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

1. Introduction

Glasses accommodating rare-earth ions have been studied from years as luminescent materials in solid state lasers, photonic, opto-electronic devices like optical amplifiers, multicolor displays and detectors. Among them glasses containing trivalent europium ion have the subject of a great deal of interest due to its intense red emission [1,2,3,4,5]. In current years large 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 glass matrix chemical composition [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 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 which make, 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 degree of distortions and type of bonding (by corners and by edges) [10,11,12].

In this work we report for 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 melt quenching method using reagent grade ZnO, WO₃, H₃BO₃ and Eu₂O₃ 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 mold to get bulk samples. Then the glasses were transferred in a laboratory electric furnace annealed at 540^oC (a temperature 10°C below the glass transition temperature) and were cooldown 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, using Cu Kα radiation in the 10 < 2Ө < 60 range. The glass transition temperature (T_g) of the synthesized glasses was determined by differential thermal analysis (DTA) using а Setaram Labsys Evo 1600 apparatus, 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 of sensitivity 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:

$${\mathit{V}}_{\mathit{m}}=\frac{\sum {\mathit{x}}_{\mathit{i}}{\mathit{M}}_{\mathit{i}}}{{\mathit{\rho }}_{\mathit{g}}}$$

(1)

$${\mathit{V}}_{\mathit{o}}={\mathit{V}}_{\mathit{m}}\times \left(\frac{1}{\sum {\mathit{x}}_{\mathit{i}}{\mathit{n}}_{\mathit{i}}}\right)$$

(2)

$$\mathit{O}\mathit{P}\mathit{D}=1000\times \mathit{C}\times \left(\frac{{\mathit{\rho }}_{\mathit{g}}}{\mathit{M}}\right)$$

(3)

where x_i is the molar fraction of each component i, M_i the molecular weight, ρ_g the glass density and n_i is the number of oxygen atoms in each oxide, C is the number of oxygen per formula units, and M is the total molecular weight of the glass compositions. Optical transmission spectra at room temperature for the glasses were measured by spectrometer (Ocean optics, HR 4000) using a UV LED light sources at 385 nm. Photoluminescence (PL) excitation and emission spectra at room temperature for all glasses were measured with Spectrofluorometer FluoroLog3-22, Horiba JobinYvon. The IR spectra of the obtained samples were measured using the KBr pellet technique on a Nicolet-320 FTIR spectrometer 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 IR bands was found as ± 3 cm⁻¹. Raman spectra were recorded at the Raman spectrometer: Delta NU, Advantage NIR 785 nm.

3. Results

3.1. XRD spectra and thermal analysis

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

The DTA data of investigated glasses are presented on Figure 2. The all curves contain an exothermic peaks over 500◦C corresponding to the glass transition temperature, T_g and an absence of glass crystallization effects. However, the T_g values of Nb₂O₅ containing glasses are slightly lower as compared with the Eu³⁺ doped Nb₂O₅-free base zinc borate glass due to the formation of weaker Nb-O bonds at the expense of stronger B-O bonds.

3.2. Raman analysis

Both IR and Raman spectroscopy techniques were used to study the effect of Nb₂O₅ addition on the structure of glass 50ZnO:50B₂O₃:0.5Eu₂O₃ The Raman spectra of the 50ZnO:(50-x)B₂O₃:xNb₂O₅:0.5Eu₂O₃, (x=0, 1, 3 and 5 mol %) glasses are shown on 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 (14–16). The most prominent band at 877 cm^-1 in the base binary glass x = 0 is assigned to the symmetric stretching of pyroborate dimmers, [B₂O₅]^4- [14,15,16]. The two shoulders at 800 cm^-1 and at 770 cm^-1 observed 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 [14]. The broad shoulder at about 705 cm^-1 contains contributions of at least 4 borate arrangements as: metaborate chains [BØ₂O^-]_n (deformation modes; Ø=bridging oxygen, O^-=nonbridging oxygen), in-plain and out of plain bending modes of both polymerized (BØ⁰) species and isolated orthoborate units (BO₃)^3-, and bending of the B-O-B connection in the pyroborate dimmers,[B₂O₅]^4- [14,15,16]. The weak lower frequency features at 270, 300 and 430 cm^-1 are related with the Zn-O vibrations, Eu-O vibrations and borate network deformation modes, respectively [14,17]. 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^- [14]. The addition of Nb₂O₅ to the 50ZnO:50B₂O₃:0.5Eu₂O₃ glass leads 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 become 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 degree of polyhedral distortion and different kinds of connection (by corners or edges) [10,18]. Slightly and highly distorted octahedral give rise to intensive bands in the regions 500-700 cm^-1 and 850-1000 cm^-1, respectively [10,18,19]. The vibration frequencies of NbO₄ tethahedra 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 [10,18,19,20]. In the 800-850 cm^-1 range stretching vibrations of Nb-O-Nb bonding in chain of corner shared NbO₆ are also reported [10,21]. On this base, 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 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 [10,18]. 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 of 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 take place [22]. In addition, the reduced intensity of the band at 800 cm^-1 observed in the glasses x = 3 and x = 5 also suggests decreasing number 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 octahedras 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 [10,14,15,16,18,23]. The nature of borate units also change with the addition of Nb₂O₅ into the base x = 0 glass, 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 obtained also 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 with 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 (Fig. 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 [24]. 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 bonds vibrations in highly distorted NbO₆ octahedra and NbO₄ tetrahedra [10,14,22,25]. The strong bands in the 750 - 500 cm^-1 range are connected with the bending modes of trigonal borate entities which overlaps with the ν₃ asymmetric stretching vibrations of corner-shared NbO₆ groups [10,22,25]. 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 with the NbO₆ stretching modes, having in mind the data in ref. 22 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 [22].

Taking into account this and as well as the Raman data above, for increasing number of NbO₆ octahedra as a result of NbO₄→NbO₆ conversion upon Nb₂O₅ content, it could be suggested the highest intensity of this band observed in the x= 3 glass spectrum evidences the higher number of NbO₆ octahedra in the vicinity of Eu³⁺ ions in its glass network. From the analysis of the IR spectra obtained above it is seen 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 observed absorption bands in the deconvoluted spectra of the investigated glasses (Figure 5) can be interpreted having in mind the band assignments proposed by Topper et al in ref. [14] for xZnO-(1-x)B₂O₃ glasses just above the metaborate stoichiometry and 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. [10,17].

The low frequency range 750-400 cm^-1 in the x = 0 glass is fitted with three bands 1 - 3 at 566 cm^-1, 646 cm^-1 and 700 cm^-1 respectively which arise from the bending modes of various trigonal borate entities [14,17]. The wide spectral contour in the region 750-1150 cm^-1 is deconvoluted into four peaks 4-7 at 841 cm^-1, 936 cm^-1, 1025 cm^-1 and 1105 cm^-1 respectively related with the B-O stretching modes of borate tetrahedral units [BØ₄]^- from ring-type superstructures that contain one or two tehtrahedral boron sites and as well to asymmetric stretching of tetrahedral metaborate groups, ν_as[BØ₄]^- [14]. The broad band in the region 1150 - 1600 cm^-1 is well fitted with five peaks. The most intense band 8 at 1236 cm^-1 is attributed mainly to the stretching vibration of BØ₃ triangles involved in various ring types superstructural borate groups (boroxol rings, tri-, tetra- and pentaborates) [14,17]. This band can be also associated with the BO₃ stretching vibration in meta-, pyro- and orthoborate units [14]. The band 9 at 1313 cm^-1 is ascribed to the B-O- stretch in pyroborate units, [B₂O₅]^4- [24]. The two bands 10, 11 at 1387 and 1472 cm^-1 respectively are due to the stretching vibration of non-bridging B-O^– bonds in metaborate units, BØ₂O^- [10,14], while the band 12 at 1535 cm^-1 originates from the stretching of B-Ø bonds in neutral BØ₃ triangles [24]. The Zn²⁺ ions are mainly located in the tetrahedral coordination in borate glasses with characteristic Zn²⁺ motion band below 400 cm^-1, (225 cm^-1) that is outside of the limit of our apparatus [15].

The addition of Nb₂O₅ in the 50ZnO:50B₂O₃ glass doped with 0.5 mol % Eu₂O₃ produces some changes in the IR spectrum. More precise after the deconvolution of the x = 1, x = 3 and x = 5 glass spectra shown in Figure 5 several new bands have appeared related with the vibrations of niobate structural units. These are: band 13 at 480 cm^-1, due to the stretching vibrations of NbO₆ [22,26]; band 14 at 670-680 cm^-1 related with the ν₃ asymmetric stretching vibrations of corner-shared NbO₆ groups; band 15 at 825 cm^-1 ascribed to the vibrations of Nb-O-Nb bonding in chains of corner shared NbO₆; band 16 at 876 cm^-1 which is interpreted as ν₁ symmetric mode of short Nb-O bonds in both distorted NbO₆ and in NbO₄ groups [10]; band 19 at 628-629 cm^-1 in the spectra of glasses x = 3 and x = 5 and band 21 at 692 cm^-1 in the deconvoluted spectrum of glass x= 5 due to the ν₃ asymmetric stretching vibrations of corner-shared NbO₆ groups [10,25]. These additional bands arise from the splitting of the ν₃ bands as a result of the lowering the symmetry of NbO₆ units [25]. On the other hand, as the IR spectrum of Eu³⁺ doped Gd₃NbO₇ is characterized with a strong band at 627 cm^-1 due to stretching vibration of NbO₆, the intenisive band 19 at 628-629 cm^-1 observed in the spectra of glasses x=3 and x=5 evidences the presence of Eu³⁺ ions located around the niobate octahedra (Nb-O-Eu bonding)[22]. In the x = 3 glass spectrum the band 13 at 480 cm^-1 (ν of NbO₆ in the vicinity of Eu³⁺) and as well as the band at 629 cm^-1 possess higher relative area, indicating the highest number of NbO₆ octahedra surrounding rare earth ions in this glass compositions (i. e. the highest number of Nb-O-Eu linkadges).

The other changes in in the deconvoluted spectra of Nb₂O₅ containing glasses, x = 1, x = 3 and x= 5 are connected with the modification of the borate oxygen network due the introduction of niobium to the binary base glass x = 0. The most obvious effects are the reduction of the numbers of the [BØ₄]^- bands in the region 750-1150 cm^-1 together with the strong decrease of the relative area of the band number 8 at 1236 cm^-1, due mainly to the stretching vibration of BØ₃ triangles involved in various ring types superstructural borate groups. At the same time new bands 17 at 1199 cm^-1-1180 cm^-1; and 18 at 1276-1258 cm^-1 appear. The higher frequency bands at 1387 cm^-1 and at 1472 cm^-1 characteristic of the B-O^- stretching in the metaborate entities, BØ₂O^- observed in the spectrum of glass x= 0 strengthen and red shift to 1370 and 1459 cm^-1 in the spectrum of glass x = 1, while in the x = 3 glass spectrum the relative area of these band strongly decreases and their peak positions shifts to the higher frequencies (1398 cm^-1 and 1462 cm^-1). The decreased number of the bands due to the [BØ₄]^- tethahedra, and the strong reduction of the 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 corresponds to the destructions of borate superstructural units containing tetrahedral groups, and increasing of the number of the BO₃ containing entities. The new strong bands 17 at 1199 cm^-1-1179 cm^-1 and 18 at 1283 -1258 cm^-1 signals for the formation of pyro- and ortoborate groups in the network of Nb₂O₅ containing glasses [24]. The observed red shift of the terminal B-O^- stretching of BØ₂O^- units (1387 – 1370 cm^-1 and 1472 - 1459 cm^-1) observed in the spectrum of x= 1 glass suggests interaction between Nb⁵⁺ ions and non-bridging oxygens, (Nb – O - B bonding), which consequently weakens the strength of the B-O^-bonding and shifts its characteristic peak positions to the lower frequency [14,24].

To summarize this section, the IR spectral analysis show that addition of Nb₂O₅ into the base zinc borate glass depolymerizes borate oxygen network causing the destruction of superstructural borate groups and their conversion to BO₃ containing borate entities. Thus the structure of Nb₂O₅-containing glasses consists mainly of [BØ₂O]^- and [BØ₄]^- metaborate groups, [B₂O₅]^4- pyroborate and [BO₃]^3- ortoborate 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 participate in the formation of mixed bridging Nb-O-B and Nb-O-Eu bonds and as well as Nb-O-Nb linkages.

3.4. Physical parameters

The observed variation in density and various physical parameters: as molar volume (V_m), oxygen molar volume (V_o) and oxygen packing density (OPD) of the investigated glasses, which are listed in Table 1 are in line with the proposed structural features, based on the Raman and IR spectral data. As it is seen the Nb₂O₅ containing glasses are characterized with higher density and OPD values, evidencing that the presence of Nb₂O₅ into the zinc borate glass causes formation of highly cross-linked and compact networks [27]. The lowest value of OPD 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 number of non-bridging atoms in the structure of this glass. With the introduction of 1 mol% Nb₂O₅ to 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 changes observed can be explained with the NbO₄→NbO₆ conversion upon Nb₂O₅ loading and formation of reticulated network because of the presence of high number of mixed bridging bonds (B-O-Nb, and Eu-O-Nb) within Nb₂O₅ containing glass networks. [28].

3.5. Determination of optical band gap

Some structural information can be also obtained from the optical band gap values (E_g) evaluated from the UV–Vis spectra on the Tauc method by plot (F(R^∞) hν)^1/n, n = 2 versus hν (incident photon energy) shown in Figure 6. [29].

It is seen that E_g of investigated glasses decreases with the increasing concentration of Nb₂O₅. Similar behavior is observed in the other glass systems containing high Nb₂O₅ content, where the reduction in the band gap energy is related to the increase of the glass overall polarizability due to the insertion of NbO₆ octahedra and their mutual linking into the glass structure [8].

3.6. EPR Spectroscopy

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

In the Figure 7 are registered 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 to the presence of Eu²⁺ ions in a highly asymmetric site environment [30,31] .The EPR spectra indicate the presence of low concentration 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 for investigated glasses are presented in Figure 8.

As can be seen all glasses are characterized with a good transmission in the visible region (around 80 %). The low intensive absorption bands at about 395 nm and 465 nm are corresponding to f-f transitions of Eu³⁺ ions between the ground and excited levels. It should be mentioned that the reduction process of the valence of niobium ions (Nb⁴⁺), produces very intense absorption peaks in the visible range due to the d-d transition. In the presented spectra there is no absorption bands corresponding to d-d transition, suggesting that Nb ions in the investigated glasses are presented as Nb⁵⁺.

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) [32].The wide excitation band in the UV-region is attributed to the charge transfer transition of Eu³⁺ (O^2-→ Eu³⁺) for x=0 [33,34,35,36]. With introduction of Nb₂O₅ in the glass composition (x=1÷5), the charge transfer band is assigned to the combined absorptions of Eu³⁺ and of NbO_n groups, NbO_n=NbO₆, NbO₄ (O^2-→Nb⁵⁺) [37,38].The contribution of these two components cannot be clearly differentiated because of the spectral overlap. The appearance of absorption of host matrix, under excitation at 612 nm (Eu³⁺ most intense emission), has been shown to play an essential role in the enhancement of the rare earth luminescence [17,39,40,41] through the occurring of non-radiative energy transfer, in particular from NbO_n structural polyhedra to the Eu³⁺ ion.

Figure 9 shows that the increasing of Nb2O5 concentration in the glass composition leads to a decrease of charge transfer band intensity, while the narrow Eu3+ peaks increases. Similar behavior was observed by Liu et al. [42] and Sreena et al. [41]. This suggest that some amount of the absorbed energy from NbO_n groups is transferred to the Eu³⁺ excitation levels. The maximum transferred energy corresponds to the lowest intensity of the charge transfer band of niobium-containing glasses. The probability of this energy transfer increases when the host absorbing groups, in our case NbO_n and the Eu³⁺ active ion are nearest neighbors in the structure [43]. 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 bands is located at 392 nm (⁷F₀→⁵L₆ transition), followed by ⁷F₀→⁵D₂ transition at 463 nm. This data signifies that the obtained phosphors can be efficiently excited with a range of excitation wavelength 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) is 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), ⁷F₄ (700 nm) in the ⁴F₆ configuration of Eu³⁺ ion [32]. As can be seen, the addition of Nb₂O₅ up to 3 mol% leads to the increase of the emission intensity. The luminescence suppression is observed at 5 mol% Nb₂O₅.

According to literature data in the spectral region 350-600 nm region is registered the broad emission band of NbO_n [33;44,45]. In the same spectral region are located the excitation bands of Eu³⁺, shown at Figure 9. In this way, the other condition for effective energy transfer, i.e. the overlap of the host group emission, in our case NbO_n and the excitation levels of the active Eu³⁺ ion, is satisfied [43]. This process is known as host sensitized energy transfer. An evidence of the non-radiative transfer is the absence of the emission band of NbO_n groups in the spectra (Figure 10). The strongest emission line, located at 612 nm, is caused by the sensitive to the small changes in the environment, forced electric dipole transition (ED) ⁵D₀ → ⁷F₂, followed by the insensitive to the surroundings magnetic dipole (MD) ⁵D₀→⁷F₁ one [32,33]. The fact that the predominant emission is from the ED transition rather than from the MD transition is an indication that Eu³⁺ ions are distributed in a non-inversion symmetry sites in the glass host. Therefore, the value of relative luminescent intensity ratio R of the two transitions (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) (Table 2) gives information of the degree of asymmetry around the Eu³⁺ ions [2,46].

The higher the value of the asymmetry parameter, the lower the local site symmetry of the active ion, the higher Eu–O covalence and emission intensity. The calculated higher R values (from 4.31-5.16) compared to the other reported in the literature (Table 2) [17,40,47,48,49,50,52], suggest more asymmetry in the vicinity of Eu³⁺ ions and stronger Eu–O covalence and thus enhanced emission intensity.

Table 2. Relative luminescencent 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 luminescencent 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	17
50ZnO.40B2O3. 5WO3.5Nb2O5.xEu2O3 (0≤x≤10)	5.09÷5.76	40
(100-y)TeO2-10Nb2O5-yPbF2 (0 ≤ y ≤ 30)	2÷4.16	47.
69TeO2 :1K2O:15Nb2O5:1.0Eu2O3	5	48.
60TeO2:19ZnO:7.5Na2O:7.5Li2O:5Nb2O5: 1Eu2O3	3.73	49.
4ZnO:3B2O3:0.5÷2.5 mol % Eu3+	3.94-2.74	50.
(99.5-x):B2O3:xLi2O:0.5Eu2O3	2.41-3.40	51
(64-x)GeO2:xSiO2:16K2O:6BaO:4Eu2O3	3.42-4.07	51
(98-x)P2O5:xCaO:2Eu2O3	3.88-3.95	51.
79TeO2+20Li2CO3+1Eu2O3	4.28	52.

Table 2. Relative luminescencent 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 luminescencent 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	17
50ZnO.40B2O3. 5WO3.5Nb2O5.xEu2O3 (0≤x≤10)	5.09÷5.76	40
(100-y)TeO2-10Nb2O5-yPbF2 (0 ≤ y ≤ 30)	2÷4.16	47.
69TeO2 :1K2O:15Nb2O5:1.0Eu2O3	5	48.
60TeO2:19ZnO:7.5Na2O:7.5Li2O:5Nb2O5: 1Eu2O3	3.73	49.
4ZnO:3B2O3:0.5÷2.5 mol % Eu3+	3.94-2.74	50.
(99.5-x):B2O3:xLi2O:0.5Eu2O3	2.41-3.40	51
(64-x)GeO2:xSiO2:16K2O:6BaO:4Eu2O3	3.42-4.07	51
(98-x)P2O5:xCaO:2Eu2O3	3.88-3.95	51.
79TeO2+20Li2CO3+1Eu2O3	4.28	52.

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) can be supposed that Nb₂O₅ addition leads Eu³⁺ to high asymmetry environments in the host, increasing the intensity of ⁵D₀→⁷F₂ transition. The most intensive emission was registered with 3 mol% Nb₂O₅. The increase of Nb₂O₅ content (5 mol%) leads to a slight diminishing in the emission intensity (Figure 10) as a result of the increasing Eu³⁺ site symmetry (lightly reducing of R) (Table 2). 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 ⁵D₀→⁷F₀ band shows that Eu³⁺ occupy sites with C_2v, C_n or C_s symmetry. [53]

3.8.1. 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 [54]. From the luminescence spectra the chromaticity coordinates of specimens were calculated using color calculator software (CIE coordinate calculator). The obtained values are listed in Table 3, whereas references are included the chromaticity coordinates of the commercial phosphor Y₂O₂S:Eu³⁺ [55] and National Television Standards Committee (NTSC) for red color. As can be seen from the Table 3, 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 with the commercially applied red phosphor Y₂O₂S:Eu³⁺ (0.658, 0.340). The calculated values are almost identical and cannot be individually separated on CIE diagram (Figure 11). This data show that the obtained glasses are characterized with high color purity.

Table 2. 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)

Table 2. 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)

The color correlated temperature (CCT) was calculated by the McCamy empirical formula:

$$\mathit{C}\mathit{C}\mathit{T}=-449{\mathit{n}}^3+3525{\mathit{n}}^2-6823\mathit{n}+5520.33 \left[56\right],$$

where n= (x−x_e)/(y−y_e) is the reciprocal slope, (x_e=0.332, y_e=0.186) are 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 [56]. The calculated CCT values of Eu³⁺ doped glasses (Table 2) ranges from 2301.26 K to 2518.60 K and these glasses can be considered as warm red light emitting materials for solid state lightening application.

4. Discussion

The Raman and IR spectral data as well as the established values of the structural sensitive physical parameters demonstrate that at smaller amount, the niobium ions (up to 5 mol %) 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 octhedra are situated around of the Eu³⁺ ions (i. e niobate groups are charge balanced by Eu³⁺ ions) and the higher number of the NbO₆ surrounding Eu³⁺ is found for the glass containing 3 mol% Nb₂O₅. Except by Eu³⁺ ions, NbO₆ octhahedra 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 is favorable for doping with Eu³⁺ active ions. Moreover, the DTA analysis shows high values of glass transition temperatures (over 500 oC) and together with the absence of glass crystallization effects are both confirming the formation of connected and stable glass networks.

The observed optical properties are discussed on the base 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 R values of Nb₂O₅ containing glasses are higher in comparison with the Nb₂O₅-free Eu³⁺: ZnO:B₂O₃ glass, indicating that Eu³⁺ are in higher asymmetry environments in the Nb₂O₅ containing glasses because of combining of niobate and borate structural units in the active ion surrounding. Thus, the introduction of Nb₂O₅ oxide in the Eu³⁺: ZnO:B₂O₃ glass, increases connectivity in the glass network and contributes to the creation of more distorted and rigid glass structure, that lowers the site symmetry of the rare earth ion and improves its photoluminescence behavior. Finally, the existence of NbO₆ groups around Eu³⁺ ions ensures an occurrence of non-radiative energy transfer from the niobate groups to the active ions that additionally improves the Eu³⁺luminescence intensity. The influence of Eu²⁺ ions on the luminescence of Eu³⁺ is negligible due to their low content.

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

5. Conclusions

The impact of the glass matrix on the Europium luminescent efficiency 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- ortoborate 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 niobate units in the active ion surrounding act as synthesizer and improve its emission as a result of the nonradiative energy transfer. 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 into 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.