Spectroscopic Properties of Erbium-Doped Oxyfluoride Phospho-Tellurite Glass and Transparent Glass-Ceramic Containing BaF2 Nanocrystals

The ErF3-doped oxyfluoride phospho-tellurite glasses in the (40-x) TeO2-10P2O5-45 (BaF2-ZnF2) -5Na2O-xErF3 system (where x = 0.25, 0.50, 0.75, 1.00, and 1.25 mol%) have been prepared by the conventional melt-quenching method. The effect of erbium trifluoride addition on thermal, structure, and spectroscopic properties of oxyfluoride phospho-tellurite precursor glass was studied by differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR), and Raman spectroscopy as well as emission measurements, respectively. The DSC curves were used to investigate characteristic temperatures and thermal stability of the precursor glass doped with varying content of ErF3. FTIR and Raman spectra were introduced to characterize the evolution of structure and phonon energy of the glasses. It was found that the addition of ErF3 up to 1.25 mol% into the chemical composition of phospho-tellurite precursor glass enhanced 2.7 µm emission and upconversion. By controlled heat-treatment process of the host glass doped with the highest content of erbium trifluoride (1.25 mol%), transparent erbium-doped phospho-tellurite glass-ceramic (GC) was obtained. X-ray diffraction analysis confirmed the presence of BaF2 nanocrystals with the average 16 nm diameter in a glass matrix. Moreover, MIR, NIR, and UC emissions of the glass-ceramic were discussed in detail and compared to the spectroscopic properties of the glass doped with 1.25 mol% of ErF3 (the base glass).


Introduction
Since Wang reported for the first time in the literature the method of obtaining transparent oxyfluoride glass-ceramic doped with Er 3+ and Yb 3+ ions [1], the dynamic development of research on transparent rare-earth-doped (RED) oxyfluoride glass-ceramics (GC) with fluoride nanocrystals has been noted [2][3][4][5]. As a result of controlled heat treatment of the precursor glass in the SiO 2 -Al 2 O 3 -PbF 2 -CdF 2 -YbF 3 -ErF 3 system, Wang and Ohwaki obtained Pb x Cd 1-x F 2 nanocrystals.

Experimental Procedure
Oxyfluoride phospho-tellurite glasses with the molar composition (40-x)TeO 2 -10P 2 O 5 -45(BaF 2 -ZnF 2 )-5Na 2 O-xErF 3 system (where x = 0.25, 0.50, 0.75, 1.00, and 1.25 mol%), denoted as TP0.25ErF 3 , TP0.50ErF 3 , TP0.75ErF 3 , TP1.00ErF 3 , TP1.25ErF 3 , respectively, were prepared by conventional melt-quenching method. High purity (99.99%) raw materials (TeO 2 , P 2 O 5 , BaF 2 , ZnF 2 , Na 2 CO 3 , and ErF 3 ) were used and each batch of 10 g was well-mixed in an agate mortar, and then melted in a covered platinum crucible at 950 • C for 90 min in ambient air. The melt was cast onto a preheated stainless steel, next annealed at 310 • C for 10 h, and then cooled down to room temperature. All the obtained samples were transparent. The glass samples were cut to the 5 mm × 5 mm size and polished to optical quality before measurements with a 2 mm thickness. Crystallized material was obtained for the addition of 1.5 mol% ErF 3 . Therefore, the glass with 1.25 mol% ErF 3 content has been chosen to obtain a transparent glass-ceramic sample. Based on the recorded DSC curve of the TP1.25ErF 3 glass, it was found that the onset crystallization temperature was 390 • C and the significant exothermic crystal peak corresponded to 456 • C. According to this data, the heat treatment temperature of TP1.25ErF 3 glass sample was determined from 395 to 460 • C. In order to obtain the BaF 2 fluoride crystalline phase in a nanometric scale, the temperature of the controlled heat treatment process of TP1.25ErF 3 glass was set at 400 • C for 3 h.
The sample with 1.25 mol% of erbium trifluoride was heat-treated for 3 h at 400 • C to obtain the glass-ceramic.
X-ray diffraction studies were carried out on the X'Pert Pro X-ray diffractometer supplied by PANalytical (Almelo, Netherlands) with Cu K α1 radiation (λ = 1.54056 Å) in the 2θ range of 10-90 • . The step size, time per step, and scan speed were as follows: 0.017 • , 184.79 s, and 0.011 • /s. The X-ray tube was operated at 40 kV and 40 mA and a scintillation detector (Almelo, Netherlands) were used to measure the intensity of the scattered X-rays. Qualitative identification of the phase composition in the glass-ceramic sample was performed with reference to the ICDD PDF-2 database. From the peak width of the X-ray pattern of the glass-ceramic sample, the crystalline size of crystals can be calculated on the basis of Scherrer's equation.
The thermal characteristic temperatures such as the glass transition (T g ), the crystallization (T c ), and melting (T m ) temperatures were measured by using the Jupiter DTA STA 449 F3 thermal analyzer (NETZSCH Thermal Analysis, Selb, Germany), operating in the heat flux DSC mode at a heating rate of 10 • C/min under synthetic air atmosphere. Measurements were carried out with an uncertainty of ±1 • C.
The FTIR spectra of the glasses were obtained with the Fourier spectrometer (Bruker Optics-Vertex70V, Rheinstetten, Germany). The measurements were done using the KBr pellet technique. Absorption spectra were recorded at 128 scans and the resolution of 4 cm −1 .
Raman spectra of all samples were obtained using a LabRAM HR spectrometer (HORIBA Jobin Yvon, Palaiseau, France) using the excitation wavelength of 532 nm. The diffraction grating was 1800 lines/mm. The spectra were recorded in several points with the standard spot of about 1 µm.
Raman and FTIR spectra have been normalized and then deconvoluted using Fityk software (0.9.8 software, open-source (GPL2+)). The coefficient of determination (R square) of all the deconvoluted FTIR and Raman spectra was 0.99. The standard deviation of the position and full width at half maximum (FWHM) of each of the component bands was ± 4 cm −1 .
The mid-infrared spectra were obtained with the Acton Spectra Pro 2300i monochromator (Princeton Instruments, Trenton, NJ, USA) in the spectral range of 2550-2850 nm using high power laser diode (λ exc = 976 nm) as a pump source and PbS detector supported by lock-in-amplifier SR510 (Stanford Research Systems, Sunnyvale, CA, USA). The NIR luminescence was performed using the Acton Spectra Pro 2300i monochromator with InGaAs detector in the range of 1.4-1.7 µm. Both measurements were carried in the transmission mode, where the laser beam was focused on the surface of the glass sample and the luminescence signal was collected on the entrance of monochromator. The upconversion luminescence spectra of the glasses and glass-ceramic were measured in a range of 500-700 nm using the Stelarnet GreenWave monochromator (Stellarnet Inc., Tampa, FL, USA) and the laser diode (λ exc = 976 nm). In this part of the experiment the luminescence signal was collected by transmitting optical fiber with NA = 0.5 and 400 µm diameter. The fiber end was located perpendicular to the excitation laser beam. All the measurements were carried out at room temperature. In order to eliminate the strong laser radiation, spectral filters FEL 1250 (Thorlabs, NJ, USA) for NIR and MIR spectra measurements and "heat glass" in the case of UC luminescence were used.

Studies of Erbium-Doped Oxyfluoride Phospho-Tellurite Glass
3.1.1. X-ray Diffraction Analysis Figure 1 presents X-ray diffraction patterns of the erbium-doped phospho-tellurite glass with the varying content of ErF 3 (from 0.25 to 1.25 mol%). The observed diffraction patterns showed the amorphous character of all the samples. The diffractograms confirmed the absence of any crystalline phase, only broad and intense broad humps in the 20-35 • range of the two theta angles are observed. X-ray diffraction analysis of the studied glasses shows that all oxyfluoride phospho-tellurite glasses with different contents of ErF 3 exhibits similar patterns and the mentioned hump does not become broader with the increasing erbium fluoride content. It suggests the absence of the evolution to a lower degree of the order of the local structure [35].

Thermal Properties of Oxyfluoride Erbium-Doped Phospho-Tellurite Glass
Thermal stability factor of the glass ΔT, defined as a resistance to crystallization during heating, is an important parameter to consider in the manufacture and technological applications of the glass. Thermal stability factor is measured as the difference between the crystallization Tc and the glass transition Tg temperatures. If the value of ΔT is higher than 100 °C, it is assumed that the glass can be considered as thermally stable [36].
Tellurite glasses combine both useful technological properties such as low melting temperatures and good thermal stability [36,37]. The phospho-tellurite glasses formed chemically stable glasses over wide compositional ranges [38][39][40][41][42]. According to the results of Rinke et al., the addition of TeO2 to sodium phosphate glass caused the increase in the glass transition temperature from 284 to 327 °C [43]. The increase in the glass transition temperature with the addition of P2O5 into the chemical composition of tellurite glass was also observed by Nandi and Jose [44]. Phosphorus pentoxide (P2O5) also plays a role as the stabilizing component, influencing the structure and the crystallization behavior in fluoride systems [45][46][47]. The influence of P2O5 on the thermal properties of fluoroindate glasses activated by Pr 3+ ions was reported by Pisarska et al. [47]. In this work, authors obtained thermally stable and transparent non-crystalline oxyfluoride glasses with high ΔT factors for low 4-12 mol% of P2O5 concentration. The stability parameter ΔT of the mentioned fluoroindate glasses increased from 114 to 155 °C with an increase in P2O5 concentration [47].
The effect of erbium doping concentration on the thermal properties of tellurite glasses was studied in the literature. These data relate mainly to oxide glasses and report that the increasing amount of erbium ions increases the characteristic temperatures and thermal stability of tellurite

Thermal Properties of Oxyfluoride Erbium-Doped Phospho-Tellurite Glass
Thermal stability factor of the glass ∆T, defined as a resistance to crystallization during heating, is an important parameter to consider in the manufacture and technological applications of the glass. Thermal stability factor is measured as the difference between the crystallization T c and the glass transition T g temperatures. If the value of ∆T is higher than 100 • C, it is assumed that the glass can be considered as thermally stable [36].
Tellurite glasses combine both useful technological properties such as low melting temperatures and good thermal stability [36,37]. The phospho-tellurite glasses formed chemically stable glasses over wide compositional ranges [38][39][40][41][42]. According to the results of Rinke et al., the addition of TeO 2 to sodium phosphate glass caused the increase in the glass transition temperature from 284 to 327 • C [43]. The increase in the glass transition temperature with the addition of P 2 O 5 into the chemical composition of tellurite glass was also observed by Nandi and Jose [44]. Phosphorus pentoxide (P 2 O 5 ) also plays a role as the stabilizing component, influencing the structure and the crystallization behavior in fluoride systems [45][46][47]. The influence of P 2 O 5 on the thermal properties of fluoroindate glasses activated by Pr 3+ ions was reported by Pisarska et al. [47]. In this work, authors obtained thermally stable and transparent non-crystalline oxyfluoride glasses with high ∆T factors for low 4-12 mol% of P 2 O 5 concentration. The stability parameter ∆T of the mentioned fluoroindate glasses increased from 114 to 155 • C with an increase in P 2 O 5 concentration [47].
The effect of erbium doping concentration on the thermal properties of tellurite glasses was studied in the literature. These data relate mainly to oxide glasses and report that the increasing amount of erbium ions increases the characteristic temperatures and thermal stability of tellurite glasses [48][49][50].
The transition temperature of the TZNE (TeO 2 -ZnO-Na 2 O-Er 2 O 3 ) glass increased by 13 • C with the increasing Er 2 O 3 content from 0.5 to 2.5 mol% [48]. According to the literature [49], doping the TZN (TeO 2 -ZnO-Na 2 O) glass up to 2 mol% of Er 2 O 3 showed the increase of the transition temperature T g and the thermal stability ∆T. For the above-mentioned concentration of erbium ions, the authors noted a decrease in the thermal stability of the analyzed glass [49]. However, the introduction of Er 2 O 3 from 0.5 to 2.8 mol% resulted in increase of the T g value and the thermal stability of the TeO 2 -Li 2 O-ZnO-Nb 2 O 5 -Er 2 O 3 glass (TLZNE) [50].
In order to analyze the effect of ErF 3 concentration on the thermal properties of the precursor glass (TP glass), DSC curves of oxyfluoride erbium-doped phospho-tellurite glass samples were measured and shown in Figure 2. Based on obtained DSC curves the characteristic temperatures such as the glass transition T g (onset), the crystallization T c (in the maximum), and the melting T m temperatures were obtained. Based on these parameters the stability factor ∆T was calculated. The results of thermal parameters for different ErF 3 concentrations into the chemical composition of the TP glass are presented in Table 1. It can be observed that the glass transition temperature T g increases from 313 ± 1 to 329 ± 1 • C with the increase of ErF 3 concentration into the chemical composition of precursor glass. The same trend was observed in the literature [51]. Jha et al. related the increase of transition temperature T g by the increase of ErF 3 to the strong bonding of erbium ions with non-bringing oxygens, which led to the increase of rigidity of the glass network [51].    All DSC curves showed the existence of exothermic peak/peaks in the maximum in the 430-485 • C range ( Figure 2, Table 1), which indicates that partial crystallization has taken place in the oxyfluoride erbium-doped phospho-tellurite glasses. In the DCS curve of glass doping with 0.25 mol% of ErF 3 (TP0.25ErF 3 glass) , the maximum of crystallization peak is located at around 438 ± 1 • C. With the increase of the erbium fluoride concentration to 0.5 mol%, the position of mentioned peak shifted to a higher temperature (458 ± 1 • C) and additionally, the second crystallization peak appeared (at 415 ± 1 • C) -glass TP0.50ErF 3 . The exothermic peak in the DSC curve of TP0.75ErF 3 glass split into two peaks, at 451 ± 1 and 483 ± 1 • C. A wide exothermic peak is visible in the DSC thermal curve of TP1.00ErF 3 glass. Related to the DSC curve of TP0.75ErF 3 glass, it can be deduced that two exothermic peaks overlapped, therefore the crystallization temperature of TP1.00ErF 3 glass has been determined to the maximum of exothermic effect and is 466 ± 1 • C. For 1.25 mol% of ErF 3 glass, two crystallization peaks were observed in the maximum at 420 ± 1 and 456 ± 1 • C, respectively. It is worth noting that the crystallization peak in the DSC curves of TP0.75ErF 3 , TP1.00ErF 3 , and TP1.25ErF 3 glasses has a greater value of full width at half maximum (FWHM) compared to the FWHM of the exothermic peak at 458 ± 1 • C (TP0.50ErF 3 ) and at 438 ± 1 • C (TP0.25ErF 3 ). The melting temperature T m seems to be not affected by the increase of ErF 3 ( Figure 2 and Table 1).
On the basis of Figure 3 it could be concluded that the increase in ErF 3 does not result in the incorporation of erbium ions in the glass network, because all FTIR spectra are very similar. Most likely, erbium ions act as the network modifier [51]. In order to better understand the influence of erbium trifluoride addition on the oxyfluoride phospho-tellurite glass structure, the deconvolution of the FTIR spectra of TP0.25ErF 3 , TP0.75ErF 3 , and TP1.25ErF 3 samples were carried out. Figures 4-6 present a deconvolution of the FTIR spectra of samples with the selected value of ErF 3 . Figure 7 presents a plot of bands position and their integral intensity (area under the curve in %) for deconvoluted TP0.25ErF 3 , TP0.75ErF 3 , and TP1.25ErF 3 spectra. The parameters of deconvoluted spectra and bands assignment to the appropriate vibrations are shown in Tables 2 and 3, respectively [68][69][70][71][72][73][74][75][76][77].
Bands A at~545 cm −1 and B at~566 cm −1 correspond to the deformation vibration of δO-P-O and δO-P-O bonds in Q 2 units [63]. The bands C and D at~606 and~680 cm −1 are due to the stretching vibrations of Te-O bonds in trigonal bipyramidal units TeO 4 (tbp) [69,70]. B and E at~740 cm −1 relates to the stretching vibrations of trigonal pyramidal units TeO 3 (tp) or TeO 3+1 polyhedra [70]. B and F at 780 cm −1 is assigned to the vibration of the continuous network composed of TeO 4 and Te-O stretching vibration of TeO 3+1 polyhedra [69] or symmetric P-O=P bonds in Q 1 units [71]. B and G at~800 cm −1 is ascribed to the asymmetric stretching vibrations of TeO 3 (tp) units or TeO 3+1 polyhedra [69].
In the FTIR spectra of phosphate glasses, there are bands associated with Q 3 -Q 0 groups (where Q is the number of bridging bonds) [72][73][74][75][76][77]. B and H at~870 cm −1 relates to the asymmetric stretching vibrations of Q 1 units [72,73]. B and I at~920 cm −1 is ascribed to the stretching vibrations of P-O-P linked with metaphosphate chain and P-F groups in Q 2 units [74,75]. The bands at~970, 1020,~1077/1094/1102 and~1135/1158/1178 cm −1 are due to the asymmetric stretching vibration of P-O − bonds in Q 0 units [68], the symmetric stretching vibrations of PO 3 groups in Q 1 units [74], the asymmetric stretching vibrations of PO 2− 3 groups in Q 1 units [76], and the asymmetric stretching vibrations of non-bridging oxygen in Q 2 units [77], respectively.
As can be seen in Figures 4-7, Tables 2 and 3, the sum of the bands intensity at~1020 and 1070-1110 cm −1 (both related to Q 1 units) increased from 27% to 41%, while the integral intensity of the bands at~970 cm −1 (related to Q 0 units) and at 1130-1180 cm −1 (related to Q 2 units) decreased (from 10% to 4% and 15% to 4%, respectively) with increasing ErF 3 concentration. This clearly indicates that the addition of ErF 3 results in an increase of Q 1 units at the expense of Q 0 and Q 2 units, according to the following equation: Q 2 + Q 0 ↔ 2Q 1 [78]. The P-(O, F)-P bonds are broken and the Er-(O, F)-P (NBO, non-bridging oxygen) bonds are formed as the number of erbium ions increased, which means that the phosphate chain is becoming shorter [79,80].  fluoride for oxide ions leading to the increase in the bond strength. This confirms the formation of F-P-F bonds [23].
As reported in the literature [87], the band at around 870-880 cm −1 is due to symmetric stretching vibration of the P-F bonds. As can be seen in   Tables 4 and 5, the integral intensity of this band increased with the addition of erbium trifluoride. This may be due to the replacement of fluorine for oxygen ions leading to the increase in bond strength [88].
All Raman spectra of glasses modified with various ErF 3 content do not have bands above 1300 cm −1 (Figure 9), the presence of which is attributed to the Q 3 units. This means that there are no Q 3 units in the structure of the analyzed glasses [89].
In summary, it can be concluded that with the increase in the content of Er 3+ and F − ions in the structure of the glasses, the integral intensity of the bands associated with Q 0 (band at~950 cm −1 ) and Q 2 (band at~1090-1150 cm −1 ) units decreased at the expense of an increase in the integral intensity of the band associated with Q 1 units (band at~1020-1040 cm −1 )- Figures 8-12, Tables 4 and 5. This confirms the conclusions from the interpretation of the FTIR spectra. Fluoride ions are embedded in the glass structure (F − replace O − anions), while erbium ions depolymerized the phosphate network and phosphate chain is becoming shorter [23,25,89,90].            In general, the luminescent properties of RE ions in the inorganic glasses depend on the chemical composition of the host glass, the activator concentration (lanthanide ions as optically active dopants), and the excitation power (pumping system). These factors and in particular the oxyfluoride environment (phonon energy) of the active ion significantly affect the emission efficiency of excited states of Er 3+ ions in the glasses [91][92][93][94]. It is also discussed in our investigated system glass and GC TP system. MIR (mid-infrared) emission spectra of oxyfluoride Er 3+ -doped phospho-tellurite glasses in the 2400-2900 nm range recorded under 976 nm LD excitation at room temperature, is shown in Figure 13. The emission peak at 2725 nm corresponding to 4 I 11/2 → 4 I 13/2 transition is observed. The changes in the intensity of this peak are characterized for all concentrations of erbium ions in oxyfluoride phosphate-tellurite glass (inset of Figure 13). The 2725 nm emission intensity increased with the increase of ErF 3 concentration (Figure 13). A similar effect was observed in tellurite glass [95]. The luminescence spectra of Er 3+ -doped phospho-tellurite glass recorded at room temperature in the wavelength of 1400-1700 nm under the excitation of 976 nm, is shown in Figure 14. The NIR intensity of emission line at 1532 nm occurred by 4 I13/2 → 4 I15/2 transition increased with the increase of ErF3 concentration up to 1.25 mol% in precursor glass-the inset of Figure 14.  The luminescence spectra of Er 3+ -doped phospho-tellurite glass recorded at room temperature in the wavelength of 1400-1700 nm under the excitation of 976 nm, is shown in Figure 14. The NIR intensity of emission line at 1532 nm occurred by 4 I 13/2 → 4 I 15/2 transition increased with the increase of ErF 3 concentration up to 1.25 mol% in precursor glass-the inset of Figure 14. The luminescence spectra of Er 3+ -doped phospho-tellurite glass recorded at room temperature in the wavelength of 1400-1700 nm under the excitation of 976 nm, is shown in Figure 14. The NIR intensity of emission line at 1532 nm occurred by 4 I13/2 → 4 I15/2 transition increased with the increase of ErF3 concentration up to 1.25 mol% in precursor glass-the inset of Figure 14.   Figure 15 shows the emission spectra of erbium-doped phospho-tellurite glasses obtained in the visible regions under excitation at 976 nm. The observed emission bands are attributed to 2 H 11/2 → 4 I 15/2 (528 nm), 4 S 3/2 → 4 I 15/2 (551 nm), and weak emission band at 668 nm to 4 F 9/2 → 4 I 15/2 transitions of Er 3+ , respectively. As can be seen in Figure 15, glasses have a strong green emission at 528 and 551 nm and a weak red emission at 668 nm. The green upconversion emission corresponding to 2 H 11/2 , 4 S 3/2 → 4 I 15/2 transition is dominant. This phenomenon can be explained as follows: Two dominant mechanisms are involved in the upconversion process: (1) the excited state absorption (ESA) and (2) energy transfer upconversion (ETU) (Figure 16). In the case of low doping level (0.25 mol% of ErF 3 in the precursor glass), erbium ions are scarcely distributed in the glassy phase. The 2 H 11/2 and 4 S 3/2 levels, corresponding to the green emission at 525 and 545 nm, are populated by the multiphonon relaxation from the 4 F 7/2 level, thus the 525 nm emission intensity is reduced and stronger emission signals at 545 nm than that at 525 nm were obtained. For high Er 3+ concentration (more than 0.5 mol% of the ErF 3 ) is greatly promoting the ETU1 process of excited Er 3+ ( 4 I 11/2 ) from one to another and ultimately excited to 4 F 7/2 level. Similar behavior was observed in oxyfluoride glass-ceramics containing LaF 3 nanocrystals [96].
The inset of Figure 15 shows the relation between the UC luminescence intensity I UP as a function of the pumping power I IR . It is well known that the relation expressed as I UP ∝ I m IR , where m determines the number of photons of the optical pump used in the conversion of excitation process which occurs in a given structure of the energy levels of RE elements [97].
The mechanism of the upconversion process was determined from the log-log dependence of the emission intensity on the excitation power and presented for the precursor glass doped with the highest content of erbium trifluoride ( Figure 17). The slopes of 1.32 and 1.24 for green and red transitions of erbium ions indicates that the 2-photon mechanism was involved in the upconversion luminescence process ( Figure 17) [96,98].  Figure 15 shows the emission spectra of erbium-doped phospho-tellurite glasses obtained in the visible regions under excitation at 976 nm. The observed emission bands are attributed to 2 H11/2 → 4 I15/2 (528 nm), 4 S3/2 → 4 I15/2 (551 nm), and weak emission band at 668 nm to 4 F9/2 → 4 I15/2 transitions of Er 3+ , respectively. As can be seen in Figure 15, glasses have a strong green emission at 528 and 551 nm and a weak red emission at 668 nm. The green upconversion emission corresponding to 2 H11/2, 4 S3/2 → 4 I15/2 transition is dominant. This phenomenon can be explained as follows: Two dominant mechanisms are involved in the upconversion process: (1) the excited state absorption (ESA) and (2) energy transfer upconversion (ETU) (Figure 16). In the case of low doping level (0.25 mol% of ErF3 in the precursor glass), erbium ions are scarcely distributed in the glassy phase. The 2 H11/2 and 4 S3/2 levels, corresponding to the green emission at 525 and 545 nm, are populated by the multiphonon relaxation from the 4 F7/2 level, thus the 525 nm emission intensity is reduced and stronger emission signals at 545 nm than that at 525 nm were obtained. For high Er 3+ concentration (more than 0.5 mol% of the ErF3) is greatly promoting the ETU1 process of excited Er 3+ ( 4 I11/2) from one to another and ultimately excited to 4 F7/2 level. Similar behavior was observed in oxyfluoride glass-ceramics containing LaF3 nanocrystals [96].
The inset of Figure 15 shows the relation between the UC luminescence intensity IUP as a function of the pumping power IIR. It is well known that the relation expressed as I ∝ I , where m determines the number of photons of the optical pump used in the conversion of excitation process which occurs in a given structure of the energy levels of RE elements [97].
The mechanism of the upconversion process was determined from the log-log dependence of the emission intensity on the excitation power and presented for the precursor glass doped with the highest content of erbium trifluoride ( Figure 17). The slopes of 1.32 and 1.24 for green and red transitions of erbium ions indicates that the 2-photon mechanism was involved in the upconversion luminescence process (Figure 17) [96,98].

X-ray Diffraction Analysis of Transparent Glass-Ceramic
The TP1.25ErF3 glass was heat-treated to induce crystallization and form glass-ceramic. To get a transparent glass-ceramic, the heat treatment temperature was selected to be 400 °C. The sample of TP1.25ErF3 glass was heat-treated at 400 °C for 3 h. The X-ray diffraction pattern of TP1.25ErF3 glass heat-treated at 400 °C is presented in Figure 18. As shown in Figure 1, the precursor phospho-tellurite glass doped with 1.25 mol% of ErF3 is amorphous with no diffraction peaks. After the heat-treated process the diffraction peaks are clearly observed, next to the broad and intense broad hump in the

X-ray Diffraction Analysis of Transparent Glass-Ceramic
The TP1.25ErF3 glass was heat-treated to induce crystallization and form glass-ceramic. To get a transparent glass-ceramic, the heat treatment temperature was selected to be 400 °C. The sample of TP1.25ErF3 glass was heat-treated at 400 °C for 3 h. The X-ray diffraction pattern of TP1.25ErF3 glass heat-treated at 400 °C is presented in Figure 18. As shown in Figure 1, the precursor phospho-tellurite glass doped with 1.25 mol% of ErF3 is amorphous with no diffraction peaks. After the heat-treated process the diffraction peaks are clearly observed, next to the broad and intense broad hump in the

X-ray Diffraction Analysis of Transparent Glass-Ceramic
The TP1.25ErF 3 glass was heat-treated to induce crystallization and form glass-ceramic. To get a transparent glass-ceramic, the heat treatment temperature was selected to be 400 • C. The sample of TP1.25ErF 3 glass was heat-treated at 400 • C for 3 h. The X-ray diffraction pattern of TP1.25ErF 3 glass heat-treated at 400 • C is presented in Figure 18. As shown in Figure 1, the precursor phospho-tellurite glass doped with 1.25 mol% of ErF 3 is amorphous with no diffraction peaks. After the heat-treated process the diffraction peaks are clearly observed, next to the broad and intense broad hump in the 20-35 • range of two theta angle ( Figure 18). These peaks are assigned in 100% to barium difluoride (BaF 2 ) cubic phase (JCPDF: 00-001-0533). The size of precipitated crystals in the obtained glass-ceramic was calculated on the basis of Scherrer's formula and evaluated to be about 16 nm [99]. The value of the calculated lattice parameter of BaF 2 crystals is 0.5899 ± 0.0002 nm and was smaller than the JCPDF value of 0.6187 nm. The similar shrinkage of barium difluoride crystals lattice was observed in the SiO 2 -ZnF 2 -BaF 2 -ErF 3 -YbF 3 glass [99] and can be ascribed to the entrance of erbium ions into BaF 2 nanocrystals because the radius of erbium ions is smaller than that of barium ions [100][101][102].
20-35° range of two theta angle (Figure 18). These peaks are assigned in 100% to barium difluoride (BaF2) cubic phase (JCPDF: 00-001-0533). The size of precipitated crystals in the obtained glassceramic was calculated on the basis of Scherrer`s formula and evaluated to be about 16 nm [99]. The value of the calculated lattice parameter of BaF2 crystals is 0.5899 ± 0.0002 nm and was smaller than the JCPDF value of 0.6187 nm. The similar shrinkage of barium difluoride crystals lattice was observed in the SiO2-ZnF2-BaF2-ErF3-YbF3 glass [99] and can be ascribed to the entrance of erbium ions into BaF2 nanocrystals because the radius of erbium ions is smaller than that of barium ions [100][101][102].

Spectroscopic Properties of the Transparent Glass-Ceramic
Rare-earth doped oxyfluoride glass-ceramics have research interest for attracted combined advantages of low phonon energy of fluoride and very good mechanical and chemical properties of the oxide matrix [103,104]. In the literature many investigations can be found, which have been done on the rare earth ions doped glass-ceramic containing lead or cadmium fluoride nanocrystals [105][106][107][108]. However, cadmium fluoride CdF2 and lead fluoride PbF2 are poisonous raw materials, which limited their application. This is reason that the rare earth doped transparent glass-ceramic containing nanocrystals without the toxic ingredients such as Pb or Cd is still evolving [109][110][111][112][113]. Much effort has been devoted to the search for novel transparent glass-ceramic. The glass-ceramic with LaF3 nanocrystals was reported for the first time by Dejneka as non-toxic glass-ceramic [114]. Next, researchers studied systemically the preparation of the rare-earth-doped glass-ceramic containing lanthanide trifluoride nanocrystals [115][116][117][118][119][120][121][122][123][124].
The optical properties of BaF2 make it a universal optical material. Optical properties of BaF2 are related to the structural and electronic properties of barium difluoride such as very large bandgap (11 eV), the low phonon energy (319 cm −1 ) and the large optical transparency from ultraviolet (UV) to far-infrared (FIR) from 0.2 to 14.0 µm [132,133].

Spectroscopic Properties of the Transparent Glass-Ceramic
Rare-earth doped oxyfluoride glass-ceramics have research interest for attracted combined advantages of low phonon energy of fluoride and very good mechanical and chemical properties of the oxide matrix [103,104]. In the literature many investigations can be found, which have been done on the rare earth ions doped glass-ceramic containing lead or cadmium fluoride nanocrystals [105][106][107][108]. However, cadmium fluoride CdF 2 and lead fluoride PbF 2 are poisonous raw materials, which limited their application. This is reason that the rare earth doped transparent glass-ceramic containing nanocrystals without the toxic ingredients such as Pb or Cd is still evolving [109][110][111][112][113]. Much effort has been devoted to the search for novel transparent glass-ceramic. The glass-ceramic with LaF 3 nanocrystals was reported for the first time by Dejneka as non-toxic glass-ceramic [114]. Next, researchers studied systemically the preparation of the rare-earth-doped glass-ceramic containing lanthanide trifluoride nanocrystals [115][116][117][118][119][120][121][122][123][124].
The optical properties of BaF 2 make it a universal optical material. Optical properties of BaF 2 are related to the structural and electronic properties of barium difluoride such as very large bandgap (11 eV), the low phonon energy (319 cm −1 ) and the large optical transparency from ultraviolet (UV) to far-infrared (FIR) from 0.2 to 14.0 µm [132,133].
In the research, it may be found that rare-earth ions like Er 3+ are the luminescence centers in BaF 2 nanocrystals in the glass-ceramic [101,[134][135][136]. Chen et al. obtained transparent glass-ceramics containing BaF 2 nanocrystals doped with Er 3+ , prepared by the sol-gel route. The authors recorded the efficient upconversion emissions around 545, 565, and 655 nm under 980 nm excitation due to the lower phonon energy environment of Er 3+ ions in glass-ceramic [85]. Transparent glass-ceramic with erbium-doped BaF 2 nanocrystals in the fluoroborate system was synthesized for Shinozaki et al. [134]. In this paper, the authors presented that crystallization enhanced the luminescence intensity 30 times compared to the precursor glass [134]. The enhanced upconversion luminescence of the Er 3+ ions in transparent oxyfluoride GCs containing BaF 2 nanocrystals in the SiO 2 -ZnF 2 -BaF 2 -ErF 3 system has been also investigated by Qiao et al. [135]. The erbium-doped germano-gallate oxyfluoride glass-ceramics containing BaF 2 nanocrystals with the high transmittance in the mid-infrared region were prepared by Zhao et al. [136]. Unfortunately, to the best of our knowledge, no reports of the transparent Er 3+ -doped BaF 2 glass-ceramic in the oxyfluoride phospho-tellurite exist in the literature. An issue which this report is intended to address. Figure 19 shows the MIR emission spectra of the TP1.25ErF 3 glass and glass-ceramic under 980 nm LD excitation recorded at room temperature. Enhanced emission at a wavelength of 2725 nm was observed in glass-ceramic. The influence of the heat-treatment process on the intensity of MIR luminescence shows that for 3 h of the annealing of the TP1.25ErF 3 glass sample, the 30% enhancement was observed for the glass-ceramic ( Figure 19). In the research, it may be found that rare-earth ions like Er 3+ are the luminescence centers in BaF2 nanocrystals in the glass-ceramic [101,[134][135][136]. Chen et al. obtained transparent glass-ceramics containing BaF2 nanocrystals doped with Er 3+ , prepared by the sol-gel route. The authors recorded the efficient upconversion emissions around 545, 565, and 655 nm under 980 nm excitation due to the lower phonon energy environment of Er 3+ ions in glass-ceramic [85]. Transparent glass-ceramic with erbium-doped BaF2 nanocrystals in the fluoroborate system was synthesized for Shinozaki et al. [134]. In this paper, the authors presented that crystallization enhanced the luminescence intensity 30 times compared to the precursor glass [134]. The enhanced upconversion luminescence of the Er 3+ ions in transparent oxyfluoride GCs containing BaF2 nanocrystals in the SiO2-ZnF2-BaF2-ErF3 system has been also investigated by Qiao et al. [135]. The erbium-doped germano-gallate oxyfluoride glass-ceramics containing BaF2 nanocrystals with the high transmittance in the mid-infrared region were prepared by Zhao et al. [136]. Unfortunately, to the best of our knowledge, no reports of the transparent Er 3+doped BaF2 glass-ceramic in the oxyfluoride phospho-tellurite exist in the literature. An issue which this report is intended to address. Figure 19 shows the MIR emission spectra of the TP1.25ErF3 glass and glass-ceramic under 980 nm LD excitation recorded at room temperature. Enhanced emission at a wavelength of 2725 nm was observed in glass-ceramic. The influence of the heat-treatment process on the intensity of MIR luminescence shows that for 3 h of the annealing of the TP1.25ErF3 glass sample, the 30% enhancement was observed for the glass-ceramic ( Figure 19). The higher NIR luminescence intensity exhibited (22% enhancement) when the TP1.25ErF3 glass sample was annealed for 3 h (Figure 20). In the case of visible emission of Er 3+ ions, the glass-ceramic was characterized by the higher intensity at 551, 528, and 668 nm ( Figure 21) in comparison to the TP1.25ErF3 glass ( Figure 16). It is worth noting that the shape of UC luminescence in the green bands is narrower for the GC sample than the parent glass. Thus, it is confirmed that the structural changes in the vicinity of erbium ions. In the case of visible emission of Er 3+ ions, the glass-ceramic was characterized by the higher intensity at 551, 528, and 668 nm ( Figure 21) in comparison to the TP1.25ErF 3 glass ( Figure 16). It is worth noting that the shape of UC luminescence in the green bands is narrower for the GC sample than the parent glass. Thus, it is confirmed that the structural changes in the vicinity of erbium ions. In the case of visible emission of Er 3+ ions, the glass-ceramic was characterized by the higher intensity at 551, 528, and 668 nm ( Figure 21) in comparison to the TP1.25ErF3 glass ( Figure 16). It is worth noting that the shape of UC luminescence in the green bands is narrower for the GC sample than the parent glass. Thus, it is confirmed that the structural changes in the vicinity of erbium ions. The intensity of MIR, NIR, and upconversion emission spectra of glass-ceramic were stronger compared to the intensity emission spectra of the TP1.25ErF 3 glass sample. This is most likely due to incorporated erbium ions in the crystalline environment of BaF 2 nanocrystals [129][130][131].

Discussion
Rare-earth doped oxyfluoride glass-ceramics have been extensively investigated due to potential applications, i.e., in solid laser and fiber amplifiers. Oxyfluoride glass-ceramics are more appropriate for practical applications compared to oxide and fluoride glasses, because the glass-ceramics have the lower phonon energy than the oxide glasses and excellent chemical durability and mechanical strength compared to the fluoride glasses.
Considering the above, authors of this paper first investigated precursor oxyfluoride phospho-tellurite glass in the TeO 2 -P 2 O 5 -BaF 2 -ZnF 2 -Na 2 O system. Addition of the second glass former (P 2 O 5 ) was aimed at improving the thermal stability and the tensile strength of tellurite glass. Next, oxyfluoride phospho-tellurite precursor glass was doped with erbium trifluoride (ErF 3 ). In this system, thermal and chemical stable glass with 0.25, 0.50, 0.75. 1.00, and 1.25 mol% of ErF 3 was obtained. Above 1.25 mol% of ErF 3 (for 1.5 mol% of ErF 3 ), we obtained non-transparent material. This showed that the agglomeration of Er 3+ ions has occurred.
The thermal, structural, and optical studies were performed for precursor TeO 2 -P 2 O 5 -BaF 2 -ZnF 2 -Na 2 O glass with varying ErF 3 concentrations (up to 1.25 mol%). The thermal characteristic of erbium-doped oxyfluoride phospho-tellurite glass provided the information of characteristic temperatures. The glass transition temperature T g increased from 313 to 329 • C with the increase of ErF 3 concentration into the chemical composition of precursor glass. This increase in the transition temperature indicates a more strongly bound network in the glass doped with ErF 3 . A stronger ionic cross-linking between modifier cations (Er 3+ ) and non-bridging oxygens (NBO) has taken place, which led to the increase of rigidity of the glass network. The existence of clear exothermic peak/peaks in the 430-485 • C range indicated that crystallization occurred in the erbium-doped oxyfluoride phospho-tellurite glasses. The crystalline phase, obtained after 1 h annealing in the maximum of each crystallization peak was only cubic barium difluoride (BaF 2 ). These data were used to synthesis and characterization of transparent glass-ceramics with barium difluoride nanocrystals. Moreover, the oxyfluoride phospho-tellurite glass with various concentration of erbium fluoride can be considered as a good thermal stability. These glasses should be suitable for optical fiber drawing.
FTIR and Raman spectra analysis confirmed the presence of tellurite and phosphate units in the precursor glass doped with ErF 3 . The replacement of fluoride for oxide leading to an increase in the bond strength confirms the formation of F-P-F bonds when TeO 2 is replaced by ErF 3. Comparison of deconvoluted spectra of TP0.25ErF 3 , TP0.75ErF 3 , and TP1.25ErF 3 glasses indicated that the erbium ions were modifier ions and depolymerized phosphate network. It was found, that the addition of ErF 3 resulted in the increase of Q 1 phosphate units at the expense of Q 0 and Q 2 phosphate units. The P-(O, F)-P bonds were broken and the Er-(O, F)-P (NBO, non-bridging oxygen) bonds were formed as the amount of ErF 3 increased up to 1.25 mol%. The phosphate chains were becoming shorter. A possible mechanism that explains the structural modifications by Er 3+ in the erbium-doped oxyfluoride phospho-tellurite glass can be related to the electronegativity of the structural units in a network of glass. According to Rada and Culea [137] the presence of multiple oxides/fluorides in the glass increases the tendency of network formers to attract oxygen/fluoride ions. This is due to the competition between the cations themselves and this preference. The units, which have higher electronegativity value pick up oxygen or fluoride ions and get modifiers. For erbium trifluoride, the erbium ions are firstly inserted in the trivalent state and they are considered as the modifiers ions due to the strong affinity of Er 3+ toward groups containing negative charged non-bridging oxygens. Both tellurium and phosphorus cations in the glass matrix attract fluoride anions, which yield a competition between them. Since phosphorus pentoxide has a higher electronegativity value than tellurium dioxide, P 2 O 5 picked up fluoride ion. Hereby, for 1.25 mol% of ErF 3 , erbium ions participated in the glass network as the modifier ions for Q 2 phosphate units [137].
The spectroscopic study showed, that the emission peak at around 2725 nm corresponding to 4 I 11/2 → 4 I 13/2 transition was observed in the mid-infrared emission spectra of all doped glasses. The near-infrared emission line at 1533 nm occurred by 4 I 13/2 → 4 I 15/2 transition. In the emission spectra in the visible regions of erbium-doped phospho-tellurite glasses bands, attributed to 2 H 11/2 → 4 I 15/2 (528 nm), 4 S 3/2 → 4 I 15/2 (551 nm), and 4 F 9/2 → 4 I 15/2 (668 nm) transitions of Er 3+ were observed, respectively. The glasses have a strong green emission at 528 and 551 nm and a weak red emission at 668 nm. The green upconversion emission corresponding to 2 H 11/2 , 4 S 3/2 → 4 I 15/2 transition is dominant. The mechanism of the upconversion process was determined from the log-log dependence of emission intensity on the excitation power and indicated that the 2-photon mechanism was involved in the upconversion luminescence process. Finally, the intensity of mid-, near-infrared emission spectra and upconversion of erbium-doped oxyfluoride glass exhibits an increased tendency with the increment of Er 3+ concentration until 1.25 mol%. The intensity of the emission peaks has been enhanced with increasing the mol% ErF 3 in the precursor glass. It suggests that the observed spectra arisen due to the presence of the erbium trifluoride in the glass and proportional to its concentration.
By annealing of the precursor oxyfluoride glass doping with the highest value of ErF 3 (1.25 mol%) for 3 h in 400 • C we obtained oxyfluoride transparent phospho-tellurite glass-ceramic with Er 3+ -doped barium difluoride nanocrystals with the size of 16 nm. Compared to the literature [55,138,139], in our transparent glass-ceramic, single nanocrystals (100% of BaF 2 ) appeared after the heat treatment.
To check if the Er 3+ ions are incorporated into the BaF 2 nanocrystals, the mid-and near-infrared emission spectra of the glass-ceramics and upconversion were measured under excitation at 976 nm in room temperature. The glass-ceramic sample has a stronger 2725 nm emission intensity than the base glass. The 30% enhancement of the intensity band at 2725 nm was observed for glass-ceramic. The near-infrared emission of the glass-ceramic that corresponds to Er 3+ : 4 I 13/2 → 4 I 15/2 was 22% enhanced compared to the glass. Under 980 nm LD pumping, the green upconversion intensity of Er 3+ in the glass-ceramic was observed much stronger than that in the glass. The cubic site of Er 3+ and a low vibration frequency of its BaF 2 environment resulted in the enhancement of spectroscopic properties of the glass-ceramic compared to the oxyfluoride phospho-tellurite glass with 1.25 mol% of ErF 3 .
The data presented in this paper first report of the transparent Er 3+ -doped BaF 2 glass-ceramic in the oxyfluoride phospho-tellurite system. The following step will be the investigation of luminescence kinetic of obtained glass-ceramic. It might provide useful information for further development of this material in both RE optimization and composition modifications leading to photonics applications.

Conclusions
In summary, Er 3+ -doped oxyfluoride phospho-tellurite glasses were prepared in the 10P 2 O 5 -45(BaF 2 -ZnF 2 )-5Na 2 O-xErF 3 system by adding x = 0.25, 0.50, 0.75, 1.00, and 1.25 mol% of ErF 3 and characterized for their thermal, structure, and spectroscopic properties. An increase in the glass transition temperature of glasses with the addition of increase of ErF 3 into the chemical composition of precursor glass indicated a stronger ionic cross-linking between Er 3+ and non-bridging oxygens (NBO) atom. FTIR and Raman analysis confirmed that the addition of ErF 3 resulted in an increase of Q 1 at the expense of Q 0 and Q 2 phosphate units. The erbium ions were modifier ions and depolymerized phosphate network. The intensity of the mid-, near-infrared and upconversion emission peaks showed the increase with the increasing mol% of ErF 3 in the precursor glass. Transparent oxyfluoride phospho-tellurite glass-ceramics containing cubic BaF 2 nanocrystals were synthesized by heat treatment of the base glass containing 1.25 mol% ErF 3 . The X-ray diffraction (XRD) analysis revealed the formation of 16 nm barium difluoride nanocrystals in the oxyfluoride phospho-tellurite glass-ceramics. Preferential incorporation of erbium ions into the BaF 2 nanocrystals was confirmed by the infrared and upconversion emission spectra. The low vibration frequency of erbium-doped