Sol-Gel Glass-Ceramic Materials Containing CaF₂:Eu³⁺ Fluoride Nanocrystals for Reddish-Orange Photoluminescence Applications

Abstract

CaF₂:Eu³⁺ glass-ceramic sol-gel materials have been examined for reddish-orange photoluminescence applications. The transformation from precursor xerogels to glass-ceramic materials with dispersed fluoride nanocrystals was verified using several experimental methods: differential scanning calorimetry (DSC), thermogravimetric analysis (TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR-ATR), energy dispersive X-ray spectroscopy (EDS) and photoluminescence measurements. Based on luminescence spectra and their decays, the optical behavior of Eu³⁺ ions in fabricated glass-ceramics were characterized and compared to those of precursor xerogels. In particular, the determined luminescence lifetime of the ⁵D₀ excited state of Eu³⁺ ions in nanocrystalline CaF₂:Eu³⁺ glass-ceramic materials is significantly prolonged in comparison with prepared xerogels. The integrated intensities of emission bands associated to the ⁵D₀ → ⁷F₂ electric-dipole transition (ED) and the ⁵D₀ → ⁷F₁ magnetic-dipole transition (MD) are changed drastically during controlled ceramization process of xerogels. This implies the efficient migration of Eu³⁺ ions from amorphous silicate sol-gel network into low-phonon energy CaF₂ nanocrystals.

1. Introduction

Transparent glass-ceramics (GCs) containing fluoride nanocrystals fabricated using high-temperature melt-quenching or low-temperature sol-gel method are suitable modern materials for lasers, optical waveguides, solid-state lighting and numerous photonic applications [1,2,3,4,5]. Depending on chemical composition and technological conditions, nano- or micro-crystals are usually well-formed during temperature-controlled crystallization of precursor glasses or xerogels. The heat-treatment process often introduces transformation from amorphous systems to transparent oxyfluoride glass-ceramic materials and rare earths play the role as optically active ions. The coordination sphere around rare earth ions changes drastically during this structural transformation, giving important contribution to the luminescence characteristics.

Among binary and ternary rare-earth fluoride nanophosphors [6], calcium fluoride CaF₂ belongs to the most important and perspective nanoparticles, which could be successfully formed during the heat treatment process. CaF₂ fluoride nanocrystals were well formed from Na₂O/K₂O/CaO/CaF₂/Al₂O₃/SiO₂ glass [7] and their mean crystallite sizes were in the range from 8 to 10.4 nm and did not get larger with time or increasing the annealing temperature. However, synthesis, structure and properties of transparent glass-ceramics containing CaF₂ nanocrystals depended critically on treating temperature and glass composition, where SiO₂ was substituted by CaO/CaF₂ [8]. A special attention has been paid to crystallization processes and spectroscopic properties of erbium-doped transparent glass ceramics containing CaF₂ nanocrystals [9,10,11]. In this CaF₂:Er³⁺ glass-ceramic system, near-IR luminescence at 1.53 µm [12,13] and up-conversion processes [14,15] of Er³⁺ ions have been examined in detail. The results for the crystallization processes and fluorescence properties of Nd³⁺-doped glass-ceramics with CaF₂ nanocrystals have been also presented and discussed [16,17]. Recently, several research groups have been focused on glass-ceramic materials containing CaF₂ nanocrystals for numerous photonic applications. Seo et al. [18] suggest that CaF₂:Dy³⁺ nano-glass-ceramics are promising for white light generation. Moreover, the oxyfluoride glass-ceramics containing CaF₂:Sm³⁺ crystals have become a new fast erasable dosimetric detector material for micro-beam radiation cancer therapy applications at the Canadian Synchrotron [19].

In this work, oxyfluoride silicate xerogels containing europium ions were heat-treated at 350 °C to obtain transparent glass-ceramics containing crystalline fluoride nano-phase CaF₂. Luminescence spectra of Eu³⁺ ions and their decays in transparent glass-ceramic samples have been studied and compared to precursor silicate xerogels. Our previously published work indicates that sol-gel glass-ceramics containing SrF₂:Eu³⁺ fluoride nanocrystals are attractive materials for reddish-orange luminescence applications [20].

2. Materials and Methods

The reagents used during proposed sol-gel synthesis were from the Aldrich Chemical Company and contained analytical purity. Deionized water was taken from an Elix 3 system (Millipore, Molsheim, France).

The Eu³⁺-doped xerogels with nominal composition (in molar ratio): TEOS:C₂H₅OH:H₂O:CH₃COOH = 1:4:10:0.5 (90 wt.%) CF₃COOH:Ca(CH₃COO)₂:Eu(CH₃COO)₃ = 5:1:0.05 (10 wt.%) were synthesized. In the presented procedure, the mixtures of tetraethoxysilane (TEOS), ethanol, water and acetic acid were put into round-bottom flasks and stirred for 30 min to perform the hydrolysis reaction. Simultaneously, Ca(CH₃COO)₂ and Eu(CH₃COO)₃ were dissolved in water and trifluoroacetic acid (TFA). In the next step, the obtained solutions were introduced into hydrolyzed tetraethoxysilane (TEOS) and mixed for another 60 min. After this time, the sols were dried at 35 °C for seven weeks to form transparent and colorless xerogels. To fabricate the glass-ceramics containing CaF₂ nanocrystals, the xerogels were heat-treated in a muffle furnace FCF 5 5SHP (Czylok, Jastrzębie-Zdrój, Poland) at 350 °C. The temperature was raised by 10 °C/min until the direct temperature (350 °C) was achieved and the xerogels were heat-treated for 10 h. Afterwards, resulted glass-ceramic materials were cooled down to room temperature in a closed furnace.

Fabricated samples were characterized by a SETARAM Labsys thermal analyzer (SETARAM Instrumentation, Caluire, France) using the thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) method. The DSC curves were acquired with heating rate of 10 °C/min and the curves were registered within temperature range from 40 °C to 500 °C. To verify the crystallization of the CaF₂ phase, the X-ray diffraction (XRD) analysis was carried out using an X’Pert Pro diffractometer supplied by PANalytical (Almelo, The Netherlands) with Cu Kα radiation. The TEM microscopy was also used and the CaF₂ nanocrystals were observed via a JEOL JEM 3010 electron transmission microscope (JEOL, Tokyo, Japan) operated at 300 kV. To determine the distribution of chemical elements in studied CaF₂:Eu³⁺ GCs, an energy dispersive X-ray spectroscopy (EDS) analysis was also performed using JEOL microscope. To examine the structural changes within silicate network during controlled heat-treatment, the infrared spectroscopy (IR-ATR) spectra were registered using the Nicolet iS50 ATR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in the frequency region 500 cm⁻¹–4000 cm⁻¹. The luminescence measurements were carried out using a Horiba Jobin-Yvon FluoroMax-4 spectrofluorimeter (Horiba Jobin Yvon, Longjumeau, France) equipped with 150 W xenon lamp. The spectral resolution was ±0.1 nm. Decay curves were detected with the accuracy of ±2 μs. All structural and photoluminescence measurements were performed at room temperature.

3. Results and Discussion

3.1. Structural and Thermal Characterization of Sol-Gel Materials

Firstly, the IR-ATR technique was used for identification the chemical bonding and functional groups inside prepared sol-gel materials and the results are presented in Figure 1. To verify the structural changes during transformation of xerogels into CaF₂:Eu³⁺ GCs, the infrared spectra were recorded in the 500 cm⁻¹–4000 cm⁻¹ frequency region. The assignment of recorded infrared signals was carried out based on current literature [21,22]. For xerogels obtained after seven weeks from synthesis, the creation of three-dimensional silicate network was confirmed. Indeed, the presence of infrared (IR) signals near 1193 cm⁻¹, 799 cm⁻¹ (Si-O-Si siloxane bridges), 1134 cm⁻¹, 1044 cm⁻¹ and 963 cm⁻¹ (Qⁿ units in SiO₄ tetrahedrons: Q⁴, Q³ and Q², respectively) indicates that polycondensation reaction occurred. In addition, we identified the presence of vicinal or geminal Si-OH groups (~3664 cm⁻¹), hydrogen-bonded Si-OH moieties (~3398 cm⁻¹) and hydrogen-bonded OH groups (~3233 cm⁻¹) originated from residual organic solvents and water. The IR signals located at 1664 cm⁻¹ (C = O groups vibrations) as well as 1447 cm⁻¹ (C-H vibrations) also confirmed that the porous sol-gel network was filled by liquids. It should be noted that the infrared peak at ~1664 cm⁻¹ frequency region was also assigned to adsorbed water and Si-OH groups. Moreover, the presence of un-decomposed calcium and europium (III) trifluoroacetates was also confirmed (1193 cm⁻¹, 1134 cm⁻¹).

To evaluate the thermal resistance of precursor xerogels and to identify the crystallization temperature, the TG/DSC analysis was performed in a temperature range from 35 °C up to 500 °C (inset of Figure 1). The thermal degradation profile with two distinguishable stages was recorded in temperature intervals: 35 °C–151 °C (1) and 151 °C–348 °C (2). For prepared xerogels, the first stage was recorded as a gentle degradation. An indicated step could be associated with desorption of ethanol and water as well as acetic acid and the accompanying weight-loss was estimated to be 5.82%. Beyond the residual acetic acid (used as a catalyst), such acid was also produced during the chemical reaction between TFA and acetate salts:

Ca(CH₃COO)₂ + 2CF₃COOH → Ca(CF₃COO)₂ + 2CH₃COOH

Eu(CH₃COO)₃ + 3CF₃COOH → Eu(CF₃COO)₃ + 3CH₃COOH

As was shown, CF₃COO⁻ anions could effectively coordinate RE³⁺ cations, therefore, the trifluoroacetate ions were chemically bonded. In fact, TFA was introduced as fluorination agent, which allowed for successful precipitation of CaF₂ crystal fraction during the heat-treatment of xerogels. Finally, the next TG step for prepared sol-gel samples could be associated with the thermal decomposition of Ca(CF₃COO)₂ and the formation of CaF₂ nanocrystals. The formation of the CaF₂ fluoride phase was realized by a homogeneous nucleation through a controlled fluorination, when the Ca-O bond cleaved and the Ca-F bond was formed [23]. The CaF₂ crystal phase was obtained during thermal decomposition of Ca(CF₃COO)₂ as follows:

Ca(CF₃COO)₂ → CaF₂ + CF₃CFO + CO₂ + CO

The decomposition of Ca(CF₃COO)₂ was observed in DSC curve as an exothermic peak with maximum at ~318 °C. Generally, such a second degradation step was observed in recorded TG curves as a relatively high weight-loss, which was estimated to 16.18%. Based on presented TG/DSC results, the T = 350 °C was chosen to perform the controlled ceramization process of precursor xerogels in order to precipitate the CaF₂ nanocrystals dispersed in silicate sol-gel host. Furthermore, the resultant sol-gel materials were almost completely thermally stable at the indicated temperature level.

As was proven by infrared measurements performed after controlled heat-treatment process, the significant changes in shape of recorded spectra were well-visible. The recorded infrared signals were identified as vibrations that originated from the silicate framework. It was observed that the broad OH-band significantly decreased in intensity, and indicated the evaporation of volatile components (disappearance of maximum located at 3233 cm⁻¹) as well as progressive polycondensation reaction of silicate network (reduction in intensity of maximum at 3390 cm⁻¹). The weak peak at ~1664 cm⁻¹ was also detected and it was associated to residual Si-OH groups. For heat-treated samples, the shoulders located at ~1193 cm⁻¹ and ~1134 cm⁻¹ were related to Si-O-Si siloxane bridges and SiO₄ tetrahedrons in Q⁴ units, respectively. Otherwise, the vibration modes originated from SiO₄ tetrahedrons inside Q³ (1051 cm⁻¹) and Q² units (951 cm⁻¹) were also identified. Indeed, it growth in density from 1.936 g/cm³ before controlled ceramization up to 2.201 g/cm³ for the heat-treated sample.

The influence of controlled heat-treatment process was also evaluated using XRD measurements and the results are depicted in Figure 2a. For xerogels, a broad halo pattern was recorded and it indicated their amorphous nature without long-range order. The diffraction lines were observed after performed controlled ceramization process at 350 °C per 10 h. The XRD patterns are in good accordance with the diffraction lines of regular CaF₂ phase from ICDD (The International Centre for Diffraction Data, PDF-2 No. 65-0535) crystallized in the Fm3m space group. The subsequent diffraction lines at 28.2°, 46.9°, 55.6°, 68.6° and 75.8° were identified as (111), (200), (311), (400) and (311) reflexes of CaF₂ phase, respectively. Observed broadening of diffraction lines clearly indicates that CaF₂ phase crystallized in nanometric range and the average crystals size was estimated using the Scherrer formula:

$$\mathrm{D}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(1)

in which D is related to the crystal size, K is a constant value (for our calculations it was taken K = 1), λ is the X-ray wavelength, β is a half width of analyzed diffraction peak and θ is the diffraction angle. The mean value of CaF₂ nanocrystals was equaled to 11.7 nm ±1.0 nm. Due to similar ionic radii of Ca²⁺ (1.00 Å) [24] and Eu³⁺ (1.07 Å) [25], Ca²⁺ cations in CaF₂ crystal lattice could be effectively substituted by trivalent Eu³⁺ ions and therefore, a very slight shift of diffraction lines was identified.

Figure 2b presents TEM image of fabricated glass-ceramics. The size of CaF₂ nanocrystals was consistent with average crystal size estimated from Scherrer equation. The CaF₂ nanocrystals with comparable size (about 10 nm) were obtained by Zhou et al. [26] in GCs fabricated during ceramization of precursor glasses with composition (45SiO₂-25CaF₂-20Al₂O₃-10CaO):4%EuF₃ (treatment conditions: 700 °C/4 h). In addition, to verify the quantitative distribution of chemical elements in fabricated glass-ceramics, we carried out the analysis from the selected sample area containing CaF₂ nanocrystal using energy dispersive X-ray spectroscopy, EDS. Generally, the content of Ca and F in CaF₂ nanocrystal were estimated to 5 wt.% and 6 wt.%, respectively; and the content of Si and O from silicate sol-gel matrix were estimated to 41 wt.% and 48 wt.%, respectively. Since Eu³⁺ ions were introduced during sol-gel synthesis as a dopant, their concentration was below the quantification limit and determination their quantitative distribution between CaF₂ nanocrystals and a silicate host was not possible. Moreover, according to another data presented in literature, we proposed the lowest treatment temperature (350 °C) to the fabrication of CaF₂:Eu³⁺ glass-ceramic materials (550 °C–570 °C [27], 620 °C–680 °C [28], 660 °C [29], 800 °C [30]). Nowadays, the low-temperature processes are preferable and therefore, the formation of CaF₂ nanocrystals at 350 °C seems to be highly desirable.

Based on weight-loss during sol-gel transformation from initial liquid sol to xerogel and taking the results from TG/DSC analysis into account, we estimated the amount of CaF₂ crystal fraction in the prepared GC sample. In general, it was found that during successive evaporation of volatile components (water and organic compounds used during synthesis) and progressive polycondensation reaction of silicate network, the remaining mass of fabricated xerogel was evaluated on 16.7 wt.% of the initial sol weight. Based on the TG curve shown in Figure 1, the total weight loss during heat-treatment could be estimated to about 22.0 wt.%. The second degradation step was strictly associated with thermal decomposition of Ca(CF₃COO)₂ and the indicated weight-loss (16.18 wt.%) should be related with evaporation of CF₃CFO, CO₂ as well as CO. Therefore, based on above results and taking the stoichiometry of thermal degradation reaction of Ca(CF₃COO)₂ into account, the amount of CaF₂ crystal fraction was estimated to about 8 wt.% (simultaneously, the estimated mass of silicate sol-gel host is about 92 wt.%). Moreover, based on performed calculations, it was assumed that thermal degradation should be complete (it should be also noted that the amount of introduced Ca(CH₃COO)₂ salt into reaction system during sol-gel synthesis was only 2.3 wt.%).

3.2. Luminescence Behavior of Fabricated Sol-Gel Materials

Figure 3 shows the photoluminescence excitation spectra (PLE) of prepared xerogels and CaF₂:Eu³⁺ glass-ceramics. The spectra were monitored at λ_em = 611 nm wavelength corresponding to the ⁵D₀ → ⁷F₂ optical transition of Eu³⁺ ions. The recorded excitation lines were identified as the intra-configurational electronic transitions inside 4f⁶ manifold of Eu³⁺ optically active ions. The bands were assigned to transitions from the ⁷F₀ ground level to the excited states: ⁵D₄ (363 nm), ⁵G_J, ⁵L₇ (372 nm–389 nm), ⁵L₆ (395 nm–xerogel, 394 nm/398 nm–CaF₂:Eu³⁺ GCs) and ⁵D₂ (465 nm–xerogel, 464 nm–CaF₂:Eu³⁺ GCs). In addition, the weak bands according to the ⁷F₀ → ⁵D₃ (416 nm) and ⁷F₀ → ⁵D₁ (525 nm) transitions were recorded. Since, the ⁷F₁ level is thermally populated at room temperature, a satellite line corresponding to the ⁷F₁ → ⁵D₁ (535 nm) was also observed.

It was observed that for xerogels the ⁷F₀ → ⁵L₆ excitation band has only one maximum, meanwhile for prepared CaF₂:Eu³⁺ GC samples such a line is a double peak with two splitted components located at 394 nm and 398 nm. Since Eu³⁺ ions could be distributed between silicate sol-gel host and CaF₂ nanocrystals in fabricated GCs, such splitting into two components could indicate that Eu³⁺ ions are residing in two different frameworks, as was suggested by A.C. Yanes for sol-gel LaF₃:Eu³⁺ GCs [31]. Thus, 394 nm component could be associated with their location within silicate network, meanwhile 398 nm component could be related with location of Eu³⁺ in fluoride nanocrystals. Based on paper by J. Pan et al. [32], the excitation spectra of pure CaF₂:Eu³⁺ crystal phase revealed only one component of the ⁷F₀ → ⁵L₆ band. Therefore, it may confirm the hypothesis that visible splitting of indicated excitation band could be related with distribution of Eu³⁺ ions between CaF₂ nanocrystals and silicate sol-gel host. Since the most intense excitation line corresponded to the ⁷F₀ → ⁵L₆ transition for fabricated sol-gel samples, the appropriate wavelengths were used to perform the emission measurements.

The photoluminescence spectrum (PL) registered for precursor xerogels is shown in Figure 4. Generally, the recorded spectrum consisted from five emission bands assigned to the transitions from the ⁵D₀ excited state into the ⁷F_J levels: ⁷F₀ (578 nm), ⁷F₁ (591 nm), ⁷F₂ (616 nm), ⁷F₃ (649 nm) as well as ⁷F₄ (697 nm). Since the relative intensities of Eu³⁺ emission bands were strictly dependent on symmetry in their local framework, the emission spectra gave valuable information about the nearest framework around them in host matrix [32]. It was observed that for silicate xerogels an emission band assigned to the ⁵D₀ → ⁷F₂ electric-dipole transition was more intense compared to a line assigned to the ⁵D₀ → ⁷F₁ magnetic-dipole transition. The ⁵D₀ → ⁷F₂ transition was very sensitive to the symmetry in the local vicinity around Eu³⁺ ions and it was hypersensitive in nature. Conversely, an intensity of the ⁵D₀ → ⁷F₁ band was rather independent on the symmetry in nearest surrounding of Eu³⁺ ions. Hence, the ratio between the ⁵D₀ → ⁷F₂ and the ⁵D₀ → ⁷F₁ emission intensities—well-known as R/O—can play the role as a useful tool for estimating the symmetry in which Eu³⁺ ions are located [33,34]. The R/O-ratio value calculated for prepared xerogels was estimated to be 4.13.

Furthermore, as was shown in the inset of Figure 4, the luminescence decay curve registered for precursor xerogels was well-fitted to the mono-exponential function and the estimated luminescence lifetime of the ⁵D₀ state was estimated to be τ = 0.46 ms. The short luminescence lifetime was strictly related with the structure of fabricated xerogels. The luminescence from the ⁵D₀ state was quenched by numerous OH groups in local framework of Eu³⁺ ions and such groups originated from silanol Si-OH moieties (3664 cm⁻¹, 3398 cm⁻¹) as well as residual water and organic solvents (3233 cm⁻¹) inside the porous silicate network. Since the ⁵D₀ → ⁷F₆ energy gap of Eu³⁺ ions was equal to ΔE = 12,500 cm⁻¹ [35], only about four OH phonons were required to promote a non-radiative deactivation of the ⁵D₀ excited level. Since CF₃COO⁻ anions were in the coordination sphere around Eu³⁺ cations, it should be noted that such non-radiative relaxation from the ⁵D₀ state could be also caused by C = O groups (1664 cm⁻¹, eight phonons) and the C-F bond (1193 cm⁻¹, ten phonons). However, among identified functional groups, we expected that OH moieties could play a major role in non-radiative deactivation. In general, the non-radiative rates of intra-configurational 4fⁿ-4fⁿ transitions of rare earths were exponentially dependent on the energy gap (ΔE) and the phonon energy in their nearest surrounding (ħω). Such correlation is well-known as the energy gap law and it states that an increase in the non-radiative decay rate is assisted by a decreasing number of phonons needed to cover the energy gap, ΔE. Hence, if there is a functional group with high vibrational energy in a local framework of Eu³⁺ ions, the probability of non-radiative multiphonon relaxation increases. The phonons with maximum energy in a host are also usually called as effective phonons (ħω_max) [36,37]. The influence of different types of functional groups in Eu³⁺-doped silicate glasses on multiphonon relaxation was discussed in work by A. Herrmann et al. [38]. The rates of multiphonon relaxation, k_NR, via OH modes (3750 cm⁻¹, k_NR = 1.4·× 10⁻² s⁻¹) and Si-O vibrations (1250 cm⁻¹, k_NR = 9.0·× 10⁻¹³ s⁻¹) clearly indicates that OH groups with maximum phonon energy in studied glasses play a major role in quenching the luminescence from the ⁵D₀ state. Moreover, the luminescence lifetime is not dependent only on phonon energy in the nearest framework around the optically active ion, but also on another factors, like symmetry. In general, the lower the symmetry is in the nearest vicinity of Eu³⁺ ion, the more allowed are the forbidden f-f transitions. Therefore, taking the symmetry aspect into account, the probability of radiative relaxation is relatively high, which results in a short luminescence lifetime.

As was presented in Figure 5, the emission spectrum of CaF₂:Eu³⁺ GCs also consisted of the characteristic bands of Eu³⁺ ions corresponding to the intra-configurational ⁵D₀ → ⁷F_J transitions within the 4f⁶ manifold. Such bands were recorded at the following wavelengths: 577 nm (J = 0), 589 nm (J = 1), 613 nm (J = 2), 648 nm (J = 3), 683 nm/689 nm/699 nm (J = 4). Compared to xerogels, a significant growth in intensity of the orange ⁵D₀ → ⁷F₁ band was observed and it was accompanied by eight-fold decrease in the R/O-ratio value (from 4.13 to 0.51). The indicated decline in the R/O-ratio value clearly points to the change in the symmetry in the nearest vicinity around dopant ions and the nature of the bonding character between Eu³⁺ ions and their nearest surrounding covalent to become more ionic. Obviously, this is related with the partial incorporation of optically active ions into the crystallized CaF₂ fluoride phase.

The ⁷F_J energy levels of Eu³⁺ ions in crystal lattice could split and the number of individual sublevels depends on the J number and the site symmetry [33]. According to paper by Brown et al. [39], Eu³⁺ ions are located in C_4v symmetry sites in the CaF₂ crystal lattice, despite the fact that Ca²⁺ cations are located in O_h symmetry sites. This effect could be explained by charge compensation, when divalent Ca²⁺ cations in CaF₂ crystal lattice are substituted by trivalent Eu³⁺ ions. Then, due to prevent the non-equilibrium charge in the crystal lattice, cation vacancies could be also formed and some fluorine anions could occupy the interstitial positions. If the Eu³⁺ ion is located in the C_4v site in the CaF₂ crystal lattice, the individual ⁵D₀ → ⁷F_J (J = 0–4) band should split into one (J = 0), two (J = 1), four (J = 2), five (J = 3) and seven (J = 4) components. However, the distinguishing of individual components was quite difficult for fabricated glass-ceramic samples, because Eu³⁺ ions are distributed both in amorphous silicate sol-gel network as well as CaF₂ nanocrystals. Hence, we assumed that the influence of the crystal field is largely masked.

The photoluminescence decay curve of the ⁵D₀ state of Eu³⁺ ions recorded for fabricated glass-ceramics was presented in the inset of Figure 5. The decay curve was well-fitted to double-exponential function and therefore, it was distinguished two different decay components: fast (τ₁ = 1.63 ms) and slow (τ₂ = 12.84 ms). The double-exponential decay clearly indicates that two different decay channels are involved in the total decay process from the ⁵D₀ excited state of Eu³⁺. The contribution of fast and slow components could be determined using fitting constants in the following equation:

$${\mathsf{\tau }}_{\mathrm{n}},\%=\frac{{\mathrm{A}}_{\mathrm{n}}}{{\mathrm{A}}_{\mathrm{n}}+{\mathrm{A}}_{\mathrm{m}}}·100\%$$

(2)

Since the A₁ and A₂ fitting constants were close to 1.9408·× 10⁶ and 2.7539·× 10⁶, respectively, a percentage contribution of τ₁ was equal to 41.34% and the contribution of τ₂ was 58.66%. The first of the luminescence lifetime, τ₁, was associated with Eu³⁺ ions located in the silicate sol-gel host consisting of Q³ units of SiO₄ tetrahedrons (~1051 cm⁻¹) and residual Si-OH groups (3390 cm⁻¹, 3656 cm⁻¹). In this way, to cover the energy gap in silicate host, four Si-OH groups or twelve Q³ units were needed. The second luminescence lifetime component, τ₂, was related with the remaining part of Eu³⁺ ions which were successfully incorporated into CaF₂ nanocrystals during the controlled heat-treatment process. Indeed, due to low-phonon energy of the CaF₂ crystal lattice (~466 cm⁻¹ [40]), about 27 Ca-F phonons were needed to cover the ⁵D₀ → ⁷F₆ energy gap. The probability of non-radiative relaxation from the ⁵D₀ state was greater within the silicate framework than in the CaF₂ crystal lattice due to higher-phonon energies. Moreover, the probability of radiative relaxation was lower in more symmetric CaF₂ than in asymmetric sol-gel host. Therefore, the τ₂ components were significantly prolonged compared to τ₁. Moreover, we suggested that the tendency of the Eu³⁺ ions to migrate into CaF₂ could be promoted by a similar ionic radii of Ca²⁺ (1.00 Å) and Eu³⁺ (1.07 Å). It should be also noted that the τ₁ luminescence lifetime component of fabricated CaF₂:Eu³⁺ GCs was prolonged compared with the τ lifetime estimated for xerogel. Both of the presented lifetimes were related with Eu³⁺ ions, which were dispersed inside the silicate sol-gel host. To explain the differences in luminescence lifetime values, we used the results from the IR-ATR spectroscopy. As was evidenced by IR measurements, the huge amounts of OH groups with high vibrational energies (above 3000 cm⁻¹) were identified in the structure of fabricated xerogels. Therefore, such numerous OH groups in the nearest vicinity of Eu³⁺ ions were mainly responsible for non-radiative quenching of luminescence from the ⁵D₀ excited state, which resulted in a relatively short lifetime. Simultaneously, the infrared measurements carried out for CaF₂:Eu³⁺ glass-ceramic indicated a strong reduction of OH groups, hence, the amount of effective ‘quenchers’ in the nearest surrounding of Eu³⁺ ions in the silicate sol-gel network was smaller than in xerogels. In consequence, the τ₁ luminescence lifetime was prolonged due to smaller amounts of OH quenchers.

4. Conclusions

Transparent glass-ceramic materials containing CaF₂ fluoride nanocrystals were prepared by a low-temperature sol-gel route. Thermal decomposition of Ca(CF₃COO)₂ in xerogels was confirmed by using TG/DSC methods. The formation of CaF₂ nanocrystals during the heat-treatment process was confirmed by X-ray diffraction measurements and TEM microscopy. The structural changes in the sol-gel silica-network were verified by the IR-ATR spectroscopy. The systematic investigations demonstrated that the Eu³⁺:CaF₂ fluoride nanocrystals dispersed in sol-gel glass-ceramic materials showed the prolonged luminescence lifetimes of the ⁵D₀ excited state of Eu³⁺ ions. The significant changes in calculated R/O-ratio values and in luminescence decay profiles clearly indicated the successful entering of Eu³⁺ ions into CaF₂ nanocrystals during the controlled ceramization process at 350 °C. It suggests that sol-gel glass-ceramic materials containing CaF₂:Eu³⁺ fluoride nanocrystals are promising candidates for reddish-orange photoluminescence applications.