A Photoluminescence Study of Eu3+, Tb3+, Ce3+ Emission in Doped Crystals of Strontium-Barium Fluoride Borate Solid Solution Ba4−xSr3+x(BO3)4−yF2+3y (BSBF)

The present study is aimed at unveiling the luminescence potential of Ba4−xSr3+x(BO3)4−yF2+3y (BSBF) crystals doped with Eu3+, Tb3+, and Ce3+. Owing to the incongruent melting character of the phase, the NaF compound was used as a solvent for BSBF crystal growth. The structure of BSBF: Eu3+ with Eu2O3 concentration of about 0.7(3) wt% was solved in the non-centrosymmetric point group P63mc. The presence of Eu2O3 in BSBF: Eu3+ leads to a shift of the absorption edge from 225 nm to 320 nm. The photoluminescence properties of the BSBF: Ce3+, BSBF: Tb3+, BSBF: Eu3+, and BSBF: Eu3+, Tb3+, Ce3+ crystals have been studied. The unusual feature of europium emission in BSBF is the intensively manifested 5D0→7F0 transition at about 574 nm, which is the strongest for BSBF: Eu3+ at 370 nm excitation and for BSBF: Eu3+, Tb3+, Ce3+ at 300 nm and 370 nm excitations. No evidence of Tb3+→Eu3+ energy transfer was found for BSBF: Eu3+, Tb3+, Ce3+. The PL spectra of BSBF: Eu3+ at 77 and 300 K are similar with CIE chromaticity coordinates of (0.617; 0.378) at 300 nm excitation and (0.634; 0.359) at 395 nm excitation and low correlated color temperature which implies application prospects in the field of lightning. Due to the high intensity of 5D0→7F0 Eu3+ transition at 370 nm excitation, the BSBF: Eu3+ emission is yellow-shifted.


Introduction
The increasing exploration of borate compounds is motivated mainly by their chemical and physical stability, a wide range of transparency from ultraviolet (UV) to infrared range, and a high laser-induced damage threshold. A number of borates demonstrate high birefringence, e.g., α-BaB 2 O 4 , Ba 2 Na 3 (B 3 O 6 ) 2 F, and outstanding nonlinear optical properties for laser frequency conversion from infrared to UV-and visible ranges, e.g., β-BaB 2 O 4 , LiB 3 O 5 [1]. Aside from optical properties, another reason for the continued attention to borates is their remarkable structural variety owing to the dual hybridization of boron atoms and the possibility for polymerization via bridging oxygen atoms.
The main object of this study is highly unconventional non-centrosymmetric (P63mc) solid solution Ba4−xSr3+x(BO3)4−yF2+3y (BSBF) exhibiting both cationic Ba 2+ ↔Sr 2+ and anionic [(BO3)F] 4− ↔[F4] 4− isomorphism [12]. Such structural flexibility allows tuning the optical properties, for instance, the value of the absorption edge [13]. Particular emphasis should be laid on the fact that crystals of Ba4−xSr3+x(BO3)4−yF2+3y are characterized by a property completely new to the class of borates. This is a reversible color change upon X-ray irradiation with the possibility of returning the crystals to their original uncolored state by irradiating with intense light in the range of 300-400 nm. X-ray irradiation is accompanied by the formation of induced color centers, which were studied by optical and electron paramagnetic resonance spectroscopy. The combination of properties of the BSBF crystals allows them to be used as a dose indicator [12,13].
The luminescent properties of undoped Ba4−xSr3+x(BO3)4−yF2+3y crystals were discussed in detail in Ref. [12]. The potential employment of fluoroborates can be enhanced by the incorporation of rare earth elements, endowing the crystals with additional luminescence properties. The present study is aimed at unveiling the luminescence potential of Ba4−xSr3+x(BO3)4−yF2+3y doped with Eu 3+ , Tb 3+ , and Ce 3+ for application in WLEDs as singlematrix or composite phosphors.

Materials and Experimental Methods
Crystal Growth. We grew crystals using NaF as a solvent owing to the incongruent melting of Ba4−xSr3+x(BO3)4−yF2+3y; BaCO3, SrCO3, SrF2, H3BO3, NaF, Ce2(CO3)3·5H2O, Tb4O7, E2O3 were used as starting reagents. The growth process was carried out in a platinum crucible 40 mm in diameter in air; the weight of the initial high-temperature solution was 40 g. As it follows from the data in Table 1, the effect of the rare earth elements addition on the liquidus temperature was minimal. Crystals were grown on a platinum loop without rotation and pulling at a cooling rate of 2 °C per day for approximately 15 days. The average weight of the crystals was about 8 g. We included the photographs of 1 mm thick plates made of the grown crystals in Table 1.  [12]. Such structural flexibility allows tuning the optical properties, for instance, the value of the absorption edge [13]. Particular emphasis should be laid on the fact that crystals of Ba4−xSr3+x(BO3)4−yF2+3y are characterized by a property completely new to the class of borates. This is a reversible color change upon X-ray irradiation with the possibility of returning the crystals to their original uncolored state by irradiating with intense light in the range of 300-400 nm. X-ray irradiation is accompanied by the formation of induced color centers, which were studied by optical and electron paramagnetic resonance spectroscopy. The combination of properties of the BSBF crystals allows them to be used as a dose indicator [12,13].
The luminescent properties of undoped Ba4−xSr3+x(BO3)4−yF2+3y crystals were discussed in detail in Ref. [12]. The potential employment of fluoroborates can be enhanced by the incorporation of rare earth elements, endowing the crystals with additional luminescence properties. The present study is aimed at unveiling the luminescence potential of Ba4−xSr3+x(BO3)4−yF2+3y doped with Eu 3+ , Tb 3+ , and Ce 3+ for application in WLEDs as singlematrix or composite phosphors.

Materials and Experimental Methods
Crystal Growth. We grew crystals using NaF as a solvent owing to the incongruent melting of Ba4−xSr3+x(BO3)4−yF2+3y; BaCO3, SrCO3, SrF2, H3BO3, NaF, Ce2(CO3)3·5H2O, Tb4O7, E2O3 were used as starting reagents. The growth process was carried out in a platinum crucible 40 mm in diameter in air; the weight of the initial high-temperature solution was 40 g. As it follows from the data in Table 1, the effect of the rare earth elements addition on the liquidus temperature was minimal. Crystals were grown on a platinum loop without rotation and pulling at a cooling rate of 2 °C per day for approximately 15 days. The average weight of the crystals was about 8 g. We included the photographs of 1 mm thick plates made of the grown crystals in Table 1.  [12]. Such structural flexibility allows tuning the optical properties, for instance, the value of the absorption edge [13]. Particular emphasis should be laid on the fact that crystals of Ba4−xSr3+x(BO3)4−yF2+3y are characterized by a property completely new to the class of borates. This is a reversible color change upon X-ray irradiation with the possibility of returning the crystals to their original uncolored state by irradiating with intense light in the range of 300-400 nm. X-ray irradiation is accompanied by the formation of induced color centers, which were studied by optical and electron paramagnetic resonance spectroscopy. The combination of properties of the BSBF crystals allows them to be used as a dose indicator [12,13].
The luminescent properties of undoped Ba4−xSr3+x(BO3)4−yF2+3y crystals were discussed in detail in Ref. [12]. The potential employment of fluoroborates can be enhanced by the incorporation of rare earth elements, endowing the crystals with additional luminescence properties. The present study is aimed at unveiling the luminescence potential of Ba4−xSr3+x(BO3)4−yF2+3y doped with Eu 3+ , Tb 3+ , and Ce 3+ for application in WLEDs as singlematrix or composite phosphors.

Materials and Experimental Methods
Crystal Growth. We grew crystals using NaF as a solvent owing to the incongruent melting of Ba4−xSr3+x(BO3)4−yF2+3y; BaCO3, SrCO3, SrF2, H3BO3, NaF, Ce2(CO3)3·5H2O, Tb4O7, E2O3 were used as starting reagents. The growth process was carried out in a platinum crucible 40 mm in diameter in air; the weight of the initial high-temperature solution was 40 g. As it follows from the data in Table 1, the effect of the rare earth elements addition on the liquidus temperature was minimal. Crystals were grown on a platinum loop without rotation and pulling at a cooling rate of 2 °C per day for approximately 15 days. The average weight of the crystals was about 8 g. We included the photographs of 1 mm thick plates made of the grown crystals in Table 1. tunable-color emission implying the Eu 2+ →Tb 3+ →Eu 3+ energy transfer [9], LaSc3(BO3)4:Eu 3+ red phosphor with zero thermal quenching and internal quantum efficiency of 88.3% [4], GdBO3: Ce 3+ , Tb 3+ , Eu 3+ broadband-excited red phosphor [10], LiBa12(BO3)4F4: Eu 3+ , Tb 3+ , Ce 3+ single-matrix white phosphor [11].
The main object of this study is highly unconventional non-centrosymmetric (P63mc) solid solution Ba4−xSr3+x(BO3)4−yF2+3y (BSBF) exhibiting both cationic Ba 2+ ↔Sr 2+ and anionic [12]. Such structural flexibility allows tuning the optical properties, for instance, the value of the absorption edge [13]. Particular emphasis should be laid on the fact that crystals of Ba4−xSr3+x(BO3)4−yF2+3y are characterized by a property completely new to the class of borates. This is a reversible color change upon X-ray irradiation with the possibility of returning the crystals to their original uncolored state by irradiating with intense light in the range of 300-400 nm. X-ray irradiation is accompanied by the formation of induced color centers, which were studied by optical and electron paramagnetic resonance spectroscopy. The combination of properties of the BSBF crystals allows them to be used as a dose indicator [12,13].
The luminescent properties of undoped Ba4−xSr3+x(BO3)4−yF2+3y crystals were discussed in detail in Ref. [12]. The potential employment of fluoroborates can be enhanced by the incorporation of rare earth elements, endowing the crystals with additional luminescence properties. The present study is aimed at unveiling the luminescence potential of Ba4−xSr3+x(BO3)4−yF2+3y doped with Eu 3+ , Tb 3+ , and Ce 3+ for application in WLEDs as singlematrix or composite phosphors.

Materials and Experimental Methods
Crystal Growth. We grew crystals using NaF as a solvent owing to the incongruent melting of Ba4−xSr3+x(BO3)4−yF2+3y; BaCO3, SrCO3, SrF2, H3BO3, NaF, Ce2(CO3)3·5H2O, Tb4O7, E2O3 were used as starting reagents. The growth process was carried out in a platinum crucible 40 mm in diameter in air; the weight of the initial high-temperature solution was 40 g. As it follows from the data in Table 1, the effect of the rare earth elements addition on the liquidus temperature was minimal. Crystals were grown on a platinum loop without rotation and pulling at a cooling rate of 2 °C per day for approximately 15 days. The average weight of the crystals was about 8 g. We included the photographs of 1 mm thick plates made of the grown crystals in Table 1. Structure solution. To assess the influence of rare earth elements on the structure, we completed a single crystal X-ray diffraction analysis of BSBF: Eu 3+ , grown from a hightemperature solution with a relatively high europium concentration of 3.5 wt%, using a STOE IPDS diffractometer with graphite-monochromatized MoKα radiation and image plate detector. The ESPERANTO protocol, CrysAlisPro [14], SUPERFLIP, and Jana2020 software [15,16] were used for row data analysis and subsequent refinement of the structure.
Optical Spectroscopy. Transmission spectra of 1 mm thick plates were recorded with UV-VIS-NIR spectrometer sUV-3101 PC, Shimadzu.
The study of photoluminescence properties and lifetime measurements was performed with a Fluorolog 3 (Horiba Jobin Yvon, Kyoto, Japan) spectrofluorometer. Optistat DN was used to investigate the luminescence property at low temperatures. The measurement of the quantum yield was performed using a G8 (GMP SA, Zurich, Switzerland) spectraloncoated integrating sphere connected to a Fluorolog 3 spectrofluorimeter. More details on the equipment used are provided in Ref. [11].
X-ray powder diffraction (XRD) analysis. X-ray powder diffraction analysis was carried out using DRON 8 (Russia) with CuKα (1.5418 Å) radiation, Mythen2 R1 D detector (Switzerland) with a step width of 0.1 • and 5 s of exposure time per position.
Energy-dispersive X-ray (EDX) microanalysis. In order to estimate europium and terbium concentration in grown crystal, the samples were examined on an MIRA 3 LMU scanning electron microscope (Tescan Orsay Holding, Brno, Czech Republic) in combination with an INCA 450 energy dispersive X-ray microanalysis system. It is worth noting that due to the intersection of the Lα lines for barium (4.84 keV) and cerium (4.47 keV), measurements of the cerium concentration were not possible. The detection (three sigma) limit for rare earth element measurements was 0.51 wt%.

Results and Discussion
Crystal Structure. The results of X-ray single crystal analysis show that BSBF: Eu 3+ is characterized by the same point symmetry as the undoped BSBF crystal, P6 3 mc. The details about the X-ray diffraction structural analysis and its results are presented in Table  S1 and the CIF file (CCDC deposition number 2254976). The X-ray powder diffraction pattern of BSBF: Eu 3+ is shown in Figure S1. The suggested geterovalent isomorphic scheme is as follows: 3(Ba, Sr) 2+ ←2Eu 3+ + , -vacancy in cationic sites. We believe that this substitution scheme is also valid for terbium and cerium ions. Refined stoichiometry of the compound 'B 3.703662 Ba 3.172464 F 2.889012 O 11.11099 Sr 3.827532 ' may be represented as In the structure of BSBF crystal, there are three nonequivalent crystallographic cationic positions: Ba 2+ , Sr 2+ (both with Cs symmetry), and isomorphic position M (C3v symmetry), in which barium and strontium are statistically distributed. The coordination number of Ba, Sr, and M positions is 15, 13, and 16, respectively ( Figure 1). The possibility of the presence of Ba 2+ , Sr 2+ , Eu 3+ , and a vacancy in the cationic positions does not make it possible to unambiguously determine the concentration of these species when refining the structure. To assess which of the positions is the most favorable for the incorporation of Eu 3+ ions, we used the bond valence sum method (Table S2). Corresponding calculations taking into account the positional disorder in X1O/X1F and X2O/X2F sites were performed using bond-valence parameters R = 2.076 for Eu 3+ −O and R = 1.961 Eu 3+ −F (R = 1.961) [17]. The resulting values for Sr 2+ , Ba 2+ , and mixed M 2+ positions are 1.8, 1.0, and 1.5, respectively (see Table S2). As the bvs for the Sr 2+ position is closer to the Eu 3+ valence, this position is slightly more favorable for substitution. Optical properties. Transmission spectra of the plates made of doped BSBF crystals are dep ure 2. The optical absorption edge of BSBF: Ce 3+ , BSBF: Tb 3+ , and BSBF: Eu approximately coincides and corresponds to 225 nm, while BSBF: Tb 3+ dem best transparency in the UV range. This value coincides with the absorption undoped Ba4Sr3(BO3)4F2 crystal, which is 225 nm (5.517 eV) for 0.5 thick plate According to EDX microanalysis, the concentration of Tb2O3 and Eu2O3 in BS BSBF: Eu 3+ , Tb 3+ , Ce 3+ is below the limit of detection, the concentration of Eu Eu 3+ is about 0.7(3) wt%. It can be seen that an increase in the Eu 3+ concentra Eu 3+ leads to a shift of the absorption edge to the long wavelength region u which might be accounted for by the high intensity of charge transfer transit Optical properties. Transmission spectra of the plates made of doped BSBF crystals are depicted in Figure 2. The optical absorption edge of BSBF: Ce 3+ , BSBF: Tb 3+ , and BSBF: Eu 3+ , Tb 3+ , Ce 3+ approximately coincides and corresponds to 225 nm, while BSBF: Tb 3+ demonstrates the best transparency in the UV range. This value coincides with the absorption edge of the undoped Ba 4 Sr 3 (BO 3 ) 4 F 2 crystal, which is 225 nm (5.517 eV) for 0.5 thick plate at 300 K [13]. According to EDX microanalysis, the concentration of Tb 2 O 3 and Eu 2 O 3 in BSBF: Tb 3+ and BSBF: Eu 3+ , Tb 3+ , Ce 3+ is below the limit of detection, the concentration of Eu 2 O 3 in BSBF: Eu 3+ is about 0.7(3) wt%. It can be seen that an increase in the Eu 3+ concentration in BSBF: Eu 3+ leads to a shift of the absorption edge to the long wavelength region up to 320 nm, which might be accounted for by the high intensity of charge transfer transitions [18].
BSBF: Ce 3+ . The 5d 1 →4f 1 luminescence of Ce ions sufficiently depends on the nature of the matrix and is in the range from the ultraviolet to the red region of the visible spectrum [19][20][21][22]. In the photoluminescence (PL) spectra of BSBF: Ce 3+ , weak nicely resolved bands corresponding to the Ce 3+ 5 D 3/2 → 2 F 7/2, 2 F 5/2 transitions are detected at 400-500 nm under 360-390 nm excitation at 77 K (Figure 3a). Transitions in the same spectral range assigned to cerium ions were reported for Ba 2 Y 5 B 5 O 17 : Ce 3+ phosphor [21]. It is also possible to distinguish an additional band with a maximum of around 460 nm, most clearly at 360-390 nm excitation. For emission at 460 nm, the maximum of the excitation band is about 360 nm (Figure 3b). A similar broadband at 400-500 nm associated with intrinsic defects is observed in the luminescence spectra of undoped crystals at 365 nm excitation [13]. ure 2. The optical absorption edge of BSBF: Ce 3+ , BSBF: Tb 3+ , and BSBF: Eu 3+ , Tb 3+ , Ce approximately coincides and corresponds to 225 nm, while BSBF: Tb 3+ demonstrates th best transparency in the UV range. This value coincides with the absorption edge of th undoped Ba4Sr3(BO3)4F2 crystal, which is 225 nm (5.517 eV) for 0.5 thick plate at 300 K [13 According to EDX microanalysis, the concentration of Tb2O3 and Eu2O3 in BSBF: Tb 3+ an BSBF: Eu 3+ , Tb 3+ , Ce 3+ is below the limit of detection, the concentration of Eu2O3 in BSBF Eu 3+ is about 0.7(3) wt%. It can be seen that an increase in the Eu 3+ concentration in BSBF Eu 3+ leads to a shift of the absorption edge to the long wavelength region up to 320 nm which might be accounted for by the high intensity of charge transfer transitions [18].   Table 1. BSBF: Ce 3+ . The 5d 1 →4f 1 luminescence of Ce ions sufficiently depends on the nature of the matrix and is in the range from the ultraviolet to the red region of the visible spectrum [19][20][21][22]. In the photoluminescence (PL) spectra of BSBF: Ce 3+ , weak nicely resolved bands corresponding to the Ce 3+ 5 D3/2→ 2 F7/2, 2 F5/2 transitions are detected at 400−500 nm under 360-390 nm excitation at 77 K (Figure 3a). Transitions in the same spectral range assigned to cerium ions were reported for Ba2Y5B5O17: Ce 3+ phosphor [21]. It is also possible to distinguish an additional band with a maximum of around 460 nm, most clearly at 360-390 nm excitation. For emission at 460 nm, the maximum of the excitation band is about 360 nm (Figure 3b). A similar broadband at 400-500 nm associated with intrinsic defects is observed in the luminescence spectra of undoped crystals at 365 nm excitation [13]. BSBF: Tb 3+ . The PL spectra of Tb 3+ ions are due to 4f 8 -4f 8 transitions, shielded from the host crystal field by electrons of outer 5s and 5p shells, and, therefore, practically insensitive to the matrix. Luminescent spectra of BSBF: Tb 3+ at 300 nm excitation consist of four relatively narrow peaks arising from 5 D4→ 7 F6,5,4,3 transitions [23], located in our case at about 489, 545, 586, and 622 nm, respectively ( Figure 4a). As the temperature increases from 77 K to 300 K, the intensity of PL decreases monotonically. The spectrum observed at 270 nm excitation distinctively reveals the transition from both 5 D3 and 5 D4 energy levels. The essential feature of the spectrum is the broadening and splitting of the bands, which is most pronounced for the 5 D4→ 7 F6 (485, 489, and 497 nm), 5 D4→ 7 F5 (538 and 545 nm), and 5 D4→ 7 F4 (580, 585, and 590 nm) transitions (Figure 4b). This may be due to the BSBF: Tb 3+ . The PL spectra of Tb 3+ ions are due to 4f 8 -4f 8 transitions, shielded from the host crystal field by electrons of outer 5s and 5p shells, and, therefore, practically insensitive to the matrix. Luminescent spectra of BSBF: Tb 3+ at 300 nm excitation consist of four relatively narrow peaks arising from 5 D 4 → 7 F 6,5,4,3 transitions [23], located in our case at about 489, 545, 586, and 622 nm, respectively (Figure 4a). As the temperature increases from 77 K to 300 K, the intensity of PL decreases monotonically. The spectrum observed at 270 nm excitation distinctively reveals the transition from both 5 D 3 and 5 D 4 energy levels. The essential feature of the spectrum is the broadening and splitting of the bands, which is most pronounced for the 5 D 4 → 7 F 6 (485, 489, and 497 nm), 5 D 4 → 7 F 5 (538 and 545 nm), and 5 D 4 → 7 F 4 (580, 585, and 590 nm) transitions (Figure 4b). This may be due to the presence of Tb 3+ ions in several cationic positions.  BSBF: Eu 3+ and BSBF: Eu 3+ ,Tb 3+ ,Ce 3+ . In the case of Eu 3+ , 4f 6 -4f 6 transitions take place between the lowest excited state 5 D0 and seven multiplets of 7 FJ (J = 0-6) ground term. Transitions to the 7 F5 and 7 F6 multiplets are at 740-770 nm and 810-840 nm, respectively [24], and often lie outside of the investigated spectral range. The PL spectra of both crystals at 300, 370, and 395 nm excitation reveal broadened and split bands, which might be caused by the presence of rare earth elements in nonequivalent crystallographic sites and crystal field splitting ( Figure 5, Table 2).
The prominent feature of PL spectra at 300 nm excitation is the high intensity of the 5 D0→ 7 F0 transition at about 574 nm. Such 0-0 transitions are forbidden according to the ΔJ selection rule. The explanations proposed so far for the breakdown of this rule assume crystal filed induced J-mixing [25][26][27] and mixing of charge-transfer state [28] into the 4f 6 wavefunctions. It is worth noting that these two mechanisms are inversely related. In Ref. [26], the authors admit that charge transfer states of Eu 3+ lie near the 4f 6 levels and, therefore, may sufficiently affect the electronic structure of Eu 3+ . Charge transfer bands (CTB) are due to electron transfer. This process implies the formal reduction in trivalent europium to bivalent state and is accompanied by a change in the ionic radius of europium and strong lattice relaxation. Since CTB in all Eu-containing BSBF crystals has a pronounced intensity (Figure 6), we believe that charge-transfer state mixing is the key factor causing the high intensity of the above-mentioned transition.
The prominent feature of PL spectra at 300 nm excitation is the high intensity of the 5 D 0 → 7 F 0 transition at about 574 nm. Such 0-0 transitions are forbidden according to the ∆J selection rule. The explanations proposed so far for the breakdown of this rule assume crystal filed induced J-mixing [25][26][27] and mixing of charge-transfer state [28] into the 4f 6 wavefunctions. It is worth noting that these two mechanisms are inversely related. In Ref. [26], the authors admit that charge transfer states of Eu 3+ lie near the 4f 6 levels and, therefore, may sufficiently affect the electronic structure of Eu 3+ . Charge transfer bands (CTB) are due to electron transfer. This process implies the formal reduction in trivalent europium to bivalent state and is accompanied by a change in the ionic radius of europium and strong lattice relaxation. Since CTB in all Eu-containing BSBF crystals has a pronounced intensity (Figure 6), we believe that charge-transfer state mixing is the key factor causing the high intensity of the above-mentioned transition.
from the luminescence spectra observed in LiBa12(BO3)4F4 (LBBF): Eu , Tb , Ce crystal grown from the high-temperature solution with exactly the same concentration of Eu 3+ , Tb 3+ , and Ce 3+ [11]. In the latter case, strong emission of cerium ions is observed at 77 K.
Transitions with J > 0 depend on the site symmetry [33]. According to Ref. [34], the number of components for 5 D 0 → 7 F j (J = 0-4) transitions is 1, 3, 5, 7, and 9 for the site with Cs symmetry, and 1, 2, 3, 3, and 5 for the C3v site. It is worth noting that the major difficulty in unambiguous assignment of the peaks is their overlap due to a small crystal field splitting. The symmetry-sensitive 5 D 0 → 7 F 1 of magnetic-dipole nature and 5 D 0 → 7 F 2 of electric-dipole nature transitions are usually taken into consideration to establish a connection between site symmetry and band splitting [24,35]. In both BSBF: Eu 3+ and BSBF: Eu 3+ , Tb 3+ , Ce 3+ , the splitting of the bands related to these transitions is observed ( Figure 5). Thus, the BSBF: Eu 3+ crystal at 300 nm excitation exhibits three well-resolved peaks at about 586, 592, and 603 nm associated with the 5 D 0 → 7 F 1 transition (Figure 5a). The intensity of various transitions in the spectra of BSBF: Eu 3+ nonmonotonically changes with temperature (Figure 5a,c,e).
In addition to the discussed peaks associated with the transitions of Eu 3+ ions, the BSBF: Eu 3+ , Tb 3+ , Ce 3+ crystal reveals weak peaks at around 545 nm under 300 nm excitation related to the 5 D 4 → 7 F 5 transition of Tb 3+ ions (Figure 5b). The emission of cerium ions expected in the region 400-450 nm is virtually absent (Figure 5d), which differs dramatically from the luminescence spectra observed in LiBa 12 (BO 3 ) 4 F 4 (LBBF): Eu 3+ , Tb 3+ , Ce 3+ crystal grown from the high-temperature solution with exactly the same concentration of Eu 3+ , Tb 3+ , and Ce 3+ [11]. In the latter case, strong emission of cerium ions is observed at 77 K.
The intensity ratio of the charge transfer band and transitions in Eu 3+ ions differs significantly for BSBF: Eu 3+ and BSBF: Eu 3+ , Tb 3+ , Ce 3+ ( Figure 6). For BSBF: Eu 3+ for emission at 613 nm, Eu 3+ transitions have a dominant intensity, which is accounted for by a relatively higher concentration of Eu 3+ in BSBF: Eu 3+ .
In order to verify the realization of energy transfer from terbium to europium, reported for a number of compounds such as La 3 (Figure 7a). Decay times for BSBF: Tb 3+ (Figure 7b) and BSBF: Eu 3+ , Tb 3+ , Ce 3+ (Figure 7c) for emission at 545 nm after excitation at 300 nm are typical of terbium ions and close enough in value, 2.58 ms and 2.64 ms, respectively, which cast doubt on the implementation of energy transfer.  The CIE chromaticity coordinates and correlated color temperature of BSBF: Tb 3+ , BSBF: Eu 3+ , and BSBF: Eu 3+ , Tb 3+ , and Ce 3+ crystals are provided in Figure 8 and Table 3. The spectra of BSBF: Eu 3+ at temperatures of 77 K and 300 K are quite similar, which results in the close values of CIE coordinates. Due to the high intensity of 5 D0→ 7 F0 Eu 3+ transition at 370 nm excitation, the BSBF: Eu 3+ emission is yellow shifted (points 5, 6, Figure 8) in comparison with emission at 300 nm (points 3, 4, Figure 8) and 370 nm (points 7, 8, Figure  8) excitation. The quantum yield for BSBF: Eu 3+ at 395 nm excitation is about 10.6%. Thus, of special interest is the dependence of luminescence intensity and quantum yield on Eu 3+ concentration, which requires further study.