Luminescence of SiO2-BaF2:Tb3+, Eu3+ Nano-Glass-Ceramics Made from Sol–Gel Method at Low Temperature

The synthesis and characterization of multicolor light-emitting nanomaterials based on rare earths (RE3+) are of great importance due to their possible use in optoelectronic devices, such as LEDs or displays. In the present work, oxyfluoride glass-ceramics containing BaF2 nanocrystals co-doped with Tb3+, Eu3+ ions were fabricated from amorphous xerogels at 350 °C. The analysis of the thermal behavior of fabricated xerogels was performed using TG/DSC measurements (thermogravimetry (TG), differential scanning calorimetry (DSC)). The crystallization of BaF2 phase at the nanoscale was confirmed by X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM), and the changes in silicate sol–gel host were determined by attenuated total reflectance infrared (ATR-IR) spectroscopy. The luminescent characterization of prepared sol–gel materials was carried out by excitation and emission spectra along with decay analysis from the 5D4 level of Tb3+. As a result, the visible light according to the electronic transitions of Tb3+ (5D4 → 7FJ (J = 6–3)) and Eu3+ (5D0 → 7FJ (J = 0–4)) was recorded. It was also observed that co-doping with Eu3+ caused the shortening in decay times of the 5D4 state from 1.11 ms to 0.88 ms (for xerogels) and from 6.56 ms to 4.06 ms (for glass-ceramics). Thus, based on lifetime values, the Tb3+/Eu3+ energy transfer (ET) efficiencies were estimated to be almost 21% for xerogels and 38% for nano-glass-ceramics. Therefore, such materials could be successfully predisposed for laser technologies, spectral converters, and three-dimensional displays.


Introduction
Barium fluoride, BaF 2 , belongs to the group of attractive nanoparticles, produced using different preparation methods and applied in numerous multifunctional applications. Nd 3+ :BaF 2 nanocrystals synthesized by the reverse microemulsion technique present interesting luminescence properties [1]. Indeed, the quenching of fluorescence intensity (λ em = 1052 nm) in nanosized Nd 3+ :BaF 2 domains was not observed even under very high dopant levels (~45 mol.% of Nd 3+ ). Further experiments revealed the crystallization of cubic and orthorhombic BaF 2 nanoparticles, and it was proven that such fluoride crystals could be quite easily transformed from the orthorhombic phase to the more thermodynamically stable cubic phase under certain preparation conditions. This effect was confirmed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) used for self-assembled monodisperse BaF 2 nanocrystals accomplished by the liquid-solid-solution (LSS) approach [2]. BaF 2 nanocrystals were also fabricated from precursor Na 2 O-K 2 O-BaF 2 -Al 2 O 3 -SiO 2 glasses via their controlled heat treatment. Their self-organized nanocrystallization processes [3] and size distribution [4] have been presented and discussed in detail. Luminescence properties of nanosized Eu 3+ -doped preparation, precursor (TEOS), ethanol, deionized water, and acetic acid (AcOH) were mixed (in a molar ratio 1:4:10:0.5) in round-bottom flasks for 30 min. TEOS, Si(OC 2 H 5 ) 4 , was used as a precursor for creating the SiO 2 silicate host, water was necessary to perform the hydrolysis reaction of TEOS, and AcOH played a role as a catalyst. Due to the significantly limited solubility of TEOS in water, ethyl alcohol was introduced into the reaction systems, enabling the hydrolysis reaction by increasing the TEOS-water contact surface. The hydrolysis could be expressed by the following reaction: Si(OC 2 H 5 ) 4 + nH 2 O Si(OH) n (OC 2 H 5 ) (4-n) + nC 2 H 5 OH, in which n ≤ 4. Simultaneously with the hydrolysis reaction, the condensation begins, which allows for the creation of a silicate network through the formation of siloxane bridges, Si-O-Si. The homocondensation could be given by the following chemical reaction: (OC 2 H 5 ) n (OH) (3-n) Si-OH + HO-Si(OC 2 H 5 ) n (OH) (3-n) (OC 2 H 5 ) n (OH) (3-n) Si-O-Si(OC 2 H 5 ) n (OH) (3-n) + H 2 O, and the heterocondensation could be expressed by: (OC 2 H 5 ) n (OH) (3-n) Si-OH + C 2 H 5 O-Si(OC 2 H 5 ) n (OH) (3-n) (OC 2 H 5 ) n (OH) (3-n) Si-O-Si(OC 2 H 5 ) n (OH) (3-n) + C 2 H 5 OH, in which n ≤ 3. The mechanisms of hydrolysis and condensation reactions of alkoxides were discussed in detail in the paper [23]. After pre-hydrolysis and pre-condensation, the solutions of Ba(AcO) 2 and RE(AcO) 3 (RE = Tb or Tb/Eu) in trifluoroacetic acid (CF 3 COOH, TFA) and deionized water were added dropwise, and the obtained mixtures were stirred for the next 60 min. Since the electrolytic dissociation of TFA acid (K a = 5.9 × 10 −1 ) is greater than for AcOH (K a = 1.8 × 10 −5 ), TFA is a much stronger acid than AcOH, and the following reaction occurs: Ba(AcO) 2 + 2TFA → Ba(TFA) 2 + 2AcOH.
For Tb 3+ -doped samples, the molar ratio of TFA:Ba(AcO) 2 :Tb(AcO) 3 was equal to 5:0.95:0.05, and for Tb 3+ , Eu 3+ co-doped materials, the molar ratio of TFA:Ba(AcO) 2 :Tb(AcO) 3 :Eu(AcO) 3 was equal to 5:0.9:0.05:0.05. The mass of TEOS, ethanol, deionized water, and acetic acid reached 90 wt.% of each sample, and the mass of the remaining part containing TFA, Ba(AcO) 2 , and RE(AcO) 3 (RE = Tb or Tb/Eu) equaled 10 wt.%. The liquid sols were dried at 35 • C for several weeks and then heat treated at 350 • C per 10 h in a muffle furnace (Czylok, Jastrzębie-Zdrój, Poland). The thermal treatment of xerogels at 350 • C aims to transform them into SiO 2 -BaF 2 nano-glass-ceramics. Indeed, TFA was introduced as a fluorination reagent, allowing for successful crystallization of BaF 2 fraction inside the silicate sol-gel host. The fabricated xerogels were denoted as XG Tb and XG Tb/Eu (for singly and doubly doped xerogels), as well as nGC Tb and nGC Tb/Eu (for singly and co-doped nano-glass-ceramics).
The thermogravimetry and differential scanning calorimetry (TG/DSC) were carried out using a Labsys Evo system with a heating rate of 10 • C/min in argon atmosphere (SETARAM Instrumentation, Caluire, France). To verify the formation of fluoride nanocrystals within the silicate sol-gel host at 350 • C, the X-ray diffraction was performed using an X'Pert Pro diffractometer equipped by PANalytical with CuK α radiation (Almelo, the Netherlands). Additionally, the fluoride nanocrystals were observed by a JEOL JEM 3010 transmission electron microscope operated at 300 kV (JEOL, Tokyo, Japan). The structural characterization was supplemented by infrared spectroscopy (IR). The experiment was performed with the use of the Nicolet iS50 ATR spectrometer (Thermo Fisher Scientific Instruments, Waltham, MA, USA), and the spectra were collected in attenuated total reflectance (ATR) configuration within the 4000-400 cm −1 as well as 500-200 cm −1 ranges (64 scans, 4 cm −1 resolution). The luminescence measurements were performed on a Photon Technology International (PTI) Quanta-Master 40 (QM40) UV/VIS Steady State Spectrofluorometer (Photon Technology International, Birmingham, NJ, USA) supplied with a tunable pulsed optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD, OPOTEK, Carlsbad, CA, USA). The laser system was coupled with a 75 W xenon lamp, a double 200 mm monochromator, and a multimode UV/VIS PMT (R928) (PTI Model 914) detector. The excitation and emission spectra were recorded with a resolution of 0.5 nm. The luminescence decay curves were recorded by a PTI ASOC-10 (USB-2500) oscilloscope. All structural and optical measurements were carried out at room temperature.

Results and Discussion
3.1. Thermal Behavior of Synthesized Xerogels Figure 1 presents the TG/DSC curves recorded for fabricated xerogels in an inert gas atmosphere in a temperature range from 45 • C to 475 • C (the heating rate during measurement was 10 • C/min). According to TG curves, there are two distinguishable degradation steps for both fabricated samples: first, identified at 45-(~205) • C, and second, observed in the temperature range (~205) • C-(~320) • C. A slight weight loss, about 2.75% (XG Tb ) and 3.56% (XG Tb/Eu ), is associated with the elimination of residual solvents (ethyl alcohol, acetic acid) and water from the porous sol-gel host. At higher temperatures, strong exothermic peaks with maxima at 305 • C (XG Tb ) and 306 • C (XG Tb/Eu ) were identified, which appear along with~17.55% weight loss. Generally, trifluoroacetates tend to decompose at temperatures near~300 • C, which is well documented and described in the current literature [24][25][26]. Thus, recorded exothermic DSC peaks are clearly correlated with thermal decomposition of Ba(TFA) 2 and crystallization of BaF 2 , which could be given by the chemical reaction:  Figure 2 presents the X-ray diffraction (XRD) patterns of the xerogels and nano-glassceramics fabricated at 350 °C. The diffractograms collected for Tb 3+ singly doped samples are depicted in Figure 2a, meanwhile, the data for Tb 3+ , Eu 3+ co-doped materials are shown in Figure 2b. The XRD patterns of the precursor xerogels revealed any sharp diffraction  The thermolysis led to cleavage of C-F bonds from −CF 3 groups, and the resultant fluorine ions (F − ) tend to react with Ba-O bonds, forming BaF 2 phase [27]. The heat exchanged during degradation of Ba(TFA) 2 in studied sol-gel materials is close to −118 J/g. J. Farjas et al. [28] pointed out that the denoted heat exchange during the degradation of trifluoroacetates depends on atmosphere (air or ambient gas) and the presence of vapored water. Our obtained value is comparable with DSC results obtained for pure Ba(TFA) 2 salt in argon atmosphere [28]. The data obtained from TG/DSC analysis for the studied sol-gel samples are shown in Tables 1 and 2. 3.2. Structural Characterization by XRD, TEM, and ATR-IR Figure 2 presents the X-ray diffraction (XRD) patterns of the xerogels and nano-glassceramics fabricated at 350 • C. The diffractograms collected for Tb 3+ singly doped samples are depicted in Figure 2a, meanwhile, the data for Tb 3+ , Eu 3+ co-doped materials are shown in Figure 2b. The XRD patterns of the precursor xerogels revealed any sharp diffraction lines, but only a broad hump with a maximum at~25 • , indicating their amorphous nature without long-range order [29]. Conversely, the intense diffraction lines were observed only after thermal treatment of xerogels at 350 • C for 10 h. The XRD patterns of prepared glassceramics are in accordance with the standard diffraction lines of cubic BaF 2 crystallized in the Fm3m space group (ICDD card no. 00-004-0452), confirming the precipitation of fluoride crystals inside the silicate sol-gel matrix. The crystalline size of BaF 2 in fabricated glass-ceramics was evaluated by calculations with the Scherrer formula given below [30]:

Structural Characterization by XRD, TEM, and ATR-IR
in which K is a shape factor (in our calculations, K = 1 was taken), λ is a wavelength of X-rays (0.154056 nm, Kα line of Cu), B is a broadening of the diffraction peak at half the maximum intensity, and θ is Bragg's angle. The average crystal sizes of BaF 2 were calculated to be 5 nm ± 0.1 nm for both nGC Tb and nGC Tb/Eu samples. The average size of BaF 2 nanocrystallites was also calculated from a Williamson-Hall plot as follows [31]: where β is half of the width of the diffraction line, whereas (∆a/a) refers to the lattice deformation.
calculated to be 5 nm ± 0.1 nm for both nGCTb and nGCTb/Eu samples. The average size of BaF2 nanocrystallites was also calculated from a Williamson-Hall plot as follows [31]: where β is half of the width of the diffraction line, whereas (Δa/a) refers to the lattice deformation. The Scherrer method makes the half-width of the diffraction line dependent only on the size of the crystallites. On the other hand, in the Williamson-Hall analysis, the internal Figure 2. XRD patterns of prepared sol-gel samples: Tb 3+ singly doped materials (a) and Tb 3+ , Eu 3+ co-doped specimens (b). The standard data for the BaF 2 cubic phase (ICDD card no. 00-004-0452) are also shown for comparison. TEM images revealed the presence of fluoride crystals in glass-ceramics singly doped with Tb 3+ (c) and co-doped with Tb 3+ , Eu 3+ (d).
The Scherrer method makes the half-width of the diffraction line dependent only on the size of the crystallites. On the other hand, in the Williamson-Hall analysis, the internal stresses reflected by the lattice deformation are additionally taken into account in the broadening of the diffraction line. The mean crystal sizes, calculated with the Williamson-Hall method, were estimated to be 4.3 nm ± 0.1 nm for nGC Tb , and 4.8 nm ± 0.1 nm for nGC Tb/Eu . Moreover, the lattice deformation was negligible and less than 0.1%. The obtained results of the average crystallite size, from both methods, reveal good agreement. They proves the lack of internal stresses in the formed BaF 2 particles. The crystal lattice parameters for BaF 2 phase were determined to be 6.188 (8) Å (Tb 3+ -doped sample) and 6.169 (7) Å (Tb 3+ , Eu 3+ co-doped sample), which are slightly smaller than the lattice parameter for undoped barium fluoride (a 0 = 6.2001 Å). Indeed, both Tb 3+ (1.04 Å) and Eu 3+ (1.07 Å) ions [32] with smaller ionic radii could substitute Ba 2+ (1.35 Å) [33] cations in BaF 2 crystal lattice, resulting in a decrease in the unit cell volume. The indicated changes in the lattice parameter are also noticeable as a slight shift of the recorded diffraction lines towards higher values of the 2θ angle (an enlargement within a 22-28 • angle, in which the (111) diffraction line was detected; shown in Figure 1). Additionally, it was observed that the shift of diffraction lines is more clearly visible for the nGC Tb/Eu sample, which evidences that the total incorporation of RE 3+ ions inside BaF 2 nanocrystals is higher than for nGC Tb . Similar results from XRD measurements were described in the literature for other oxyfluoride optical systems, e.g., SiO 2 -LaF 3 :Er 3+ sol-gel nano-glass-ceramics [34] and germano-gallate glass-ceramics containing BaF 2 :Er 3+ nanocrystals [11]. Figure 2c,d display the TEM images of prepared nano-glass-ceramic samples singly doped with Tb 3+ and co-doped with Tb 3+ , Eu 3+ ions, respectively. The size of BaF 2 nanocrystals was average, estimated from the Scherrer equation and the Williamson-Hall method.
The ATR-IR spectrum within the 4000-400 cm −1 range for a representative XG Tb/Eu sample is shown in Figure 3a, and the assignment of individual IR peaks was carried out based on the literature [35,36]. The recorded infrared signals confirmed the formation of a polycondensed silicate network created by Q 2 (949 cm −1 ), Q 3 (1045 cm −1 ), and Q 4 (1134 cm −1 ) units of SiO 2 tetrahedrons as well as Si-O-Si siloxane bridges (1192 cm −1 , 801 cm −1 ). On the other hand, the signals recorded at~3665 cm −1 and~3400 cm −1 , according to vicinal/geminal and hydrogen-bonded Si-OH groups, respectively, clearly pointed to the presence of unreacted silanol groups. Indeed, xerogels are highly porous materials [37], hence, the IR bands originating from Si-OH groups are expected. Indeed, the next recorded infrared band, 1659 cm −1 , revealed the vibrations of Si-OH groups, but also oscillations within C=O carbonyl groups (from residual AcOH and unreacted TFA), as well as molecular water. A signal at~3200 cm −1 was interpreted as vibrations from hydrogen-bonded OH groups in organic compounds and water, and may confirm that pores inside the silicate network are filled with liquids. It should be noted that peaks located near~1134 cm −1 and~1192 cm −1 could be assigned, despite Q 4 units and Si-O-Si bridges, to oscillations of C-F bonds in Ba(TFA) 2 and unreacted TFA. Indeed, a comparison of ATR-IR spectra in this region for XG Tb/Eu xerogel and an analogous sample prepared without the addition of Ba(AcO) 2 and TFA revealed that the signals are more intense for XG Tb/Eu (inset of Figure 3a). This could point to the presence of additional oscillators that contribute to overall signals recorded at~1134 cm −1 and~1192 cm −1 . The signal recorded at~420 cm −1 was assigned to the O-Si-O bending vibration.
The ATR-IR spectrum within the 4000-400 cm −1 region registered for a representative nGC Tb/Eu sample obtained at 350 • C is shown in Figure 3b. Compared with the ATR-IR spectrum for XG Tb/Eu , the intensities of signals at >3000 cm −1 and 1659 cm −1 weakened significantly, which allows us to make conclusions about evaporation of volatile chemical components from the sol-gel host and progressive reactions between unreacted Si-OH groups. Additionally, it was also observed that for the nGC Tb/Eu nano-glass-ceramic sample, the intensity of the IR signal near~949 cm −1 is weaker than for the XG Tb/Eu xerogel sample. An indicated effect may also suggest a continuation of polycondensation because Q 2 units probably transformed into Q 3 and Q 4 ones, which favor the creation of a more cross-linked sol-gel host. It was also observed that intensities of IR signals near 1134 cm −1 and~1192 cm −1 weakened compared with those recorded for the xerogel. This effect could be explained by thermal decomposition of Ba(TFA) 2 compound into BaF 2 crystals within the prepared silicate sol-gel host in the proposed heat-treatment conditions. Indeed, the Ba-F vibrations might be observed at lower frequencies (inset of Figure 3b), which agrees with the IR spectrum recorded for pure BaF 2 [38]. The peak with a maximum at~440 cm −1 was recorded for both nGC Tb/Eu nano-glass-ceramics and an analogous sample prepared without the addition of Ba(AcO) 2 and TFA, confirming that such a band is not related to the fluoride fraction but to the oscillations within the silicate host (O-Si-O vibration). It should be noticed that this band shifts toward a higher frequency for nano-glass-ceramics in comparison with the xerogel. The reason for such spectral behavior could be explained by differences in the inter-tetrahedra angle of SiO 4 units in xerogels and glass-ceramics, as was stated in the literature [39].
the prepared silicate sol-gel host in the proposed heat-treatment conditions. Indeed, the Ba-F vibrations might be observed at lower frequencies (inset of Figure 3b), which agrees with the IR spectrum recorded for pure BaF2 [38]. The peak with a maximum at ~440 cm −1 was recorded for both nGCTb/Eu nano-glass-ceramics and an analogous sample prepared without the addition of Ba(AcO)2 and TFA, confirming that such a band is not related to the fluoride fraction but to the oscillations within the silicate host (O-Si-O vibration). It should be noticed that this band shifts toward a higher frequency for nano-glass-ceramics in comparison with the xerogel. The reason for such spectral behavior could be explained by differences in the inter-tetrahedra angle of SiO4 units in xerogels and glass-ceramics, as was stated in the literature [39].  Figure 4a shows the excitation spectra of the prepared XGTb/Eu samples. The spectra were registered within the 340-520 nm spectral range on collecting the luminescence at 541 nm and 612 nm wavelengths. The excitation spectrum, while monitoring the green emission line at 541 nm, revealed the characteristic bands for Tb 3+ ions according to the following transitions within the near-UV and VIS scope: 7 F6 → 5 L9 (352 nm), 7 F6 → 5 L10 (370 nm), 7 F6 → 5 D3 (379 nm), and 7 F6 → 5 D4 (488 nm). Meanwhile, the spectrum recorded by Figure 3. ATR-IR spectra recorded for xerogel XG Tb/Eu (a) and nano-glass-ceramic nGC Tb/Eu (b) co-doped with Tb 3+ , Eu 3+ ions. Figure 4a shows the excitation spectra of the prepared XG Tb/Eu samples. The spectra were registered within the 340-520 nm spectral range on collecting the luminescence at 541 nm and 612 nm wavelengths. The excitation spectrum, while monitoring the green emission line at 541 nm, revealed the characteristic bands for Tb 3+ ions according to the following transitions within the near-UV and VIS scope: 7 F 6 → 5 L 9 (352 nm), 7 F 6 → 5 L 10 (370 nm), 7 F 6 → 5 D 3 (379 nm), and 7 F 6 → 5 D 4 (488 nm). Meanwhile, the spectrum recorded by collecting the red luminescence at 612 nm showed the excitation lines of Eu 3+ related to the electronic transitions from the 7 F 0 ground level into the following excited states: 5 G J (376 nm), 5 L 7 (384 nm), 5 L 6 (394 nm), 5 D 3 (418 nm), and 5 D 2 (464 nm). However, it was observed that the spectrum recorded for XG Tb/Eu contains some additional weak bands, which did not appear for the sample singly doped with Eu 3+ (for better visibility, an enlargement of the 340-390 nm scope is presented in the inset of Figure 2a, and the bands are marked by asterisks). It should be noted that the recorded additional bands correspond to the contribution of excitation lines originating from Tb 3+ ions ( 7 F 6 → 5 L 9 (352 nm), 7 F 6 → 5 L 10 (379 nm), and 7 F 6 → 5 D 4 (488 nm)). Moreover, a slight shift of the 7 F 0 → 5 L 7 band (from 384 nm to 382 nm) was also denoted, which could be related to its overlapping with the 7 F 6 → 5 D 3 excitation line originating from Tb 3+ co-dopant. Hence, the obtained results could suggest the occurrence of Tb 3+ → Eu 3+ ET. A similar interpretation of excitation spectra was described for lead borate glasses co-doped with Tb 3+ and Eu 3+ ions [40].

Luminescence of Amorphous Silicate Xerogels
observed that the spectrum recorded for XGTb/Eu contains some additional weak bands, which did not appear for the sample singly doped with Eu 3+ (for better visibility, an enlargement of the 340-390 nm scope is presented in the inset of Figure 2a, and the bands are marked by asterisks). It should be noted that the recorded additional bands correspond to the contribution of excitation lines originating from Tb 3+ ions ( 7 F6 → 5 L9 (352 nm), 7 F6 → 5 L10 (379 nm), and 7 F6 → 5 D4 (488 nm)). Moreover, a slight shift of the 7 F0 → 5 L7 band (from 384 nm to 382 nm) was also denoted, which could be related to its overlapping with the 7 F6 → 5 D3 excitation line originating from Tb 3+ co-dopant. Hence, the obtained results could suggest the occurrence of Tb 3+ → Eu 3+ ET. A similar interpretation of excitation spectra was described for lead borate glasses co-doped with Tb 3+ and Eu 3+ ions [40].  The fluorescence spectra of prepared sol-gel specimens are displayed in Figure 4b. The emission spectrum recorded for the XG Tb/Eu sample under excitation at 394 nm (presented as a red line) consisted of several emission lines at 574 nm ( 5 D 0 → 7 F 0 ), 590 nm ( 5 D 0 → 7 F 1 ), 612 nm ( 5 D 0 → 7 F 2 ), 648 nm ( 5 D 0 → 7 F 3 ), and 696 nm ( 5 D 0 → 7 F 4 ) within the reddishorange light area. It was observed that the 5 D 0 → 7 F 2 red emission band is the most prominent luminescence line, and the spectrum is similar to other Eu 3+ -doped typical glassy-like optical materials described in the literature [41,42]. Based on the collected spectrum, the R/O ratio (red-to-orange) was calculated using the areas of the 5 D 0 → 7 F 2 (R) and the 5 D 0 → 7 F 1 (O) bands. The R/O ratio value estimated for precursor silicate xerogel is relatively high and equals 3.92. It indicates that Eu 3+ ions are far from an inversion center, which is characteristic for amorphous materials. In the luminescence spectrum of the XG Tb sample (marked as a green line), the bands centered at 486 nm, 541 nm, 580 nm, and 618 nm were attributed to the 5 D 4 → 7 F J (J = 6-3) electronic transitions, respectively.
To verify the occurrence of ET between Tb 3+ and Eu 3+ ions in the studied silicate xerogels, the emission spectrum for the XG Tb/Eu sample was recorded upon excitation at a 352 nm wavelength (shown as a blue line). The spectrum consisted of the following emission bands in the VIS spectral range: blue (486 nm), an intense green (541 nm), yellowish-orange (584 nm), and red (616 nm). The same bands within the blue-green light area were detected for the XG Tb xerogel, and the mentioned emission lines were ascribed to the 5 D 4 → 7 F 6 and the 5 D 4 → 7 F 5 electronic transitions, respectively. Although the positions of these emission bands are the same, their intensity is slightly lower for the co-doped XG Tb/Eu sample than for the singly doped XG Tb one. Simultaneously, an increase in luminescence intensity within the yellowish-orange as well as red ranges was observed, and-compared with emissions recorded for the XG Tb xerogel-the maxima of these bands were slightly shifted (from 580 nm to 584 nm, and from 618 nm to 616 nm). Thus, based on this observation, we could conclude that the indicated shift is a result of the superimposition of the yellow ( 5 D 4 → 7 F 4 , 580 nm) and red band ( 5 D 4 → 7 F 3 , 618 nm) of Tb 3+ ions with orange ( 5 D 0 → 7 F 1 , 590 nm) and red ( 5 D 0 → 7 F 2 , 612 nm) luminescence originating from Eu 3+ .
Hence, our experimental results indicate the occurrence of Tb 3+ /Eu 3+ ET upon excitation at a 352 nm wavelength when Tb 3+ ions are excited from the 7 F 6 ground state. Then, the electrons at the 5 L 9 level decay rapidly through the 5 G 5 , 5 L 10 , and 5 D 3 states by the multiphonon relaxation process until the 5 D 4 level is populated. Since there is the energetical resemblance of the 5 D 4 (Tb 3+ ) and the 5 D 1 / 5 D 0 (Eu 3+ ) levels, the Tb 3+ /Eu 3+ energy migration is feasible, and the excitation energy is transferred from Tb 3+ to the adjacent Eu 3+ ion. The acceptor ions relax from the 5 D 0 state to the 7 F J levels, promoting the light emission within the reddish-orange spectral region [43]. The ET is schematized in the level diagram presented in Figure 5.
where τ0 and τ are the lifetimes of the 5 D4 (Tb 3+ ) state for sample singly doped with Tb 3+ , and the sample co-doped with Tb 3+ , Eu 3+ ions, respectively. In the case of the studied xerogels, the efficiency of Tb 3+ /Eu 3+ ET was estimated to be about 21%, and the comparable values were denoted for, e.g., fluoroborate glass (ηET = 20%) [45].

Luminescence of SiO2-BaF2 Nano-Glass-Ceramics
The excitation spectra recorded for the nGCTb/Eu sample are shown in Figure 6a. The spectra emerged by monitoring the green luminescence characteristic for Tb 3+ (541 nm), and the red emission originating from Eu 3+ ions (612 nm). The luminescence of Tb 3+ ions (541 nm) could be efficiently excited by the following wavelengths from the near-UV scope: 352 nm ( 7 F6 → 5 L9), 369 nm ( 7 F6 → 5 L10), and 377 nm ( 7 F6 → 5 D3), as well as from the The decay curves were registered for the green light at 541 nm, upon excitation at 352 nm from the near-UV range (inset in Figure 4b). For xerogels, a mono-exponential fit was used to evaluate the lifetimes of Tb 3+ , and the fitted curves are marked with a black line, while the collected experimental data are shown as green and blue lines for XG Tb and XG Tb/Eu , respectively. A slight shortening in the decay time of the 5 D 4 (Tb 3+ ) state from 1.11 ms (XG Tb ) to 0.88 ms (XG Tb/Eu ) was identified. An indicated decline in a lifetime for co-doped xerogel could be explained by introducing an additional decay pathway via Eu 3+ ions. Indeed, the ET from Tb 3+ to Eu 3+ enhances the decay rate of the excited Tb 3+ ions, resulting in the shortening of the 5 D 4 (Tb 3+ ) lifetime. Hence, the analysis of luminescence decay curves also enables calculation of the efficiency of Tb 3+ /Eu 3+ ET, based on the following equation [44]: where τ 0 and τ are the lifetimes of the 5 D 4 (Tb 3+ ) state for sample singly doped with Tb 3+ , and the sample co-doped with Tb 3+ , Eu 3+ ions, respectively. In the case of the studied xerogels, the efficiency of Tb 3+ /Eu 3+ ET was estimated to be about 21%, and the comparable values were denoted for, e.g., fluoroborate glass (η ET = 20%) [45].

Luminescence of SiO 2 -BaF 2 Nano-Glass-Ceramics
The excitation spectra recorded for the nGC Tb/Eu sample are shown in Figure 6a. The spectra emerged by monitoring the green luminescence characteristic for Tb 3+ (541 nm), and the red emission originating from Eu 3+ ions (612 nm). The luminescence of Tb 3+ ions (541 nm) could be efficiently excited by the following wavelengths from the near-UV scope: 352 nm ( 7 F 6 → 5 L 9 ), 369 nm ( 7 F 6 → 5 L 10 ), and 377 nm ( 7 F 6 → 5 D 3 ), as well as from the VIS range: 485 nm ( 7 F 6 → 5 D 4 ). In the case of the excitation spectrum recorded at a 612 nm emission wavelength, an intense line appeared at 394 nm ( 7 F 0 → 5 L 6 , Eu 3+ ), but a few weaker bands at 376 nm ( 7 F 0 → 5 G J ), 384 nm ( 7 F 0 → 5 L 7 ), 418 nm ( 7 F 0 → 5 D 3 ), and 465 nm ( 7 F 0 → 5 D 2 ) were also detected. Similarly, as for xerogels, the recorded additional excitation lines at 352 nm, 369 nm, and 485 nm-marked in Figure 6a by asterisks-are typical for the 7 F 6 → 5 L 9,10 , 5 D 4 transitions of Tb 3+ ions, which could suggest the occurrence of Tb 3+ /Eu 3+ ET in the studied nano-glass-ceramic samples. Similar results were found for other Tb 3+ , Eu 3+ co-doped fluoride-based optical systems, e.g., pure CaF 2 nanocrystals [46], and glass-ceramics containing SrF 2 [47], as well as NaYF 4 nanocrystals [48]. Figure 6b depicts the emission spectra collected for nGC Tb/Eu and nGC Tb samples, recorded upon excitation at 352 nm (a blue line for nGC Tb/Eu , and a green line for nGC Tb ) and 394 nm (a red line) wavelengths. An excitation of the nGC Tb sample using 352 nm results in registration of the visible emissions ascribed to the 5 D 4 → 7 F 6 (487 nm), 5 D 4 → 7 F 5 (541 nm), 5 D 4 → 7 F 4 (580 nm, 587 nm), and 5 D 4 → 7 F 3 (619 nm) transitions characteristic for Tb 3+ ions. Subsequently, when the nGC Tb/Eu co-doped sample was excited by a 394 nm wavelength, the luminescence bands originating from Eu 3+ ions centered at 589 nm ( 5 D 0 → 7 F 1 ), 611 nm/614 nm ( 5 D 0 → 7 F 2 ), 648 nm ( 5 D 0 → 7 F 3 ), and 688 nm/696 nm ( 5 D 0 → 7 F 4 ) were observed. One can see that, in contrast to xerogel, the 5 D 0 → 7 F 1 magnetic dipole transition dominates the spectrum, which indicates that Eu 3+ ions are placed at sites close to an inversion symmetry [49]. According to the calculated R/O ratio value (3.92 for XG Tb/Eu and 0.51 for nGC Tb/Eu ) and the literature [18], the observed change in emission profile clearly suggests that Eu 3+ ions tend to embed into the BaF 2 fluoride nanocrystal lattice by substituting Ba 2+ cations. The decrease in the R/O ratio value was denoted for other Eu 3+ -doped oxyfluoride glass-ceramic systems described in the literature [50][51][52].
inversion symmetry [49]. According to the calculated R/O ratio value (3.92 for XGTb/Eu and 0.51 for nGCTb/Eu) and the literature [18], the observed change in emission profile clearly suggests that Eu 3+ ions tend to embed into the BaF2 fluoride nanocrystal lattice by substituting Ba 2+ cations. The decrease in the R/O ratio value was denoted for other Eu 3+ -doped oxyfluoride glass-ceramic systems described in the literature [50][51][52]. The emission spectrum of the nGCTb/Eu sample, collected upon 352 nm excitation, revealed an intense orange (589 nm) and red (611 nm/615 nm, and 647 nm) luminescence corresponding to the transitions of Eu 3+ from the 5 D0 level. Along with those bands, two emission lines with relatively low intensity were found within the blue-green scope and Figure 6. The excitation spectra recorded for Tb 3+ (λ em = 541 nm) and Eu 3+ (λ em = 612 nm) ions in prepared SiO 2 -BaF 2 nano-glass-ceramics. For the latter, the additional lines originated from Tb 3+ ions were marked by asterisks (a). The registered emission spectra for nGC Tb (green line, λ exc = 352 nm) as well as nGC Tb/Eu glass-ceramics (blue line, λ exc = 352 nm; red line, λ exc = 394 nm). Inset shows the decay curves recorded for the 5 D 4 (Tb 3+ ) state in nano-glass-ceramics (b).
The emission spectrum of the nGC Tb/Eu sample, collected upon 352 nm excitation, revealed an intense orange (589 nm) and red (611 nm/615 nm, and 647 nm) luminescence corresponding to the transitions of Eu 3+ from the 5 D 0 level. Along with those bands, two emission lines with relatively low intensity were found within the blue-green scope and were assigned to the emissions originating from the 5 D 4 state of Tb 3+ ions. Therefore, compared with XG Tb/Eu , the luminescence in the reddish-orange spectral range is particularly enhanced for nGC Tb/Eu . Based on this observation, we could conclude that the distance between interacting Tb 3+ and Eu 3+ ions in the prepared nano-glass-ceramics might be significantly shorter than in xerogels. Such shortening in the inter-ionic distance, strictly related to the segregation of rare earths inside BaF 2 nanocrystals precipitated at 350 • C, could be responsible for a more efficient transfer of excitation energy from Tb 3+ to Eu 3+ ions.
For the SiO 2 -BaF 2 nano-glass-ceramics, the luminescence decay from the 5 D 4 level follows a double-exponential function with two different decay lifetimes. It results from the distribution of RE 3+ ions between either the sol-gel host (described by faster τ 1 component) and BaF 2 nanocrystals (described by longer τ 2 lifetime). The results are presented in the inset of Figure 6b, and the fitted decay curves are labeled with a black line, whereas the experimental data are tagged as green and blue lines for nGC Tb and nGC Tb/Eu , respectively. For the sample singly doped with Tb 3+ ions, the lifetime components are equal to τ 1 = 2.51 ms and τ 2 = 6.97 ms, while for the sample co-doped with Tb 3+ , Eu 3+ the decay times are equal to τ 1 = 1.05 ms and τ 2 = 4.53 ms. Based on lifetime components, the average decay times, τ avg , were calculated from the following formula [53]: Thus, the average luminescence lifetime of the 5 D 4 (Tb 3+ ) state for nGC Tb/Eu was determined to be τ avg = 4.06 ms, and for nGC Tb it equaled τ avg = 6.56 ms. The analysis of luminescence decay curves showed a noticeable prolongation in lifetimes for SiO 2 -BaF 2 nano-glass-ceramics compared with xerogels. It suggests that the amount of OH groups characterized by high vibrational energy (>3000 cm −1 ) should be significantly reduced in glass-ceramics. Moreover, the dopant ions tend to enter into the BaF 2 nanocrystals characterized by low phonon energy (~319 cm −1 [54]), making the radiative relaxation from the 5 D 4 level more prominent compared with xerogels.
Additionally, based on luminescence lifetimes, the calculated ET efficiency for prepared SiO 2 -BaF 2 nano-glass-ceramic exceeds 38%. In such a case, the distance between interacting RE 3+ ions entering into BaF 2 nanocrystals decreased, resulting in a reinforced transfer of energy absorbed by Tb 3+ to Eu 3+ . Indeed, it is related to creating an energy transfer net among the donor and acceptor ions, causing the ET to become more frequent. Comparable values of ET efficiency were described for glass-ceramics containing YF 3 :1Tb 3+ , 0.5Eu 3+ (mol.%) nanophase (η ET ≈ 39%) [55].
Summarizing, due to the unique properties of BaF 2 , e.g., a broad region of transparency from 0.14 µm up to 14 µm, wide bandgap (11 eV), and low maximum phonon energy (~319 cm −1 ), the oxyfluoride glass-ceramics containing BaF 2 nanophase are extensively applied to generate an efficient up- [11] and down-conversion luminescence [9], or white light emission [20]. Therefore, such materials could be successfully used for laser technologies, spectral converters, and three-dimensional displays [10]. Since Eu 3+ ions emit within the red or reddish-orange light area, and Tb 3+ ions are well known as green emitters, the fabricated SiO 2 -BaF 2 :Tb 3+ , Eu 3+ nano-glass-ceramics are able to generate multicolor luminescence. Thus, sol-gel materials might be considered for use as optical elements in RGB lighting optoelectronic devices operating upon near-UV excitation.

Conclusions
This work presented the fabrication of Tb 3+ , Eu 3+ co-doped oxyfluoride glass-ceramics at 350 • C from xerogels prepared via the sol-gel technique. The analysis of the thermal behavior of xerogels was performed using TG/DSC measurements, and the structural properties were determined based on ATR-IR spectroscopy. The crystallization of BaF 2 at the nanoscale was confirmed by XRD and TEM measurements. The characterization of sol-gel samples involved an excitation of the prepared sol-gel materials upon near-UV irradiation at 352 nm which showed the Tb 3+ /Eu 3+ energy transfer, resulting in strengthening the luminescence within the reddish-orange light scope due to additional emission from Eu 3+ ions. Nevertheless, for xerogels, the blue-green luminescence ( 5 D 4 → 7 F 5,6 of Tb 3+ ) dominated, meanwhile, the reddish-orange emission ( 5 D 0 → 7 F 0-4 of Eu 3+ overlapped with 5 D 4 → 7 F 4,3 bands of Tb 3+ ) was particularly enhanced for SiO 2 -BaF 2 nano-glass-ceramics. The luminescence decay kinetics showed that in the co-doped sol-gel materials, the energy transfer from Tb 3+ to Eu 3+ ions occurred with an efficiency that varied from 21% for xerogels to 38% for nano-glass-ceramics. An indicated increase in energy transfer efficiency for prepared nano-glass-ceramics could be explained by shortening the distance between interacting Tb 3+ and Eu 3+ ions embedded into the BaF 2 nanocrystal lattice. The obtained results suggest that the fabricated SiO 2 -BaF 2 :Tb 3+ , Eu 3+ nano-glass-ceramics could be predisposed to application in selected technologies, e.g., three-dimensional displays and color screens.