Rare earth (RE) doped oxyfluoride glass ceramics have been widely studied as luminescence materials, because they exhibit high stability due to the good thermal and mechanical properties of the oxide-glass matrix, in addition to high quantum efficiency due to the low phonon energy of the fluoride crystals [1–6]. Various studies have been carried out to improve their luminescence properties and quantum efficiency, by modifying the RE ions and crystalline phases within the oxyfluoride glass ceramics [1–10]. Fluoride nanocrystals such as LaF₃, CaF₂, BaF₂, and SrF₂ have been successfully formed within oxyfluoride glasses and have shown potential as lead-free ultraviolet light emitting diode (UV-LED) color converters, with enhanced quantum efficiency of the doped RE ions [1–6, 11–14.].

Among the various RE ions, europium ions have been widely used as activators for oxyfluoride glasses or glass ceramics, due to their unique luminescence properties and high photoluminescence (PL) efficiency [15, 16]. Eu can easily change its valence from the 2+ to the 3+ state, producing various emission wavelengths depending on the local environment. Lee et al. [17] fabricated an oxyfluoride glass ceramic containing Eu²⁺ and Eu³⁺ at the same time by controlling the reduction atmosphere and Eu concentration, and demonstrated white emission under 400-nm LED excitation. Recently, a Eu²⁺-doped nepheline phase was formed within oxyfluoride glass ceramic and showed a highly improved PL quantum yield (PL-QY) of up to 78%, which is comparable to that of commercial phosphors [9]. Thanks to this high PL-QY, they successfully demonstrated the feasibility of glass ceramic as a UV-LED color converter by mounting the material on top of a 400-nm UV-LED [9]. It was believed that the LaF₃ crystals were formed first and mediated the crystallization of the nepheline phase, as observed by the peak shift of Eu²⁺ emission. Since other nanocrystals such as CaF₂, BaF₂ and SrF₂ can easily form within oxyfluoride glass, as previously reported [18–20], they can also alter the formation of crystalline phases and Eu²⁺ emission when they are introduced.

While most oxyfluoride glass ceramics are studied for use as visible and near-infrared color converters, which have short decay lifetimes of less than 10⁻³ seconds, phosphorescent materials with afterglow emission are also required for various applications, such as emergency-route signs, high-energy irradiation detectors, optical storage media, and medical sensors for in vivo imaging [21–26]. Considering that the phosphorescence of RE ions is mostly associated with the electronic and defect structures of their host crystals [27–29], phosphorescent oxyfluoride glass ceramics can also be expected to form, by properly adjusting the crystalline phase and RE ions.

In this study, BaF₂ was introduced into a Eu²⁺-doped oxyfluoride glass ceramic containing LaF₃, to modify the crystallization behavior and corresponding Eu²⁺ emission characteristics. Crystalline phases within the glass ceramics and PL properties of Eu²⁺ such as emission intensity and decay time were monitored and controlled according to the BaF₂ content. X-ray diffraction (XRD) and transmission electron microscopy with energy dispersive spectroscopy (TEM-EDS) revealed the crystalline phases within the glass ceramics after heat treatment, and time-resolved emission-intensity change was used to observe the afterglow emission. Finally, a possible mechanism for the change in Eu²⁺ emission characteristics was discussed, based on the spectroscopic and structural analyses.

The nominal glass composition was (in mol%) 45SiO₂-15Na₂O-20Al₂O₃-(20-x)LaF₃-xBaF₂ (x = 0-20), and 0.5 mol% of EuF₃ was additionally doped, as reported previously [9]. The high purity (>99.99%) raw materials were weighed and thoroughly mixed. 20 wt% of carbon was placed on top of the mixture of raw materials within an alumina crucible, to provide a reducing atmosphere for the Eu ions during melting. The glasses were fabricated by a conventional melt-quenching method at 1,450 ℃ for 1 h under ambient atmosphere, and the obtained glasses were then annealed at 450 ℃ for 2 h. The glasses were crystallized at 800 ℃ for 10 h, which yielded the highest PL-QY in the previous report [9]. The glasses and glass ceramics were then cut into squares of about 10 × 10 mm (and 1 mm thick) and optically polished.

An XRD (D/MAX-2500U; Rigaku Co., Akishima, Japan) and a field-emission transmission electron microscope (FE-TEM, JEM-2100F; JEOL Ltd., Akishima, Japan) equipped for energy-dispersive spectroscopy (EDS) were used to inspect the crystalline phases formed within the glass matrix. The glasses and glass ceramics were each mounted on top of a UV-LED chip with a 5050 package (λ_center = 405 nm), and a spectrometer equipped with an integration sphere (DARSA-5200; PSI, Suwon, Korea) was used to examine the visible emission spectra of the packaged LEDs. A fluorescence and phosphorescence spectrometer (FS-2; Scinco, Seoul, Korea) was employed to measure the time-resolved spectra.

In the previous study [9], Eu²⁺-doped oxyfluoride glasses and glass ceramics were successfully fabricated and showed cyan emission centered at around 480 nm, due to the Eu²⁺:4f ⁶5d→4f transition. When the glasses were heat-treated at 800 ℃ for different durations, the emission peak not only shifted to 540 nm but also highly improved in intensity. The maximum intensity was observed with the highest PL-QY (78%) under heat treatment at 800 ℃ for 10 h. Based on the structural analysis with CL mapping, TEM/EDS, and XRD, the formation of Eu²⁺-doped nepheline on the surface of LaF₃ crystals was responsible for the emission-peak shift and the significant improvement in PL-QY.

Pristine glasses without heat treatment and of varying BaF₂ content (from 0 to 20 mol%) were prepared and mounted on top of a 405-nm UV-LED, to demonstrate their feasibility as LED color converters and to monitor their emission spectra. All of the glasses showed a broad emission centered around 478 nm, as depicted in Fig. 1, which exhibited a gradual decrease in emission intensity with increasing BaF₂ content to replace LaF₃. Considering that in the present glass the cyan emission mostly comes from the Eu²⁺ ions within the glass host matrix [9], the decrease in Eu²⁺ emission implies that the formation of the Eu²⁺ state has been disturbed by the introduction of BaF₂. Since Ba2+ and Eu²⁺ have a similar coordination number of 9, while La³⁺ prefers 9 or 11 in the glass structure [30–32], although further study is required it seems plausible to assume that Ba2+ preferentially took the place of Eu²⁺ sites, inhibiting the formation of Eu²⁺ and reducing the emission intensity.

Figure 1. Visible emission spectra and actual photos of 0.5 mol% EuF₃ doped glasses of varying BaF₂ content, after being mounted on top of a 405-nm ultraviolet light emitting diode (UV-LED).

The glasses were then heat treated to form crystals within the glass matrix. As seen in Fig. 2, the glass ceramics had a yellow emission centered at around 550 nm, implying the formation of Eu²⁺-doped nepheline crystals after the heat treatment. The clear yellow emission from the packaged LED, as displayed on the right side of the figure, demonstrated this glass’s feasibility as a UV-LED color converter.

Figure 2. Visible emission spectra and actual photos of 0.5 mol% EuF₃ doped glass ceramics of varying BaF₂ content, which were heat treated at 800 ℃ for 10 h and then mounted on top of a 405 nm UV-LED.

However, it should be noted that the yellow emission’s intensity also decreased with increasing BaF₂ content, and almost disappeared with 20 mol% of BaF₂. This result clearly suggests that the formation of Eu²⁺-doped nepheline was effectively suppressed by the incorporation of BaF₂ within the glass network. Recalling that the nepheline was formed on the surface of LaF₃ crystals in the previous study [9], the reduction in LaF₃ content could be a decisive reason for the change in emission intensity. However, BaF₂ can also form crystalline phases, which can be other nucleation sites for nepheline crystals. Thus, possible changes in the crystalline-phase formation during heat treatment with increasing BaF₂ content could be another reason for the suppressed growth of the nepheline phase, which will be discussed with XRD analysis.

It is interesting to note that along with the decrease in emission intensity, the glass ceramics with BaF₂ showed afterglow emission after being excited by the UV light source. To show its effect visually, a 365 nm UV lamp was employed to excite the glass ceramics at the same time, as shown in Fig. 3(a). When the glass ceramics were irradiated with this lamp, all samples showed yellow emission, due to the Eu²⁺-doped nepheline crystal. It should also be noted that unlike the glass ceramics with 5 and 10 mol% of BaF₂ (Ba5 and Ba10 respectively), the glass ceramics with 15 and 20 mol% of BaF₂ (Ba15 and Ba20 respectively) also showed weak blue emission under UV excitation, as can be observed in Fig. 3(a). Although it was hard to obtain a PL spectrum even with and integration sphere, due to the weak emission intensity, a blue emission implies a possible change in the Eu²⁺ environment, or the formation of another crystalline phases within the glass ceramics. However, the emission continued for 10 seconds after the lamp was removed. This afterglow effect improved depending on the BaF₂ content, with the longest afterglow emission observed for Ba20.

Figure 3. Actucal photos and emission decay time of oxyfluoride glass ceramic: (a) Actual photos of glass ceramics with varying BaF₂ content, under 365-nm ultra violet (UV)-lamp irradiation and then after the lamp was turned off; and (b) the emission-intensity decay with time of glass ceramics with varying BaF₂ content, monitored at 540 nm under 365-nm excitation. The inset shows the emission-intensity change at the early stage of decay.

When their emission intensities were monitored at 540 nm under a 375 nm excitation source, the change in decay time depending on BaF₂ content could be clearly observed, as demonstrated in Fig. 3(b). When BaF₂ content was less than 10 mol%, the glass ceramics showed only a short decay of less than about 1 μsec, while a slow decay longer than 30 s was observed for Ba15 and Ba20. The emission from Ba20 could be observed with the naked eye even after as long as 20 minutes, demonstrating its phosphorescence properties. As shown in the inset figure of Fig. 3(b), the average decay time was less than 10 nsec and increased with BaF₂ content; the decay curves were fitted by a double exponential function in the early stage of decay.

However, as summarized in Table 1, it is worth noting that the second exponential component significantly increased for the Ba15 and Ba20 samples, suggesting a possible change in emission mechanism, which induced the afterglow effect.

TABLE 1. Fitting results for emission-intensity decay of Eu²⁺-doped oxyfluoride glass ceramics varying BaF₂ content, for the double exponential function (I = I₁exp(−t / τ₁) + I₂exp(−t / τ₂)).

Sample	I1 (nsec)	τ1	I2 (nsec)	τ2	Aver. (nsec)
B0	16.31	0.0163	5.65	0.9837	6.17
B5	17.83	0.0226	5.38	0.9774	6.27
B10	24.75	0.0186	5.52	0.9814	7.03
B15	23.26	0.0333	7.87	0.9667	9.29
B20	24.56	0.0672	7.43	0.9328	10.72

To elucidate the spectral change in the glass ceramics, the structural evolution of the glass ceramic with BaF₂ replacing LaF₃ was monitored by XRD; the results are displayed in Fig. 4(a). Characteristic crystalline peaks from nepheline (PDF#-01-088-1231) and LaF₃ (PDF#-01-076-1500) were observed for the glass ceramic without BaF₂ (Ba0), as reported previously [9]. When 5 mol% of BaF₂ was added (Ba5), crystalline peaks corresponding to BaAl₂ Si₂O₈ (PDF#-01-088-1049) appeared, and grew as BaF₂ content increased.

Figure 4. X-ray diffraction patterns of 0.5 mol% EuF₃ doped oxyfluoride glass ceramics: (a) with varying BaF₂ content (0, 5, 10, 15, and 20 mol%) and a fixed heat treatment of 800 ℃ for 10 h, and (b) with varying heat-treatment temperature and fixed duration time of 10 h, and BaF₂ content of 20 mol% (Ba20).

It should be noted that BaF₂ (PDF#-00-004-0452) was also been formed when BaF₂ content was higher than 15 mol%, while LaF₃ and the nepheline phases were effectively reduced. The noticeable decrease in nepheline phase with BaF₂ can explain the decrease in the yellow emission from the Eu²⁺-doped nepheline in Fig. 2. Considering the chemical composition of nepheline (NaAlSiO₄), the evolution of the BaAl₂Si₂O₈ phase implies the consumption of Al, Si, and O, which would impede the formation of the nepheline phase in the glass matrix. Thus the reduction in nepheline can be attributed to the lack of nucleation sites (LaF₃) and constituent elements (Al, Si, and O), with the introduction of BaF₂ replacing LaF₃.

Although BaF₂ crystals have been formed, BaF₂ crystals unlike LaF₃ have little role as nucleation sites for nepheline. As found in Fig. 4(b), which shows the XRD results for Ba20 with varying heat treatment temperature, BaAl₂SiO₈ has predominantly formed, consuming Al and Si, followed by the growth of BaF₂ crystals. Thus little chance for nepheline phase formation can be anticipated. However, it is noteworthy that the crystalline peaks corresponding to nepheline can still be found, in trace amounts. The result suggests that a limited amount of nepheline phase has also formed, and is responsible for the weak yellow emission under UV excitation.

For visual inspection of the crystalline phases within the glass ceramics, FE-TEM and EDS were employed. As depicted in Fig. 5, in Ba20 (which showed afterglow emission) large crystals were observed, implying high crystallinity of the glass ceramic. When the elemental distribution of crystals was examined by TEM-EDS [Figs. 5(b)–5(h)], a Ba- and F-rich phase was observed, implying the formation of BaF₂ crystals. Another region was rich in Si, Al, Ba, and O, suggesting the formation of a BaAl₂SiO₈ phase. It should be noted that although Eu ions were located throughout the samples, they were mostly found in the BaF₂ region, showing that the fluoride crystals were the preferential location of RE ions, as observed in other oxyfluoride glass ceramics [8, 9, 11, 17, 33]. Considering the distribution of Na, it seems the nepheline phase also can be formed with BaAl₂ SiO₈, but it could not be clearly identified as in XRD.

Figure 5. Field-emission transmission electron microscope (FE-TEM) and energy dispersive spectroscopy (EDS) results of oxyfluoride glass ceramic: (a) FE-TEM image and (b)–(h) EDS elemental-mapping results for the 0.5 mol% EuF₃-doped oxyfluoride glass ceramic with 20 mol% BaF₂ which was heat treated at 800 ℃ for 10 h.

Based on the XRD and TEM-EDS results, it is clear that the formation of BaF₂ and BaAl₂Si₂O₈ crystal phases is responsible for the spectral change with BaF₂ content. In previous reports, Eu²⁺-doped BaF₂ nanocrystals within oxyfluoride glass ceramics and Eu²⁺-doped monoclinic BaAl₂ Si₂O₈ ceramic phosphors both emitted blue luminescence under UV excitation [34–36]. Thus the weak blue emission observed in Ba15 and Ba20 in Fig. 3(a) can be attributed to Eu²⁺-doped BaF₂ or BaAl₂Si₂O₈ crystals.

Although further detailed study is required, the afterglow emission from Ba15 and Ba20 can also be understood by the change in crystalline phases. When the BaAl₂Si₂O₈ was doped with Eu²⁺ and other RE ions such as Dy²⁺, a long blue afterglow emission was observed [37]. Electron-trap sites within the BaAl₂Si₂O₈ crystal were responsible for the delayed emission. Since Eu²⁺-doped nepheline shows only fluorescence with a short lifetime of less than 1 msec, and has broad absorption bands up to about 500 nm [9], the delayed blue emission from the Eu²⁺-doped BaAl₂Si₂O₈ can be reabsorbed by the Eu²⁺-doped nepheline, contributing to the yellow afterglow emission.

Figure 6(a) depicts the schematic PL excitation (PLE) spectrum of the Eu²⁺-doped nepheline [9] and the PL spectrum of the Eu²⁺-doped BaAl₂Si₂O₈ [37], clearly showing the spectral overlap for reabsorption. Recently, Jiang et al. [31] reported that Eu²⁺/Tm³⁺-codoped Ba_13.35Al_30.7Si_5.3O₇₀ can also emit yellow phosphorescence under UV excitation. Internal electron-trap sites along with the external trap sites related to the Tm³⁺ within the crystal were also responsible for the afterglow.

Figure 6. photoluminescence excitation (PLE) and photoluminescence (PL) spectra of oxyfluoride glass ceramic and schematic diagram of afterglow mechanism in the oxyfluoride glass ceramic: (a) PLE spectrum of glass ceramic with Eu²⁺-doped nepheline (after [7]) and the PL spectrum of Eu²⁺-doped BaAl₂Si₂O₈ ceramic phosphor (after [35]). (b) Schematic diagram of the afterglow mechanism in the Eu²⁺-doped glass ceramic with BaF₂.

Although a crystalline peak has not been clearly observed in XRD, considering the similarity of composition between Ba_13.35Al_30.7Si_5.3O₇₀ and BaAl₂Si₂O₈, the possible formation of Eu²⁺-doped Ba_13.35Al_30.7Si_5.3O₇₀ and its direct yellow afterglow emission cannot be completely ruled out. These possible mechanisms for the afterglow emission have been schematically drawn in Fig. 6(b).

Eu²⁺-doped oxyfluoride glass ceramics containing a nepheline phase were modified with BaF₂, and the resulting changes in their spectral properties were investigated. When BaF₂ replaced LaF₃ up to 20 mol%, the PL emission from the Eu²⁺-doped glasses and glass ceramics decreased as BaF₂ content increased. The decrease in cyan emission from the Eu²⁺-doped oxyfluoride glass was attributed to the preferential occupation of Ba2+ ions for Eu²⁺ sites within the glass matrix. The noticeable decrease in yellow emission was induced by suppressing the formation of Eu²⁺-doped nepheline. The preferential formation of BaAl₂Si₂O₈ and reduction of LaF₃ crystals as nucleation sites within the glass network were responsible for the decrease in emission intensity. However, the glass ceramic with a high content of BaF₂ showed afterglow emission, which lasted for 20 minutes after the lamp was turned off. Although further study is required, it is believed that the delayed blue emission from Eu²⁺-doped BaAl₂Si₂O₈ was reabsorbed by Eu²⁺-doped nepheline, and that this was responsible for the yellow afterglow emission. The possible formation of Eu²⁺-doped Ba_13.35Al_30.7Si_5.3O₇₀ has also been suggested as the source of the afterglow emission.

The authors declare no conflicts of interest.

Data underlying the results presented in this paper are not publicly available at this time of publication, which may be obtained from the authors upon reasonable request.

National Research Foundation of Korea (NRF) grant; Korean government (MIST) (No. NRF-2019R1A2C100 7621).