gd₃NbO₇, which consists of infinite chains of GdO₈ units alternately with the NbO₆ units i.e., evidencing the presence of Eu³⁺ ions located around the niobate octahedra (Nb-O-Eu bonding) [23]. In the x = 3 glass spectrum, the band 13 at 480 cm⁻¹ (ν of NbO₆ in the vicinity of Eu³⁺) as well as the band 19 at 629 cm⁻¹ possess higher relative area, indicating the highest number of NbO₆ octahedra surrounding rare-earth ions in this glass composition (i.e., the highest number of Nb-O-Eu linkages).

Thus, the IR spectral analysis shows that addition of Nb₂O₅ into the base zinc borate glass depolymerizes the borate oxygen network, causing the destruction of superstructural borate groups and their conversion to BO₃-containing borate entities. The structure of Nb₂O₅-containing glasses consists mainly of [BØ₂O]⁻ and [BØ₄]⁻ metaborate groups, [B₂O₅]⁴⁻ pyroborate and [BO₃]³⁻ orthoborate units, isolated NbO₄ tetrahedra and corner-shared NbO₆. The presence of niobium increases the disorder and the degree of connectivity between the various structural units in the glass network, as it participates in the formation of mixed bridging Nb-O-B and Nb-O-Eu and as well as Nb-O-Nb linkages.

3.4. Physical Parameters

The observed variation in density and various physical parameters, such as molar volume (V_m), oxygen molar volume (V_o) and oxygen packing density (OPD), of the investigated glasses are listed in

Table 2. Values of physical parameters of glasses 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xNb₂O₅, (x = 0, 1, 3 and 5 mol%): density (ρ_g), molar volume (V_m), oxygen molar volume (V_o), oxygen packing density (OPD). Optical band gap (E_g) values of glasses 50ZnO:(50 − x)B₂O₃:0.5Eu₂O₃:xNb₂O₅, (x = 0, 1, 3 and 5 mol%).

Sample ID	ρg (g/cm3)	Vm (cm3/mol)	Vo (cm3/mol)	OPD (g atom/L)	Eg (eV)
x = 0	3.413 ± 0.001	22.634	11.261	88.804	3.80
x = 1	3.567 ± 0.001	22.208	10.940	91.408	3.78
x = 3	3.663 ± 0.001	22.697	10.965	91.201	3.67
x = 5	3.665 ± 0.001	23.755	11.258	88.823	3.66

. They are in line with the proposed structural features, based on the Raman and IR spectral data. The Nb₂O₅-containing glasses are characterized by higher density and OPD values, evidencing that the presence of Nb₂O₅ in the zinc borate glass causes the formation of highly cross-linked and compact networks [28]. The lowest OPD value of the glass having the highest Nb₂O₅ content (x = 5), as compared with the OPD values of other Nb₂O₅-containing glasses, indicates decreasing cross-link efficiency of niobium ions and higher numbers of non-bridging atoms in the structure of this glass. With the introduction of 1 mol% Nb₂O₅ into the base zinc-borate glass, the molar volume V_m and oxygen molar volume V_o decrease, while with the further increase in Nb₂O₅ content (x = 3 and x = 5), both parameters start to increase. These observed changes can be explained with the NbO₄ → NbO₆ conversion upon Nb₂O₅ loading and the formation of a reticulated network because of the presence of high numbers of mixed bridging bonds (B-O-Nb, and Eu-O-Nb) within Nb₂O₅-containing glass networks [29].

3.5. Determination of Optical Band Gap

Some structural information also can be obtained from the optical band gap values (E_g) evaluated from the UV-Vis spectra with the Tauc method by plotting (F(R_∞) hν)^1/n, n = 2 versus hν (incident photon energy), as shown in Figure 6 [30]. It is accepted that in metal oxides, the creation of non-bonding orbitals with higher energy than bonding ones shifts the valence band to higher energy, which results in E_g decreasing [31]. Therefore, the increase in the concentration of the NBOs (non-bridging oxygen ions) reduces the band gap energy. As seen from Figure 6, the E_g values decrease with increasing Nb₂O₅ content, indicating an increasing number of non-bridging oxygen species in the glass structure. This suggestion is in agreement also with the IR and Raman data obtained for the depolymerization of the borate network with the addition of Nb₂O₅ into the base ZnO-B₂O₃ glass. On the other hand, for the glasses containing Nb₂O₅, the reduction in E_g values is related to the increase in the glass’s overall polarizability due to the insertion of NbO₆ octahedra and their mutual linking into the glass structure [8]. Thus, the same E_g values of x = 5 and x = 3 glasses show that there is an increasing number of polymerized NbO₆ groups in the structure of glass x = 5.

3.6. EPR Spectroscopy

EPR analysis was carried out to provide insightful information about the Eu²⁺ ions in the studied glasses.

Figure 7 shows several dominant signals with g-values at g = 2.7, g = 4.6, g = 6.0. The most intensive feature is assigned to the impurities of isolated Mn²⁺ ions. The observed resonance signals in the spectral range 0–300 mT are assigned to the presence of Eu²⁺ ions in a highly asymmetric site environment [32,33]. The EPR spectra indicate the presence of low concentrations of Eu²⁺ ions in the obtained glasses, based on the comparison between the background spectrum and the analyzed spectra.

3.7. Optical Studies

The optical transmittance spectra and absorption coefficient data for investigated glasses are presented in Figure 8a,b.

As seen from Figure 8a, all glasses are characterized by good transmission in the visible region at around 80%. The low-intensity absorption bands at about 395 nm and 465 nm correspond to f-f transitions of Eu³⁺ ions between the ground and excited states. It should be mentioned that the reduction process of the valence of niobium ions (Nb⁵⁺ → Nb⁴⁺) produces very intense absorption peaks in the visible range due to the d-d transition. In the obtained spectra, there are no absorption bands corresponding to d-d transition, suggesting that Nb ions in the investigated glasses are present as Nb⁵⁺ only. The absorption coefficient (α) has been calculated with the following equation:

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

where “T” is the percentage transmission and “t” is thickness of the glass. Figure 8b shows the absorption coefficients versus wavelength spectra. The maximum absorption values of the glasses increase with the increase in Nb₂O₅ content and vary between 290 and 316 nm.

3.8. Luminescent Properties

The excitation spectra (Figure 9) of the obtained glasses, monitored at 612 nm, consist of a wide excitation band below 350 nm and some narrow transitions of Eu³⁺ located at 317 nm (⁷F₀ → ⁵H₃), 360 nm (⁷F₀ → ⁵D₄), 375 nm (⁷F₀ → ⁵G₂), 380 nm (⁷F₁ → ⁵L₇), 392 nm (⁷F₀ → ⁵L₆), 413 nm (⁷F₀ → ⁵D₃), 463 nm (⁷F₀ → ⁵D₂) 524 (⁷F₀ → ⁵D₁), 530 nm (⁷F₁ → ⁵D₁) and 576nm (⁷F₀ → ⁵D₀) [34].The wide excitation band in the UV region is attributed to the charge transfer transition of Eu³⁺ (O²⁻ → Eu³⁺) [35,36,37,38] and host absorbing ZnO_n groups (O²⁻ → Zn²⁺) [39] and NbO_n groups (O²⁻ → Nb⁵⁺) [40]. Their contribution cannot be clearly differentiated due to the spectral overlap.

Figure 9 shows that the increase in the Nb₂O₅ concentration in the glass composition leads to an increase in both charge transfer band intensity and narrow Eu³⁺ peaks. On the basis of structural analysis, it can be assumed that Nb₂O₅ modifies the glass network and makes it convenient for accommodation of Eu³⁺ ions. Hence, the incorporation of niobium into Eu³⁺-doped 50ZnO:50B₂O₃ host materials is favorable for achieving proper excitation, since, in general, Eu³⁺ bands are weak due to the parity-forbidden law. As can be seen from Figure 9, the strongest band is located at 392 nm (⁷F₀ → ⁵L₆ transition), followed by ⁷F₀ → ⁵D₂ transition at 463 nm. These data signify that the obtained phosphors can be efficiently excited with a range of excitation wavelengths of the commercially available near ultraviolet—NUV (250–400 nm) and blue LED chips (430–470 nm).

The emission spectra of Eu³⁺-doped 50ZnO:(50 − x)B₂O₃: xNb₂O₅:0.5Eu₂O₃:, x = 0, 1, 3 and 5 mol% glasses (Figure 10) were acquired upon excitation at 392 nm (⁷F₀ → ⁵L₆ transition). The observed bands are due to the intra-configurational transitions of the excited ⁵D₀ state to the ground states ⁷F₀ (578 nm), ⁷F₁ (591 nm), ⁷F₂ (612 nm), ⁷F₃ (651 nm), and ⁷F₄ (700 nm) in the ⁴F₆ configuration of the Eu³⁺ ion [34]. The energy at 392 nm is not sufficient to excite the host optical groups, as their absorption is located below 350 nm, and thus, the non-radiative energy transfer to active ions cannot be expected. In detail, the excited ⁵L₆ energy-level electrons relax into the first excited metastable singlet state ⁵D₀ from ⁵D₃, ⁵D₂, and ⁵D₁ states without visible emissions. In other words, the absorbed energy relaxes to the ⁵D₀ state by the non-radiative process, and then, the emission of Eu³⁺ occurs by the radiative process.

The addition of Nb₂O₅ up to 3 mol% leads to an increase in the emission intensity. The luminescence suppression is observed at 5 mol% Nb₂O₅.

The strongest emission line, located at 612 nm, is caused by the forced electric dipole transition (ED) ⁵D₀ → ⁷F₂, sensitive to small changes in the environment, followed by the magnetic dipole (MD) ⁵D₀ → ⁷F₁ transition insensitive to the surroundings [34,35]. An indication that Eu³⁺ ions are distributed in a non-inversion symmetry sites in the glass host is the fact that the predominant emission is from the ED transition rather than from the MD transition. Therefore, the value of relative luminescent intensity ratio R of the two transitions (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) (

Table 3. Relative luminescent intensity ratio (R) of the two transitions (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) for glasses with different Nb₂O₅ content and of other reported Eu³⁺-doped oxide glasses.

Glass Composition	Relative Luminescent Intensity Ratio, R	Reference
50ZnO:50B2O3:0.5Eu2O3	4.31	Present work
50ZnO:49B2O3:1Nb2O3:0.5Eu2O3	4.89	Present work
50ZnO:47B2O3:3Nb2O3:0.5Eu2O3	5.16	Present work
50ZnO:45B2O3:5Nb2O3:0.5Eu2O3	5.11	Present work
50ZnO:40B2O3:10WO3:xEu2O3 (0 ≤ x≤ 10)	4.54–5.77	[18]
50ZnO:40B2O3:5WO3:5Nb2O5:xEu2O3 (0 ≤ x ≤ 10)	5.09–5.76	[42]
(100 − y)TeO2-10Nb2O5-yPbF2 (0 ≤ y ≤ 30)	2–4.16	[43]
69TeO2:1K2O:15Nb2O5:1.0Eu2O3	5	[44]
60TeO2:19ZnO:7.5Na2O:7.5Li2O:5Nb2O5:1Eu2O3	3.73	[45]
4ZnO:3B2O3:0.5–2.5 mol% Eu3+	3.94–2.74	[46]
(99.5 − x):B2O3:xLi2O:0.5Eu2O3	2.41–3.40	[47]
(64 − x)GeO2:xSiO2:16K2O:6BaO:4Eu2O3	3.42–4.07	[47]
(98 − x)P2O5:xCaO:2Eu2O3	3.88–3.95	[47]
79TeO2 + 20Li2CO3 + 1Eu2O3	4.28	[48]

) gives information on the degree of asymmetry around the Eu³⁺ ions [2,41]. The higher the value of the asymmetry parameter, the lower the local site symmetry of the active ion, and the higher Eu–O covalence and emission intensity. The calculated higher R values (from 4.31 to 5.16), compared to the others reported in the literature (Table 3) [18,42,43,44,45,46,47,48], suggest more asymmetry in the vicinity of Eu³⁺ ions, stronger Eu–O covalence, and thus enhanced emission intensity.

Comparing the R values of the synthesized zinc borate glass without Nb₂O₅ (4.31) and glass samples containing 1–5 mol% Nb₂O₅ (4.89–5.16), it can be assumed that Nb₂O₅ addition leads Eu³⁺ to a high-asymmetry environment in the host, increasing the intensity of ⁵D₀ → ⁷F₂ transition. The most intensive emission was registered with 3 mol% Nb₂O₅. Increasing the Nb₂O₅ content (5 mol%) leads to a slight decrease in the emission intensity (Figure 10) as a result of the increasing Eu³⁺ site symmetry (a slight reduction in R value) (Table 3). An additional indication of the Eu³⁺ location in non-centrosymmetric sites is the appearance of the ⁵D₀ → ⁷F₀ transition in the emission spectra. Based on the standard Judd-Ofelt theory, this transition is strictly forbidden. According to Binnemans, the observation of the ⁵D₀ → ⁷F₀ band shows that Eu³⁺ ions occupy sites with C_2v, C_n or C_s symmetry [49].

CIE Color Coordinates and CCT (K) Values

To characterize the emission color of Eu³⁺-doped glasses, the standard Commission International de l’Eclairage (CIE) 1931 chromaticity diagram was applied [50]. From the luminescence spectra, the chromaticity coordinates of specimens were calculated using color calculator software SpectraChroma (CIE coordinate calculator) [51]. The obtained values are listed in

Table 4. CIE chromaticity coordinates, dominant wavelength, color purities and correlated color temperature (CCT, K) of 50ZnO:(50 − x)B₂O₃: xNb₂O₅:0.5Eu₂O₃, x = 0, 1, 3 and 5 mol%.

Glass Composition	Chromaticity Coordinates (x, y)	CCT (K)
50ZnO:B2O3:0.5Eu2O3 (x = 0)	(0.645, 0.346)	2301.26
50ZnO:49B2O3:1Nb2O3:0.5Eu2O3 (x = 1)	(0.656, 0.344)	2479.99
50ZnO:47B2O3:3Nb2O3:0.5Eu2O3 (x = 3)	(0.656, 0.343)	2505.78
50ZnO:45B2O3: 5Nb2O3:0.5Eu2O3 (x = 5)	(0.657, 0.343)	2518.60
NTSC standard for red phosphors	(0.67, 0.33)
Y2O2S:Eu3+	(0.658, 0.340)

, whereas references are included for the chromaticity coordinates of the commercial phosphor Y₂O₂S:Eu³⁺ [52] and National Television Standards Committee (NTSC) for red color. As can be seen from Table 4, the chromaticity coordinates of the niobium-containing glasses are very close to the standard recommended by NTSC (0.67, 0.33) values and nearly equivalent to the commercially applied red phosphor Y₂O₂S:Eu³⁺ (0.658, 0.340). The calculated values are almost identical and cannot be individually separated on the CIE diagram (Figure 11). These data show that the obtained glasses are characterized by high color purity.

The color-correlated temperature (CCT) was calculated by the McCamy empirical formula [53]:

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

where n = (x − x_e)/(y − y_e) is the reciprocal slope, (x_e = 0.332, y_e = 0.186) is the epicenter of convergence, and x and y are the chromaticity coordinates. The phosphors with CCT values below 3200 K are generally considered as a warm light source, while those with values above 4000 K, as a cold light source [53]. The calculated CCT values of Eu³⁺-doped glasses (Table 4) range from 2301.26 K to 2518.60 K, and these glasses can be considered as warm red light-emitting materials for solid-state lighting applications.

4. Discussion

The Raman and IR spectral data as well as the established values of the structurally sensitive physical parameters demonstrate that at smaller concentrations (up to 5 mol%), the niobium ions are embedded into the base Eu³⁺: ZnO:B₂O₃ glass as isolated NbO₄ tetrahedra and corner-shared NbO₆ with increasing distortion upon Nb₂O₅ loading. NbO₄ tetrahedral units play a network-forming role and strengthened the host glass structure through B-O-Nb bonding. NbO₆ octahedra are situated around the Eu³⁺ ions (i.e., niobate groups are charge-balanced by Eu³⁺ ions), and the higher numbers of NbO₆ surrounding Eu³⁺ are found for the glass containing 3 mol% Nb₂O₅. Other than by Eu³⁺ ions, NbO₆ octahedra are also charge-balanced by Zn²⁺ ions. Hence, the incorporation of Nb₂O₅ into Eu³⁺: ZnO:B₂O₃ glass creates more disordered and reticulated glass networks, which are favorable for doping with Eu³⁺ active ions. Moreover, the DTA analysis shows high values of glass transition temperatures (over 500 °C) and also an absence of glass crystallization effects—both confirming the formation of connected and stable glass networks.

The observed optical properties are discussed on the basis of the glass structural features. The most intensive Eu³⁺ emission peak, corresponding to the hypersensitive ⁵D₀ → ⁷F₂ transition, along with the high values of the luminescent ratio R, evidence that Eu³⁺ ions are located in low site symmetry in the host matrix. This emission peak intensity and the R values of Nb₂O₅-containing glasses are higher in comparison with the Nb₂O₅-free Eu³⁺: ZnO:B₂O₃ glass, indicating that Eu³⁺ ions are in higher-asymmetry environments in the Nb₂O₅-containing glasses because of the combination of niobate and borate structural units in the active ion surroundings. Thus, the introduction of Nb₂O₅ oxide into the Eu³⁺: ZnO:B₂O₃ glass increases connectivity in the glass network and contributes to the creation of a more distorted and rigid glass structure that lowers the site symmetry of the rare-earth ion and improves its photoluminescence behavior. The influence of Eu²⁺ ions on the luminescence of Eu³⁺ is negligible due to their low content.

The results of these investigations show that Nb₂O₅ is an appropriate constituent for modification of zinc borate glass structure and for enhancing the luminescent intensity of the doped Eu³⁺ ion.

5. Conclusions

The impact of the glass matrix on the luminescent efficiency of europium has been studied. According to IR and Raman data, the structure of glasses consists of [BØ₂O]⁻ and [BØ₄]⁻ metaborate groups, [B₂O₅]⁴⁻ pyroborate and [BO₃]³⁻ orthoborate units, isolated NbO₄ tetrahedra and corner-shared NbO₆. The local environment of the Eu³⁺ ions in the Nb₂O₅-containing ZnO:B₂O₃ glasses is dominated by the interaction with both, borate and NbO₆ octahedral structural groups. The luminescent properties of the obtained Eu³⁺-doped glasses revealed that they could be excited by 392 nm and exhibit pure red emission centered at 612 nm (⁵D₀ → ⁷F₂ transition). The incorporation of niobium oxide into the ZnO:B₂O₃ glass enhances the luminescent intensity, making it a desirable component in the glass structure. It was established that the optimum Nb₂O₅ concentration to obtain the most intensive red luminescence is 3 mol%. The structure–optical property relationship studied in this work will be favorable for the elaboration of novel red-emitting materials.