Processing and Optical Properties of Eu-Doped Chloroborate Glass-Ceramic

Abstract

An europium doped BaO–B₂O₃–BaCl₂ chloroborate glass-ceramic containing a BaCl₂ nanocrystalline phase was produced by melt-quenching followed by glass crystallization during annealing. Structural and morphological investigations using x-ray diffraction and scanning electron microscopy have shown fvBaCl₂ nanocrystals of about tens of nm size accompanied by a smaller amount of the BaB₂O₄ crystalline phase. Photoluminescence spectra have indicated the reduction of Eu³⁺ to Eu²⁺ during processing in air or a reducing atmosphere. The spectra analysis showed the presence of Eu³⁺ ions in the borate glass matrix, while the Eu²⁺ were incorporated in both the BaCl₂ nanocrystals and glass matrix. Thermoluminescence properties were due to the recombination of F(Cl) centers and Eu²⁺ related hole centers produced by irradiation within the BaCl₂ nanocrystals. The color impression of the samples and the photoluminescence quantum efficiency were influenced by the glass processing.

1. Introduction

Glass-ceramics are composite materials formed by a nano-crystalline phase dispersed within a glass matrix. The precursor glass is converted into a glass-ceramic during a subsequent annealing through a controlled nucleation and crystallization processes. Transparent glass-ceramics offer balanced properties between a glassy material and crystals. By proper doping with luminescent rare-earth (RE) ions, they can combine the fluorescence properties and optical transparency due to lack of scattering. The new optical functionalities added are very important for photonics related applications (optical amplifiers, optical waveguides, white LEDs, etc.) at the industrial scale ([1,2] and references therein).

Most studies have been focused on the preparation and characterization of RE-doped transparent oxyfluoride glass-ceramics containing nanocrystalline fluorides such as PbF₂, CaF₂, BaF₂, LaF₃, NaLaF₄, etc. embedded in an aluminosilicate glass matrix ([3] and references therein). The thermal stability and chemical advantages of oxide glasses combine with those of the fluoride environment of dopant ions that assures a high luminescence efficiency because of low phonon energies and low non-radiative decay rates. Transparent fluorozirconate-based glass-ceramics containing RE³⁺-doped BaCl₂ nanocrystals represent new developments in the field, and their crystallization behavior [4,5] and optical properties have been investigated [6]. Eu²⁺-doped glass-ceramics are of special interest [7,8,9] because of their remarkable scintillating and x-ray storage phosphor properties as shown by the Eu²⁺-doped BaCl₂ crystal [9,10,11]. Nevertheless, fluorozirconate glass synthesis is performed at relatively high temperature and controlled atmosphere, and therefore other glass matrix options have been considered such as such as borate glasses. The synthesis and crystallization properties of barium borate glass-ceramics in the BaO–B₂O₃ system have been studied, and their structural and optical properties related to the borate nanocrystals (BaB₂O₄, BaB₄O₇, Li₂B₄O₇) [12,13,14,15] and BaCl₂ [16,17] nanocrystals have been investigated.

The aim of the present study was to perform a structural and morphological investigation of the Eu-doped chloroborate glass ceramic in the BaO–B₂O₃–BaCl₂ system and to evaluate the influence of the ceramization process on its optical properties: optical absorption, photoluminescence, and thermoluminescence.

2. Materials and Methods

2.1. Samples Preparation

Glass samples with the composition of (34BaO–30B₂O₃-28BaCl₂-1Eu₂O₃) (mol%) [16] were prepared from high purity BaCO₃, B₂O₃, BaCl₂, and Eu₂O₃ precursors, as detailed in [11]. The constituent chemicals were carefully mixed, dried at 80 °C, and melted in a covered corundum crucible at 1130 °C in air for 1 h, followed by fast pouring onto a brass preheated plate. Then, in order to relinquish the inner stress, the glass samples were annealed for 1 h at 400 °C (i.e., below the glass temperature T_g of 500 °C (see thermal analysis section from below)). Glass ceramization has been obtained during subsequent annealing at different temperatures above T_g for various times in air or reducing atmosphere (5H₂–95Ar gas mixture). The initial glass was clear-transparent, but after annealing, it was converted to milky-white as a consequence of the crystallization processes evidenced by the XRD measurements.

2.2. Characterization Methods

For the thermal analysis by thermogravimetry (TG) and differential scanning calorimetry (DSC), we used a SETARAM Setsys Evolution 18 Thermal Analyzer in synthetic air (80% N₂/20% O₂) and a heating rate of 10 °C/min. Structural characterization by X-ray diffractometry (XRD) was carried out by using a BRUKER D8 ADVANCE type X-ray diffractometer in focusing geometry, equipped with copper target x-ray tube and LynxEye one-dimensional detector. The XRD pattern was recorded in the 20 to 60° range, with 0.04° step and 2 s integration time. For the phase composition analysis and crystallographic characteristic, we used the Powercell dedicated software [18]. The morphological characterizations of the samples were carried out by using a Zeiss MERLIN (Jena, Germany) Compact scanning electron microscope (SEM) with GEMINI column and equipped with an energy dispersive x-ray system (EDX) analyzer. The photoluminescence, excitation spectra, and photoluminescence decay curves were recorded at room temperature by using a FluoroMax 4P spectrophotometer (HORIBA Jobin Yvon, Kyoto, Japan); for the chromaticity and quantum efficiency analysis, we used the Quanta-Phy accessory. For thermoluminescence (TL) measurements, we used a Harshaw 3500 TL reader in the 50–350 °C temperature range with a heating rate of 2 °C/s. The TL measurements were performed after x-ray irradiation at room temperature for 30 min (using a copper anode at 40 kV and 40 mA) and storage for 25 min.

3. Results and Discussion

3.1. Thermal Analysis

The thermogravimetry (TG) curve recorded on the prepared glass sample showed a monotone increase with the temperature, but several peaks were observed on the DSC curve (Figure 1). The weak endothermic decrease at about 500 °C was assigned to the glass transition temperature T_g and the sharp exothermic peak at about 710 °C to the bulk crystallization of the glass (T_cr.gl). Between 525 and 600 °C, we observed a composed exothermic peak T_cr. that was assigned to the glass ceramization associated with the precipitation of BaCl₂ and β-BaB₂O₄ phases within the glass matrix [11], as was shown by XRD analysis (see below). The inset of the crystallization observed at 530 °C (Figure 1) agreed with the value of 521 °C recorded on a barium borate glass with a similar composition (i.e., the ratio ΣBa: 2B = 1.7) [19].

3.2. Structural and Morphological Characterization

The XRD pattern of the initial glass showed several broad peaks at 21.5, 31, and 41.5°, consistent with its amorphous structure (Figure 2). The glass ceramization started at 535 °C (i.e., right below the composed exothermic peak T_cr). The XRD patterns of the glass ceramic samples (Figure 2) showed the diffraction peaks assigned to the precipitation of the orthorhombic BaCl₂ phase accompanied by the peaks due to the β-BaB₂O₄ crystallites [11]; weaker peaks due to Al₂O₃ from the crucibles were also observed [11]. The stable phase of BaCl₂ has an orthorhombic PbCl₂ structure (space group D_2h¹⁶, Pnma) with the lattice parameters a = 7.865 Å, b = 4.731 Å, and c = 9.421 Å (PDF 00-024-094) [20].

According to the results of the XRD pattern analysis (

Table 1. The results of the x-ray diffraction (XRD) pattern analysis of Eu-doped chloroborate glass-ceramic annealed at 570 °C in an air and reducing atmosphere (H₂–Ar); the lattice parameters for BaCl₂ [20] and BaCl₂:Eu²⁺ nanocrystals [21] are included for comparison.

Sample	a (Å)	b (Å)	c (Å)	Cell Volume (Å3)
Annealed in air	7.903	4.751	9.422	353.9
Annealed in H2-Ar	7.906	4.752	9.426	354.2
BaCl2 [20]	7.8813 (2)	4.7369 (1)	9.4360 (2)	352.27
BaCl2:Eu2+ [21]	7.878 (5)	4.735 (8)	9.428 (1)	351.68

), the cell parameters of BaCl₂:Eu²⁺ nanocrystals in the glass ceramic showed a clear increase compared to the corresponding nanocrystalline powder. For Eu²⁺-doped BaCl₂ nanocrystals where the Ba²⁺ ions are substituted by the Eu²⁺ dopants [20,21], there is a relatively small distortion of the BaCl₂ crystalline structure assigned to the ionic radii difference of 1.47 Å for Ba²⁺ and 1.3 Å for Eu²⁺ [22]. However, for the glass-ceramic, we observed a strong crystalline lattice expansion effect (Table 1). Therefore, we assigned it to the particularities of the nanocrystal growth process in the glass matrix, which is strongly influenced by the ionic environment and ionic impurities incorporated during the growth process [23].

The scanning electron microscopy (SEM) image analysis indicated volume crystallization of the sample as a relatively uniform, compact, and broad distribution of nanoparticles of about tens of nm size (Figure 3); their aggregation and collapse resulted in bigger particles, of about hundreds of nm in size. Previous studies have indicated a surface crystallization in the glasses containing BaO and B₂O₃ [24], but in the present case, the SEM image (Figure 3) indicated a ceramization process within the glass sample volume.

The EDX spectral analysis of the glass-ceramic samples showed the presence of elements from the precursor chemicals: 7 at%(C), 72 at%(O), 9 at%(Ba), 2.5 at%(Al), 9 at%(Cl), and 0.5 at%(Eu). There was a deviation from the batch composition that was assigned to the chlorine loss during the melting caused by the chemical interaction of the oxychloride melt with water vapor or oxygen [16].

3.3. Optical Absorption UV–Vis

The effect of glass ceramization can be clearly observed in the absorption spectra recorded on the initial and glass ceramic samples (Figure 3). The initial glass showed high transparency in the whole blue-visible region, but for the glass-ceramic, the spectrum was dominated by the scattering on the nanocrystals; two small peaks at about 394 and 463 nm were due to the ⁵F₀→⁵L₆ and ⁵F₀→⁵D₂ transitions of the Eu³⁺ ions [25]. The absorption spectrum recorded on the glass-ceramic followed a 1/λⁿ wavelength dependence, with n ≈ 4 (Figure 4, dotted curve) consistent with a Rayleigh scattering on nanocrystals much smaller than the wavelength (i.e., <1/10 of the wavelength) [26]. Superimposed on the Rayleigh scattering curve, we observed a weak “bump” at about 480 nm assigned to the Mie scattering on nanocrystal agglomerates larger than the wavelength.

3.4. Photoluminescence Properties

The luminescence properties of the europium ion are strongly dependent on its valence state (Eu²⁺ or Eu³⁺) and on the nature of the host lattice; sharp visible luminescence peaks were observed for Eu³⁺ and broad blue luminescence for Eu²⁺. The PL and PL excitation spectra recorded on the Eu-doped chloroborate glass and glass-ceramic samples are presented in Figure 5 and Figure 6, respectively.

The PL spectra of the initial glass showed weak Eu³⁺-related luminescence peaks at 578, 592, and 620 nm assigned to the ⁵D₀→⁷F_0–3 radiative transitions [25], which increased after glass ceramization due to the partial absorption of the UV-blue luminescence of the Eu²⁺ ions (see below). The PL excitation spectrum of the 620 nm luminescence showed sharp peaks at 363, 377, 394, 465, and 526 nm assigned to the 4f→4f transitions between the ⁷F₀ ground state to various excited states [25]. For the dopant level used (1%), we supposed a uniform distribution of the europium dopants within the amorphous structure of the glass (Figure 2), which was characterized by no regular arrangement of the constituent ions. Hence, in the glass matrix, the luminescence spectra of the RE³⁺-ions showed broadening due to the random positions and random surroundings of the dopant ions. When the RE³⁺-ions are incorporated within the crystalline structure of the precipitated nanocrystals (i.e., during the glass ceramization process), this effect is removed, and for the Eu³⁺ ions, Stark splitting of the ⁷F_J final states [27] is expected, which, however, in the present case, could not be observed (Figure 5). Moreover, for lower phonon frequencies of the precipitated nanocrystals (i.e., reduced non-radiative relaxation rate), longer RE³⁺ luminescence lifetimes are expected. The phonon energies of borate glasses of about ~1300–1500 cm⁻¹ were much higher compared to barium chloride (~300 cm⁻¹). However, the PL decay curves recorded on the glass and glass ceramic bulk sample (not presented) showed single decays with relatively close characteristic lifetimes of 1.86 ± 0.02 ms and 2.20 ± 0.02 ms, respectively (

Table 2. The results of the J-O parameter analysis of the Eu-doped chloroborate glass and glass-ceramic annealed at 570 °C in an air and reducing atmosphere (H₂–Ar); the J-O parameters for the (29PbO–70B₂O₃-0.5Eu2O₃) lead borate glass are included [32] for comparison.

Sample	Ω2 (×10−20 cm2)	Ω4 (×10−20 cm2)	PL Lifetime (ms)
Initial glass	5.9	3.9	1.86 ± 0.02
Annealed in air	5.8	3.9	2.20 ± 0.02
Annealed in H2-Ar	5.1	3.9	2.20 ± 0.02
Lead borate glass [32]	6.03	4.29

). From all these experimental facts, we can infer that in glass-ceramics, the Eu³⁺ ions are incorporated dominantly in the glass matrix.

Further insight into the local structure and bonding in the vicinity of the Eu³⁺ ions is given by the analysis of the Judd–Ofelt (J-O) intensity parameters Ω_2,4,6. The Ω₂ parameter is sensitive to the local RE environment and large values were observed for chemical bonds with covalent character; and the Ω₄ and Ω₆ parameters are related to the viscosity and rigidity of the material (for doped glasses), respectively [28]. The JO parameters Ω₂ and Ω₄ were computed from the experimental luminescence spectra and by using the Einstein coefficients A_0-λ for spontaneous emission [29,30]. The experimental spontaneous emission coefficients A_0–2 and A_0–4 are obtained from the relation:

A_0–λ = A_0–1 ⋅ (S_0–λ/S_0–1) (σ₂/σ₁)   with λ = 2,4

(1)

where S_0–2 and S_0–4 are the integrated areas of the luminescence curves related to the ⁵D₀→⁷F_2,4 transitions, and σ₁, σ₂, and σ₄ are the energies of the ⁵D₀→⁷F_1,2,4 transitions. The ⁵D₀→⁷F₁ transition is the only MD transition, and therefore we used A_0–1 = 50 s⁻¹ [31]. The computed results for the Ω_2,4 parameters are tabulated in Table 2, where a very good agreement with the Ω₂, Ω₄ parameters was reported for the Eu³⁺ doped lead borate glass [32] and a small decrease in the glass ceramic as a consequence of structural transformations during the ceramization process could be observed. We suppose that in the glass ceramic, a very small fraction of the Eu³⁺ ions is incorporated within the crystalline lattice, and hence we observed a decrease in the Eu bonds’ covalency degree and of the Ω₂ parameter.

In the glass ceramic, the Eu³⁺-related luminescence peaks were accompanied by broad blue luminescence peaks at 407 and 480 nm (Figure 5) assigned to the Eu²⁺ ion luminescence in different environments. The Eu²⁺ luminescence occurs from the lowest crystal-field component of the 4f⁶5d excited configuration to the ⁸S_7/2 ground state (parity-allowed). This d-f type emission has a broad band character and is strongly dependent on the host lattice (i.e., the coordination number and symmetry of the site occupied by the Eu²⁺ ions) [33]. The Eu²⁺-related luminescence was not observed in the initial glass, but only after the ceramization process as a consequence of the Eu³⁺→Eu²⁺ reduction processes in the glass matrix [34,35,36], and was enhanced by the reduction atmosphere (Figure 5). As the Eu²⁺ luminescence in BaB₂O₄ crystals was observed at 380 nm [37] and given that the 407 nm luminesence was close to that observed in bulk orthorhombic BaCl₂:Eu²⁺ crystals at 399 nm [38], we could assign the observed features to the Eu²⁺ luminescence in the BaCl₂ nanocrystalline phase from the glass ceramic.

The excitation spectrum of the 407 nm luminescence showed two broad bands at about 275 and 330 nm (Figure 6) assigned to energy splitting between the t_2g and e_g one electron states of the 5d orbital. Its shape was different from the spectrum recorded on the BaCl₂:Eu²⁺ nanocrystalline powder [39], where a broad band in the 250 to 375 nm range was observed. The energy splitting indicated a lower crystal field symmetry at the Ba²⁺ sites occupied by the Eu²⁺ ions (compared to the nanocrystalline powder), which was assigned to the stress and distortion induced by the growth process (see Section 3.2). Such effects might also be responsible for the spectral shift of the 407 nm luminescence, compared to the Eu²⁺ luminescence observed at 396 nm in the nanocrystalline powder [39]. By comparison, the excitation spectrum of the 470 nm luminescence showed a broad and structureless band, which could be tentatively assigned to the Eu²⁺ ions in the glass matrix.

3.5. Colorimetry Analysis

The color impression of the samples was influenced by the glass processing. Figure 7 shows the Commission Internationale de l’Eclairage (CIE) chromaticity diagram of the initial and glass-ceramized (in an air or reducing atmosphere) Eu-doped chloroborate samples, respectively. Under 345 nm excitation, the coordinates x = 0.677 and y = 0.274 corresponded to the region of red Eu³⁺ luminescence, but after glass ceramization (in air or reducing atmosphere), these changed to x = 0.404 and y = 0.244 as a consequence of the Eu²⁺ blue luminescence. The photoluminescence quantum efficiency was about 16% for the glass, but decreased to only 4% for the glass-ceramic due to the scattering losses [13].

3.6. Thermoluminescence Properties

Thermoluminescence (TL) represents the light emission by a solid sample when it is heated after irradiation by ionizing radiation such as UV light, x-rays, gamma-rays, etc. [40]. According to the basic model of TL, charge carriers (electrons and holes) are produced during the irradiation, which are subsequently trapped on local energy levels (such as vacancies, interstitials, or impurities) within the band gap; during heating, their recombination is thermally activated, resulting in the emission of light (i.e., TL) [40,41].

Thermoluminescence curves recorded on undoped and Eu-doped chloroborate glass-ceramics are depicted in Figure 8. The glow curve recorded on undoped glass ceramic and BaB₂O₄ showed a broad band between 75 and 400 °C [42], which was assigned to the thermal recombination of defects within the BaB₂O₄ nanocrystalline phase. The glow curve recorded on the Eu-doped glass-ceramic (annealed in air) showed two peaks at 97 and 134 °C with a higher magnitude than in the undoped sample, but much weaker than in the Eu²⁺-doped BaCl₂ nanocrystalline powder. The same glow peaks were observed in the glass-ceramic annealed in a reducing atmosphere, where 134 °C was the dominant feature, accompanied by a shoulder at about 180 °C. The high temperature peak at 134 °C has been observed on Eu²⁺-doped BaCl₂ nanocrystalline powder, and it has been assigned to the recombination of F(Cl) centers in the BaCl₂ crystalline lattice [21]. During the heating, the electrons are thermally released from the F(Cl) trapping centers and recombine with holes trapped as Eu³⁺/(Eu²⁺- hole) centers giving rise to the excited (Eu²⁺)* ions and radiative emission according to:

(Eu²⁺- hole) + é–k_BT→(Eu²⁺)*→Eu²⁺ + hν

(2)

The nature of the 97 and 180 °C glow peaks remains unclear, but it might be related to the recombination of F-type centers perturbed by impurities [43] or other structural defects, as was inferred from the XRD analysis.

4. Conclusions

An europium doped BaO–B₂O₃–BaCl₂ chloroborate glass-ceramic was produced by melt-quenching followed by glass crystallization at T = 570 °C. X-ray diffraction and scanning electron microscopy measurements revealed the formation of orthorhombic BaCl₂ nanocrystals of about tens of nm size accompanied by a smaller amount of BaB₂O₄ nanocrystals. Structural analysis indicated strong distortions of the BaCl₂ crystalline lattice, which was associated with the ionic impurities and other defects during the nanocrystal growth process in the glass matrix.

The photoluminescence properties analysis showed that in the glass-ceramics, the Eu³⁺ ions are incorporated dominantly in the glass matrix. In addition, a reduction process of Eu³⁺ to Eu²⁺ was revealed by the presence of broad blue luminescence peaks at 407 and 480 nm, assigned to the Eu²⁺ ion luminescence in different environments: perturbed sites in the BaCl₂ nanocrystals and in the glass matrix, respectively.

Thermoluminescence curves recorded on the Eu-doped glass-ceramic showed two glow peaks at 97 and 134 °C. The high temperature peak was due to the F(Cl) center recombination in the BaCl₂ nanocrystals, and the low temperature peak at 97 °C was tentatively assigned to the recombination of perturbed F-centers.

In conclusion, glass-ceramic materials based on stabilized rare-earth doped BaCl₂ nanoparticles embedded in a glass matrix were demonstrated as attractive potential candidates for phosphor related applications. They offer a viable solution to overcome the disadvantages of the BaCl₂ crystal phosphor: raw material purification, clean environment control requirements during crystal growth, and chemical instability to water. Hence, its remarkable performances as a scintillator, x-ray storage phosphor, or thermoluminescence dosimeter can be used for practical applications of radiation detection. In particular, new developed chloroborate glass-ceramics can be processed at lower temperature and in an open atmosphere compared to the fluorozirconate glass-ceramics. However, further improvements of the optical properties and transparency through a better control of the processing parameters are required as a prerequisite condition for future applications.