Optical Properties of Transparent Rare-Earth Doped Sol-Gel Derived Nano-Glass Ceramics

Rare-earth doped oxyfluoride glass ceramics represent a new generation of tailorable optical materials with high potential for optical-related applications such as optical amplifiers, optical waveguides, and white LEDs. Their key features are related to the high transparency and remarkable luminescence properties, while keeping the thermal and chemical advantages of oxide glasses. Sol-gel chemistry offers a flexible synthesis approach with several advantages, such as lower processing temperature, the ability to control the purity and homogeneity of the final materials on a molecular level, and the large compositional flexibility. The review will be focused on optical properties of sol-gel derived nano-glass ceramics related to the RE-doped luminescent nanocrystals (fluorides, chlorides, oxychlorides, etc.) such as photoluminescence, up-conversion luminescence, thermoluminescence and how these properties are influenced by their specific processing, mostly focusing on the findings from our group and similar ones in the literature, along with a discussion of perspectives, potential challenges, and future development directions.


Introduction
Glass ceramics are inorganic, non-metallic materials prepared by controlled crystallization of precursor glasses with at least one type of functional crystalline phase, that may vary from ppm to almost 100% is embedded in residual glass [1]. The basic requirements for optical transparency include: nanocrystals size smaller than the light wavelength, very close refractive index of amorphous and crystalline phases and low birefringency [1]. Transparent oxyfluoride nano-glass ceramics are produced by fluoride nanocrystals precipitation in a silica glass matrix during a controlled thermal processing. They have shown a high potential for applications in various devices (optical amplifiers, optical waveguides, white LEDs, etc.) since they combine the optical phenomena with optical transparency due to lack of scattering.
The conventional way to obtain glass ceramics is by melt-quenching (at around 1400 • C for oxy-fluoride glasses or a lower temperature in a controlled atmosphere for fluoride glasses) followed by crystallization of the precursor glass during a controlled annealing step throughout a heat-treatment at lower temperatures. Nucleation and growth of fluoride nanocrystals occurs in the glass matrix during heat treatment and rare earth ions can preferentially segregate into the fluoride nanocrystals. Hence, the partition of the optically active rare-earth (RE) ions into the precipitated fluoride nanocrystals can be obtained, with low phonon frequencies, but keeping good chemical and mechanical stability of the oxide glass (see the review of de Pablos-Martin et al. [2]). An alternative route is based on sol-gel chemistry, well known as a flexible approach to preparing various materials such as glass, ceramics, and organic-inorganic hybrids by using colloidal solutions (sol) as starting materials [3,4]. The glass ceramics' synthesis by using the sol-gel method has several advantages such as lower processing temperature, the ability to control the purity and homogeneity of the final materials on a molecular level, and the large compositional

Sol-Gel Method for Glass Ceramic Synthesis
The pioneering contribution on sol-gel derived oxyfluoride glass ceramic preparation came from the first works of Fujihara et al. [5][6][7] performed in two steps: (i) preparation of a silica sol by using classic route with metal alkoxides (ii) followed by mixing with a second trifluoroacetates solution formed by trifluoroacetic (TFA) acid reaction with rare earth precursors (acetates). The liquid mixture is dried and aged at room temperature under sealed containers for several days or weeks. The nanocrystalline phase precipitation is obtained during subsequent calcination of dried xerogel matrix above the temperature required for thermal decomposition of corresponding trifluoracetates at around 300 • C: 2 (1)

RE(CF 3 COO) 3 → REF 3 + (CF 3 CO) 2 O + CO + CO
Since then, the number of the publications on that topic has increased steadily and many compositions and crystalline phases have been studied using different Ln 3+ ions as dopants to obtain enhanced linear and nonlinear optical processes for a wide application range.

Crystallization Mechanism
The first investigations of the crystallization processes of sol-gel derived glasses were done by Fujihara et al., with MgF 2 thin films synthesis from trifluoracetic acid (TFA) and Mg-ethoxide (Mg(OC 2 H 5 ) 2 as precursors. It was assumed that the Mg 2+ ions released during the thermal decomposition of the Mg(OC 2 H 5 ) 2 complexes form the xerogel and react with thermally activated fluorine in CF 3 COO − ions to form MgF 2 crystals [7].
The investigations of the SiO 2 -LaF 3 sol gel system evolution during the heat treatment by using Fourier transform infrared spectroscopy (FTIR) analysis indicated decomposition of trifluoroacetic acid (TFA), used as a fluorine precursor, together with the formation of fluoride lattice bonding [5,6] (Figure 1). The formation of LaF 3 nanocrystals (of about 10 ± 30 nm in size after calcination above 300 • C of the precursor silica xerogel) was assigned to the screening effect of the CF 3 COO − ions coordinating the La 3+ ions that prevented the La 3+ ions incorporation into the SiO 2 matrix [5]. The smallness of the LaF 3 nanocrystals (tens of nm size) even for higher temperatures calcination was assigned to the low nanocrystals growth rate because of the uniform distribution of the nanocrystals [5]. For higher annealing temperatures above 800 • C, the formation of LaF 3 , La 2 O 3 , and LaOF nanocrystals was observed [5].
Later on, the studies of the crystallization kinetics of sol gel-derived glasses were extended by using multiple techniques (structural, magnetic, optical), which indicated a decomposition chemical reaction accompanied by the precipitation of nanocrystals, and not a diffusion-controlled nucleation and growing process, as in the melt-quenched glasses [6][7][8][9][10]. For longer treatment times, the loss of crystal structure and a partial/total dissolution of crystals in the glass matrix was observed. Higher annealing temperature does not result in better crystallinity or bigger nanocrystals but for even higher temperatures (1000 • C) the Si-O-Si network is heavily destroyed due to precipitation of the SiO 2 crystalline phase, so the nano-crystals size grow quickly [11]. It was shown that the nanocrystal growth process in the glass matrix is strongly influenced by the ionic environment and ionic impurities incorporated during the growth process, and energy dispersive X-ray (EDX) analysis indicated the presence of oxygen ions in the nanocrystals [12]. aterials 2021, 14, x FOR PEER REVIEW Figure 1. The XRD patterns of SiO2-LaF3 gel and glass ceramics obtained after calcinat temperatures. (Reproduced from reference [5].) Later on, the studies of the crystallization kinetics of sol gel-derived gla tended by using multiple techniques (structural, magnetic, optical), which in composition chemical reaction accompanied by the precipitation of nanocry a diffusion-controlled nucleation and growing process, as in the melt-quen [6][7][8][9][10]. For longer treatment times, the loss of crystal structure and a partial/ tion of crystals in the glass matrix was observed. Higher annealing tempera result in better crystallinity or bigger nanocrystals but for even higher tempe °C) the Si-O-Si network is heavily destroyed due to precipitation of the SiO phase, so the nano-crystals size grow quickly [11]. It was shown that the growth process in the glass matrix is strongly influenced by the ionic envi ionic impurities incorporated during the growth process, and energy disp (EDX) analysis indicated the presence of oxygen ions in the nanocrystals [12 Structural and morphological studies of the crystallization mechanism Structural and morphological studies of the crystallization mechanism [13] of the SiO 2 -YF 3 glass ceramic showed that the crystallization of YF 3 nanocrystalline phase is due to decomposition of Y(CF 3 COO) 3 , according to the previous image of the crystallization mechanism. Moreover, transmission electron microscopy (TEM) investigations showed that YF 3 nanophase crystallization started with formation of big congeries particles in the glassy matrix consisting of many randomly oriented mono-crystallites sized around 5 nm ( Figure 2). Higher calcination temperature (up to 600 • C) promotes the separation of the initial nanocrystals by reducing stress and lowering system energy. A similar conclusion was reached by Yunlong Yu et al. (2006) [11], who showed that the formation of SrF 2 nanocrystals 8-10 nm distributed homogenously among the glassy matrix. The nanocrystals size remains almost unchanged up to 800 • C annealing due to the interfacial interaction of SrF 2 nano-crystals with the glass matrix, which hinders their further growth. Structural and optical investigations of the crystallization process in SiO2-BaF2 solgel indicated a homogenous crystallization mechanism, with BaF2 small nanocrystals (a few nm size) acting as nucleation centers that resulted from thermal decomposition of Batrifluoroacetate at about 300 °C [8]. The investigations of thermal behavior of the silicate xerogels by using differential scanning calorimetry (DSC) and thermogravimetry (TG) showed a three-stage thermal degradation profile related to the glass ceramization process [8] (Figure 3). The first step corresponding to the temperature range from 120 °C to 200 °C is due to the removal of volatile components. A second step from 280 to 360 °C is closely related to the Ba trifluoroacetate decomposition [14] with the formation of tiny BaF2 nanocrystalline seeds (few nm size) and is accompanied by a strong DSC peak at about 307 °C. The next weight loss in the TG curve occurs in the temperature range from 400 to 500 °C and is due to the pyrolysis of organic groups bonded to silicon (residual organics and carbon). The formation of the BaF2 nanocrystalline phase in Eu 3+ -doped SiO2-BaF2 and crystallinity improvement was associated to a broad and weak DSC peak at 685 °C ( Figure 3) and shows a slight dependence (within few degrees) on the RE-dopant ions incorporation. A similar peak was reported at 663 °C in 95SiO2-5CaF2 [15] and at 700 °C in 95SiO2-5SrF2 [11] xerogel, but not in the undoped one, indicating its strong relation to the nature of the nanocrystalline phase. Therefore, the general characteristic of the sol-gel glasses crystallization process is the thermal decomposition reaction of the metal trifluoroacetates, revealed as a strong Structural and optical investigations of the crystallization process in SiO 2 -BaF 2 solgel indicated a homogenous crystallization mechanism, with BaF 2 small nanocrystals (a few nm size) acting as nucleation centers that resulted from thermal decomposition of Ba-trifluoroacetate at about 300 • C [8]. The investigations of thermal behavior of the silicate xerogels by using differential scanning calorimetry (DSC) and thermogravimetry (TG) showed a three-stage thermal degradation profile related to the glass ceramization process [8] (Figure 3). The first step corresponding to the temperature range from 120 • C to 200 • C is due to the removal of volatile components. A second step from 280 to 360 • C is closely related to the Ba trifluoroacetate decomposition [14] with the formation of tiny BaF 2 nanocrystalline seeds (few nm size) and is accompanied by a strong DSC peak at about 307 • C. The next weight loss in the TG curve occurs in the temperature range from 400 to 500 • C and is due to the pyrolysis of organic groups bonded to silicon (residual organics and carbon). The formation of the BaF 2 nanocrystalline phase in Eu 3+ -doped SiO 2 -BaF 2 and crystallinity improvement was associated to a broad and weak DSC peak at 685 • C ( Figure 3) and shows a slight dependence (within few degrees) on the RE-dopant ions incorporation. A similar peak was reported at 663 • C in 95SiO 2 -5CaF 2 [15] and at 700 • C in 95SiO 2 -5SrF 2 [11] xerogel, but not in the undoped one, indicating its strong relation to the nature of the nanocrystalline phase. Structural and optical investigations of the crystallization process in SiO2gel indicated a homogenous crystallization mechanism, with BaF2 small nanocr few nm size) acting as nucleation centers that resulted from thermal decompositio trifluoroacetate at about 300 °C [8]. The investigations of thermal behavior of th xerogels by using differential scanning calorimetry (DSC) and thermogravime showed a three-stage thermal degradation profile related to the glass ceramizat cess [8] (Figure 3). The first step corresponding to the temperature range from 1 200 °C is due to the removal of volatile components. A second step from 280 to closely related to the Ba trifluoroacetate decomposition [14] with the formation BaF2 nanocrystalline seeds (few nm size) and is accompanied by a strong DSC about 307 °C. The next weight loss in the TG curve occurs in the temperature ran 400 to 500 °C and is due to the pyrolysis of organic groups bonded to silicon organics and carbon). The formation of the BaF2 nanocrystalline phase in Eu 3+ -dop BaF2 and crystallinity improvement was associated to a broad and weak DSC pe °C ( Figure 3) and shows a slight dependence (within few degrees) on the RE-dop incorporation. A similar peak was reported at 663 °C in 95SiO2-5CaF2 [15] and at 95SiO2-5SrF2 [11] xerogel, but not in the undoped one, indicating its strong relati nature of the nanocrystalline phase. Therefore, the general characteristic of the sol-gel glasses crystallization p the thermal decomposition reaction of the metal trifluoroacetates, revealed as  Therefore, the general characteristic of the sol-gel glasses crystallization process is the thermal decomposition reaction of the metal trifluoroacetates, revealed as a strong DSC peak between 280 and 350 • C [10,16]. Higher calcination temperature results in a weak increase of the nanocrystals size by promoting the separation and growth of the initial nanocrystals [13]. These processes are revealed by a second and weaker DSC peak, which is strongly dependent of the fluoride nanocrystals nature. However, the mechanism is expected to be more complicated for glass ceramics containing complex compounds such as NaYF 4 , NaLaF 4 , NaGdF 4, KLaF 4, KYF 4 , LiYF 4 , BaYF 5 , and BaGdF 5 (refer to the next section).

Optical Properties: Photoluminescence, Up-Conversion Luminescence, and Thermoluminescence
The calcination of dried xerogel affects not only structural and morphological properties (i.e., nanocrystals formation), but also the optical ones. During xerogel calcination, a silica network is formed due to the progressive enhancement of dehydration-condensation reactions [8,15,17], accompanied by the precipitation of the fluoride nanocrystalline phase, where a fraction of the RE 3+ ions was incorporated. It was shown that in Er 3+ -doped SiO 2 -LaF 3 sol-gel glass ceramic the effective concentration of rare-earth ions estimated by X-ray absorption spectroscopy was 91% and 9% in a fluoride and glass environment, respectively [18].
In the amorphous matrix, the RE 3+ -ions dopants are distributed within the amorphous structure of the glass, characterized by unregular arrangement of the constituent ions. Hence, in the glass matrix, the photoluminescence spectra of the RE 3+ -ions shows broadening due to the random positions and random surroundings of the dopant ions. The new luminescence features observed in the glass ceramic were assigned to the RE 3+ions incorporated into the precipitated nanocrystals. The luminescence spectra in the glass ceramic shows Stark splitting due to the degeneracy lifting by the local crystal field where the RE 3+ -ion are incorporated, an effect clearly observed for Eu 3+ ions [19,20]; Figure 4. Because of the nanocrystals smallness in the glass ceramic, the nanosize-related effects can have a strong influence on the PL spectra (i.e., broadening effects), as was observed in the nanocrystals [21,22]. Moreover, on the nanocrystals surface, we expect a high concentration of surface atoms and defects [21,22] acting as trapping centers and non-radiatively dissipating energy. Therefore, the PL spectra are assigned to a superposition between the RE 3+ ions emission located in the glass matrix and in the crystalline phase, inside the nanocrystals and on their surface [19]. Photoluminescence decay measurements are also highly sensitive to the RE 3+ -ions environment. Lower phonon energy of fluorides and the dehydration processes reduce the probability of non-radiative de-excitations and as a result a luminescence signal enhancement and longer luminescence lifetimes compared to the xerogels were observed [8,18,23]. Defects with relatively large vibration energies like hydroxyl groups from the water, solvent, and silanol groups are efficient non-radiative relaxation channels for the excited states and therefore multiphonon relaxation rate is rather large, thereby decreasing the luminescence lifetime and efficiency.
Among the RE 3+ ions, Eu 3+ ions are widely used as green-red light emitting activators, where the characteristic luminescence properties are to the intra-configurational 4f 6 -4f 6 transitions ( 5 D 0 → 7 F J , J = 0-4); see Figure 4. Moreover, they are widely studied as probe-ions for local site symmetry in various materials [24,25] and have been used to investigate the sol-gel process [8,[26][27][28]. The electric-dipole 7 D 0 → 5 F 2 transition is sensitive to the environment and therefore the intensity ratio between the two visible emissions 5 D 0 -7 F 2 / 5 D 0 -7 F 1 is highly dependent on the Eu 3+ -ion environment. The lower this ratio, the closer is the local symmetry to the one having an inversion center [29]. In particular, the intensity ratio has been used to monitor and discuss the sol-gel process and xerogel transformation in various Eu 3+ -doped glass ceramics [8,20,23,30]. Moreover, luminescence spectra recorded under proper excitation wavelengths and luminescence decay measurements recorded in SiO 2 -LaF 3 glass ceramic have allowed discerning between ions residing in precipitated nanocrystals and those remaining in a glassy environment [23]. The 464 nm excitation wavelength is inhibited for Eu 3+ ions in high symmetry sites, whereas the broad luminescence spectrum and shorter luminescence decay lifetime reflect the glassy environment [23]. By comparison, the luminescence spectra recorded under 394 nm excitation (the 7 D 0 → 5 F 2 transition) shows Stark splitting and longer decay time characteristic to the crystalline environment [23]. A similar approach was applied to the Eu 3+ -doped SiO 2 -BaF 2 glass ceramic (prepared in ref. [8]) and the spectra showed structured luminescence spectra for both excitation wavelengths but different red to green visible emissions ratios ( Figure 5). The spectra are consistent with dominant incorporation of the Eu 3+ ions within the BaF 2 nanocrystals [8], exhibiting two different crystalline sites with higher and lower local symmetry.
Among the RE 3+ ions, Eu 3+ ions are widely used as green-red light emitting ac where the characteristic luminescence properties are to the intra-configurationa transitions ( 5 D0 → 7 FJ, J = 0-4); see Figure 4. Moreover, they are widely studied a ions for local site symmetry in various materials [24,25] and have been used to inv the sol-gel process [8,[26][27][28]. The electric-dipole 7 D0 → 5 F2 transition is sensitive to vironment and therefore the intensity ratio between the two visible emissions 5 D0 7 F1 is highly dependent on the Eu 3+ -ion environment. The lower this ratio, the clos local symmetry to the one having an inversion center [29]. In particular, the intens has been used to monitor and discuss the sol-gel process and xerogel transform various Eu 3+ -doped glass ceramics [8,20,23,30]. Moreover, luminescence spectra r under proper excitation wavelengths and luminescence decay measurements rec SiO2-LaF3 glass ceramic have allowed discerning between ions residing in prec nanocrystals and those remaining in a glassy environment [23]. The 464 nm ex wavelength is inhibited for Eu 3+ ions in high symmetry sites, whereas the broad l cence spectrum and shorter luminescence decay lifetime reflect the glassy envir [23]. By comparison, the luminescence spectra recorded under 394 nm excitation → 5 F2 transition) shows Stark splitting and longer decay time characteristic to the line environment [23]. A similar approach was applied to the Eu 3+ -doped SiO2-Ba ceramic and the spectra showed structured luminescence spectra for both excitatio lengths but different red to green visible emissions ratios ( Figure 5). The spectra sistent with dominant incorporation of the Eu 3+ ions within the BaF2 nanocrystal A special case is represented by the up-conversion luminescence (UC) sh rare-earth ions (or ion pairs) that is an anti-Stokes luminescence process where effects of light excitation and mutual interactions between ions produce hig A special case is represented by the up-conversion luminescence (UC) shown by the rare-earth ions (or ion pairs) that is an anti-Stokes luminescence process where synergistic effects of light excitation and mutual interactions between ions produce higher energy emission photons [31]. For rare-earth doped materials, the UC mechanism is based on the large absorption cross-section in the NIR region around 1000 nm of Yb 3+ and a very efficient energy transfer (ET) to one of the Er 3+ /Ho 3+ or Tm 3+ ions. The absorption of infrared light photons by the Yb 3+ ions ( 2 F 7/2 → 2 F 5/2 transition) is followed by a two-step energy transfer process to neighboring Er 3+ ions with their characteristic green (( 2 H 11/2 , 4 S 3/2 ) → 4 I 15/2 ) and red ( 4 F 9/2 → 4 I 15/2 ) luminescent emissions (see Figure 6). A special case is represented by the up-conversion luminescence (UC) shown by the rare-earth ions (or ion pairs) that is an anti-Stokes luminescence process where synergistic effects of light excitation and mutual interactions between ions produce higher energy emission photons [31]. For rare-earth doped materials, the UC mechanism is based on the large absorption cross-section in the NIR region around 1000 nm of Yb 3+ and a very efficient energy transfer (ET) to one of the Er 3+ / Ho 3+ or Tm 3+ ions. The absorption of infrared light photons by the Yb 3+ ions ( 2 F7/2 → 2 F5/2 transition) is followed by a two-step energy transfer process to neighboring Er 3+ ions with their characteristic green (( 2 H11/2, 4 S3/2) → 4 I15/2) and red ( 4 F9/2 → 4 I15/2) luminescent emissions (see Figure 6). Nanocrystalline phase formation has a great effect on the optical response to ionizing radiation, and this can be characterized by using thermoluminescence measurement technique. Thermoluminescence (TL) represents the light emission by a solid sample during controlled heating after irradiation by ionizing radiation such as UV light, X-rays, gammarays, etc. [32] and it has proved to be a useful and sensitive tool for the study of radiation effects in various materials. According to the basic model, charge carriers (electrons and holes) produced by irradiation (stage a) are trapped in local energy levels Tr, (stage b), such as vacancies, interstitials, or impurities within the band gap; during the heating (stage c), they are thermally released and recombine at the recombination centers R giving rise to TL [33]; the thermal activation energy is E. In particular, TL has proved an effective Figure 6. The green ( 2 H 11/2 , 4 S 3/2 → 4 I 15/2 ) and red ( 4 F 9/2 → 4 I 15/2 ) Er 3+ up-conversion luminescences excited at 980 nm in various Yb 3+ /Er 3+ -doped oxyfluoride glass ceramics [32] and the energy level schemes of Yb 3+ and Er 3+ with the main energy transfer processes.
Nanocrystalline phase formation has a great effect on the optical response to ionizing radiation, and this can be characterized by using thermoluminescence measurement technique. Thermoluminescence (TL) represents the light emission by a solid sample during controlled heating after irradiation by ionizing radiation such as UV light, X-rays, gammarays, etc. [33] and it has proved to be a useful and sensitive tool for the study of radiation effects in various materials. According to the basic model, charge carriers (electrons and holes) produced by irradiation (stage a) are trapped in local energy levels Tr, (stage b), such as vacancies, interstitials, or impurities within the band gap; during the heating (stage c), they are thermally released and recombine at the recombination centers R giving rise to TL [33]; the thermal activation energy is E. In particular, TL has proved an effective tool for the study of rare-earth induced levels in material and their behavior as electron or hole trap [35][36][37].
The TL properties of Eu 3+ -doped materials are due to the thermal release of the electrons from deep Eu 3+ -related traps followed by the recombination with the hole traps, resulting in the emission of light. TL properties were used to monitor the densification of Eu 3+ doped SiO 2 -BaF 2 dried xerogel prepared in ref. [8] to the glass ceramic (Figure 7-right). The investigations evidenced two glow peaks at 309 and 350 • C in the xerogel and a single dominant one at 370 • C in the glass ceramic, different from the glow peak observed in the Eu 3+ -silica glass at 400 • C [38]. This change was associated with structural changes during the ageing process of the gel and glass crystallization (Figure 7-right). tion to glass and then glass ceramic (Figure 7). The investigations evidenced two glow peaks at 309 and 350 °C in the xerogel and a single dominant one at 400 °C in the glass ceramic. This change was associated with structural changes during the ageing process of the gel (Figure 7-right); a further shift of the glow peak in the glass ceramic at 370 °C is due to the RE 3+ -ions incorporation in the crystalline matrix [15,38].

Oxyfluoride Glass ceramic
In the following sections, we present the most relevant results about the optical properties (mainly photo-and up-conversion luminescence properties) for RE 3+ doped oxyfluoride glass ceramics, depending on the nature of the nanocrystalline phase.

SiO2-MeF2 Oxyfluoride Glass ceramic (with Me = Mg, Ca, Sr, Ba, Pb)
Alkali-earth fluorides are well-known optical materials with various applications for radiation detection (as scintillators and for thermoluminescence dosimetry) or as laser media due to excellent optical properties such as high transmittance from ultraviolet to mid infrared spectral range and easy incorporation of RE 3+ -ions.

Oxyfluoride Glass Ceramic
In the following sections, we present the most relevant results about the optical properties (mainly photo-and up-conversion luminescence properties) for RE 3+ doped oxy-fluoride glass ceramics, depending on the nature of the nanocrystalline phase. Alkali-earth fluorides are well-known optical materials with various applications for radiation detection (as scintillators and for thermoluminescence dosimetry) or as laser media due to excellent optical properties such as high transmittance from ultraviolet to mid infrared spectral range and easy incorporation of RE 3+ -ions.
The Eu 3+ ions environment in the silica glass and SiO 2 -CaF 2 glass ceramic was investigated by using phonon side bands (PSB) measurements [15,17] (Figure 8). The local structures around Eu 3+ ions give rise to local vibration modes that can be observed as vibronic lines associated to excitation peaks, i.e., PSB. The PSB peaks were assigned to the vibrations of the SiO 4 tetrahedra units of the glass network (above ∼ =500 cm −1 ) and Ca-F bonds vibrations in the precipitated CaF 2 nanocrystalline phase (below ∼ =500 cm −1 ). The presence of Si-O, Eu-O, and Eu-F bonds is consistent with the Eu 3+ ions partition in both silica glass matrix and CaF 2 nanocrystaline phase, i.e., in the non-centrosymmetric sites of the CaF 2 nanocrystals structure [15].
bonds vibrations in the precipitated CaF2 nanocrystalline phase (below ≅500 cm −1 ). The presence of Si-O, Eu-O, and Eu-F bonds is consistent with the Eu 3+ ions partition in both silica glass matrix and CaF2 nanocrystaline phase, i.e., in the non-centrosymmetric sites of the CaF2 nanocrystals structure [15].
New evidence on the Eu 3+ ions partition was provided by the thermoluminescence measurements. Thermoluminescence investigations of SiO2-CaF2 glass ceramic revealed a glow peak at 370 °C, assigned to the recombination of the electrons released from the Eu 3+electron traps in the CaF2 nanocrystals, which is shifted from the peak observed in the Eu 3+ -doped silica glass at 400 °C [38]; the broadening of the glow peaks is consistent with multiple Eu 3+ -ion sites.

SiO2-SrF2 Oxyfluoride Glass ceramic
Luminescence properties of Eu 3+ /Tb 3+ co-doped SiO2-SrF2 glass ceramics have been studied and showed cross-relaxation in materials with Tb 3+ dopant as well as energy transfer from Tb 3+ to Eu 3+ . White luminescence can be achieved by combining blue emission of SiO2, green light emitted by Tb 3+ and red one by Eu 3+ , resulting in a white phosphor-like behavior [41] (Figure 9). New evidence on the Eu 3+ ions partition was provided by the thermoluminescence measurements. Thermoluminescence investigations of SiO 2 -CaF 2 glass ceramic revealed a glow peak at 370 • C, assigned to the recombination of the electrons released from the Eu 3+ -electron traps in the CaF 2 nanocrystals, which is shifted from the peak observed in the Eu 3+ -doped silica glass at 400 • C [38]; the broadening of the glow peaks is consistent with multiple Eu 3+ -ion sites.

SiO 2 -SrF 2 Oxyfluoride Glass Ceramic
Luminescence properties of Eu 3+ /Tb 3+ co-doped SiO 2 -SrF 2 glass ceramics have been studied and showed cross-relaxation in materials with Tb 3+ dopant as well as energy transfer from Tb 3+ to Eu 3+ . White luminescence can be achieved by combining blue emission of SiO 2 , green light emitted by Tb 3+ and red one by Eu 3+ , resulting in a white phosphor-like behavior [41] (Figure 9). Up-conversion luminescence spectra of Er 3+ -doped SiO2-SrF2 oxyfluoride glass ceramic showed resolved Stark components of 4 I13/2 band in the glass ceramics compared to the xerogel, assigned to the change of the environment around Er 3+ from the glass to nanocrystals [42]. Moreover, the introduction of Al 3+ to the SiO2 network caused an improve- Figure 9. Photoluminescence spectra recorded on SrF 2 :Eu/Tb@SiO 2 glass ceramics after under 368 nm excitation wavelength; blue broad emission of SiO 2 is accompanied by the green light peaks of Tb 3+ and red light peaks of Eu 3+ (reproduced from reference [41]).
Up-conversion luminescence spectra of Er 3+ -doped SiO 2 -SrF 2 oxyfluoride glass ceramic showed resolved Stark components of 4 I 13/2 band in the glass ceramics compared to the xerogel, assigned to the change of the environment around Er 3+ from the glass to nanocrystals [42]. Moreover, the introduction of Al 3+ to the SiO 2 network caused an improvement of the optical properties: better transparency in the UV region due to lower pore content and intense visible UC luminescence as compared to the one with only SiO 2 [42].

SiO 2 -BaF 2 Oxyfluoride Glass Ceramic
Transparent glass ceramics containing Er 3+ -doped BaF 2 nano-crystals doped were prepared by sol-gel route and upconversion luminescence was assigned to the Er 3+ -ions incorporated within the BaF 2 nanocrystals of about 2-15 nm size [43]. Further investigations of RE 3+ -doped SiO 2 -BaF 2 glass ceramic (RE = Ho, Dy, Eu, Sm) showed that in the glass ceramic material, a large fraction of RE 3+ optically active ions is partitioned into BaF 2 nanocrystals of about 10 nm size [8,19]. The Eu 3+ -luminescence signal enhancement is accompanied by an increase of luminescence lifetime, from 0.27 ms in dried xerogel to 4.7 ms in the glass ceramic [8].
Thermoluminescence measurements recorded after X-ray irradiation of RE 3+ -doped SiO 2 -BaF 2 glass ceramic indicated that new deep trap levels are introduced by the RE 3+doping: ∼ =140 • C (for Ho 3+ , Dy 3+ ), 340 • C (for Sm 3+ ), and 370 • C (for Eu 3+ ); in undoped glass ceramic, the TL peak was observed at 383 • C. The glow peaks were assigned to the recombination of RE 3+ -related electron traps located mainly inside the BaF 2 nanocrystals (Figure 10). Within the series, the trivalent lanthanide ions act as increasingly deeper electron trapping centers [34][35][36][37][38] and this can be observed as glow peaks shift in the temperature scale [36]. The glow peaks energy within the energy levels model proposed for the crystals might be influenced by the nanosize-related effects on the band gap energy.

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11 of 23 Figure 10. TL curves recorded on undoped (dotted line) and RE 3+ -doped glass ceramics (solid lines) after X-ray irradiation at room temperature (reproduced from reference [44]); the energy levels scheme of lanthanides in YPO4 (reproduced from reference [34]).

SiO2-PbF2 Oxyfluoride Glass ceramic
The first study on Er 3+ -doped SiO2-PbF2 oxyfluoride glass ceramic made by Luo et al. [44] assigned the crystallization of β-PbF2 phase to a diffusion-controlled process of threedimensional growth with decreasing nucleation. It was assumed that Er 3+ -ions segregated at the surface of the crystallites and hindered the growth of β-PbF2, thus postponing the Figure 10. TL curves recorded on undoped (dotted line) and RE 3+ -doped glass ceramics (solid lines) after X-ray irradiation at room temperature (reproduced from reference [44]); the energy levels scheme of lanthanides in YPO 4 (reproduced from reference [34]).

SiO 2 -PbF 2 Oxyfluoride Glass Ceramic
The first study on Er 3+ -doped SiO 2 -PbF 2 oxyfluoride glass ceramic made by Luo et al. [44] assigned the crystallization of β-PbF 2 phase to a diffusion-controlled process of three-dimensional growth with decreasing nucleation. It was assumed that Er 3+ -ions segregated at the surface of the crystallites and hindered the growth of β-PbF 2 , thus postponing the crystallization. Later on, the studies were extended to crystallization behavior, structural investigation, and optical properties of other rare earth doped SiO 2 -PbF 2 oxyfluoride glass ceramics.
The up-conversions properties of transparent 0.3Yb 3+ /0.1Er 3+ (mol%) co-doped 90SiO 2 -10PbF 2 oxyfluoride glass-ceramics were extensively studied by J. del-Castillo et al. [47,48]. X-ray diffraction and electron microscopy analysis showed the precipitation of cubic β-PbF 2 nanocrystals varying from 5 to 25 nm depending on heat treatment conditions at low temperatures, i.e., 300-400 • C. The up-conversion luminescence spectra showed green (( 2 H 11/2 , 4 S 3/2 ) → 4 I 15/2 ) and red ( 4 F 9/2 → 4 I 15/2 ) luminescence at 520-540 and 660 nm, respectively, accompanied by weaker blue luminescence at 410 nm ( 2 H 9/2 → 4 I 15/2 ) all assigned to the Er 3+ transitions; the Stark splitting is consistent with the ions incorporation within the nanocrystals that assure an efficient energy transfer between ions ( Figure 11). The analysis of the dynamics of the up-conversion emissions indicated that distinct energy transfer excitation pathways are responsible for populating the luminescent levels: two and three photon processes for red-visible emission at 520-540 nm and 660 nm, and Figure 11. Up-conversion emission spectra of Er 3+ -doped SiO 2 -PbF 2 glass ceramic under 980 nm IR light pumping at different powers; the inset shows emission spectrum excited at 378 nm (reproduced from reference [47]).
The analysis of the dynamics of the up-conversion emissions indicated that distinct energy transfer excitation pathways are responsible for populating the luminescent levels: two and three photon processes for red-visible emission at 520-540 nm and 660 nm, and blue emission at 410 nm, respectively. Moreover, white up-conversion luminescence was obtained by combining the blue and red upconversion emissions band of Tm 3+ with the upconversion green emission bands of Er 3+ or Ho 3+ and by using different pump powers and ratios between co-dopants [48].

SiO 2 -MeF 3 Oxyfluoride Glass Ceramic (with Me = La, Y, Gd, Ce)
Rare-earth doped trifluorides and the corresponding oxyfluoride glass ceramics are attractive optical materials because of their wide band-gap, high solubility of rare-earth ions without additional charge compensation, and possible energy transfer (ET) processes to other co-dopant ions (for Gd 3+ and Ce 3+ ).

SiO 2 -LaF 3 Oxyfluoride Glass Ceramic
The first studies on the glass ceramization investigations in the SiO 2 -LaF 3 system revealed the importance of controlling the synthesis and heat treatment parameters [5][6][7]. Recent investigations on the crystallization process of SiO 2 -LaF 3 glass-ceramics indicated a chemical reaction, followed by the fast precipitation of LaF 3 crystals and not diffusioncontrolled nucleation and growing process, as in the melt-quenched glasses [10]. Structural and optical investigations of nano-structured Eu 3+ -doped SiO 2 -LaF 3 transparent glassceramics calcinated at 800 • C [49] were used to investigate the Eu 3+ ions partition in both glassy and crystalline phases. Site selective luminescence spectroscopy revealed the 393 nm excitation peak assigned to the Eu 3+ ions in the silica glassy phase, whereas the 396 nm peak would correspond to Eu 3+ ions partitioned into LaF 3 nanocrystals (Figure 12). It is concluded that about half the Eu 3+ ions are partitioned into fluoride nanocrystals, while the rest remains in the glassy phase. 393 nm excitation peak assigned to the Eu 3+ ions in the silica glassy phase, whereas the 396 nm peak would correspond to Eu 3+ ions partitioned into LaF3 nanocrystals ( Figure 12). It is concluded that about half the Eu 3+ ions are partitioned into fluoride nanocrystals, while the rest remains in the glassy phase. Figure 12. Excitation spectra of Eu 3+ -doped LaF3-SiO2 glass ceramics recorded at indicated wavelengths; the spectra of 89.9SiO2-10LaF3-0.1EuF3 glassy sample and SiO2:Eu 3+ sol-gel glass detected at 590 nm are also included (reproduced from reference [49]).
The liquid sol transformation to xerogel and then Eu-doped SiO 2 -LaF 3 glass ceramic was investigated by using photoluminescence spectroscopy [20,50,51]. The Eu 3+ luminescence spectra and luminescence decay analysis showing a double-exponential character (with τ 1 ( 5 D 0 ) = 2.07 ms, τ 2 ( 5 D 0 ) = 8.07 ms and τ 1 ( 5 D 0 ) = 0.79 ms, τ 2 ( 5 D 0 ) = 9.76 ms for powders and glass ceramics, respectively) indicated the incorporation of optically active Eu 3+ ions from amorphous silica framework into the low phonon energy LaF 3 nanocrystalline phase. Moreover, the intensity ratio between the two visible emissions ( 5 D 0 -7 F 2 / 5 D 0 -7 F 1 ) was used to analyze the local symmetry around Eu 3+ ions: the observation of the ratio value decrease is consistent with the ions' incorporation within the LaF 3 nanocrystals.

SiO 2 -YF 3 Oxyfluoride Glass Ceramic
Structural and morphological investigations of Eu 3+ -doped SiO 2 -YF 3 sol-gel nanoglass ceramics have indicated the precipitation of YF 3 nanocrystals with an average diameter of about 15 nm [57]. The reducing of the ( 5 D 0 -7 F 2 / 5 D 0 -7 F 1 ) intensity ratio value from 2.82 to 0.66 as well as bi-exponential character of decay curves (with shorter and longer lifetimes, 0.86 and 1.14 ms) was assigned to the incorporation of Eu 3+ ions in both glassy and YF 3 nanocrystalline phases, respectively.
An exhaustive analysis of the luminescence properties of (Eu 3+ , Sm 3+ ) doped SiO 2 -YF 3 structured glass-ceramic containing nanocrystals of around 11 nm was performed by A. Carlos Yanes et al. [23]. Selective excitation wavelengths and luminescence decay measurements allowed discerning between ions residing in precipitated YF 3 nanocrystals and those remaining in a glassy environment; a large fraction of optically active ions is efficiently partitioned into nanocrystals. Moreover, for Yb 3+ -Tm 3+ co-doped samples, bright and efficient up-conversion was observed, as well as very intense high-energy emissions in the UV range strongly dependent on the Yb 3+ doping level opening the way to developing short-wavelength solid-state lasers for various photonic related applications [23]. The investigations of Tb 3+ /Eu 3+ co-doped sol-gel glass ceramic materials containing MF 3 (M = Y, La) nanocrystals showed Tb 3+ → Eu 3+ energy transfer process (ET) accompanied by multicolor emission due to the visible 4f n -4f n transitions of Tb 3+ and Eu 3+ ions [58].

SiO 2 -MeF 3 Oxyfluoride Glass Ceramic (with Me = Gd, Ce)
Oxyfluoride glass ceramics containing GdF 3 or CeF 3 nanocrystals dispersed in the SiO 2 matrix have been obtained and the optical properties of RE 3+ -ions studied. The energy levels overlap between the 6 P J states of Gd 3+ and the UV-excited states of RE-ions promote an efficient energy transfer from Gd 3+ to the RE 3+ ions resulting in their characteristic photoluminescence. Crystalline phases analysis of RE 3+ -doped SiO 2 -GdF 3 oxyfluoride glass ceramics has shown the precipitation of both hexagonal and orthorhombic GdF 3 nanocrystals [59,60] or only orthorhombic one [61][62][63] in the silica matrix, depending on the synthesis path and the chemical composition of the system [50].
The luminescence spectra and luminescence decay measurements recorded on Eu 3+ /Tb 3+ doped SiO 2 -GdF 3 oxyfluoride glass ceramics indicate the incorporation of the RE 3+ ions within the nanocrystalline phase and silica glass matrix (Figure 14). The energy transfer process from Gd 3+ to Eu 3+ or Tb 3+ ions was evidenced by the excitation spectra and luminescence spectra recorded under Gd 3+ ions excitation wavelength at 273 nm [63]. by multicolor emission due to the visible 4f n -4f n transitions of Tb 3+ and Eu 3+ ions [58].

SiO2-MeF3 Oxyfluoride Glass ceramic (with Me = Gd, Ce)
Oxyfluoride glass ceramics containing GdF3 or CeF3 nanocrystals dispersed in the SiO2 matrix have been obtained and the optical properties of RE 3+ -ions studied. The energy levels overlap between the 6 PJ states of Gd 3+ and the UV-excited states of RE-ions promote an efficient energy transfer from Gd 3+ to the RE 3+ ions resulting in their characteristic photoluminescence. Crystalline phases analysis of RE 3+ -doped SiO2-GdF3 oxyfluoride glass ceramics has shown the precipitation of both hexagonal and orthorhombic GdF3 nanocrystals [59,60] or only orthorhombic one [61][62][63] in the silica matrix, depending on the synthesis path and the chemical composition of the system [50].
The luminescence spectra and luminescence decay measurements recorded on Eu 3+ /Tb 3+ doped SiO2-GdF3 oxyfluoride glass ceramics indicate the incorporation of the RE 3+ ions within the nanocrystalline phase and silica glass matrix (Figure 14). The energy transfer process from Gd 3+ to Eu 3+ or Tb 3+ ions was evidenced by the excitation spectra and luminescence spectra recorded under Gd 3+ ions excitation wavelength at 273 nm [63]. Recently we used Fujuhara's approach [5] for the synthesis of Eu-doped and Yb/Er co-doped SiO 2 -GdF 3 and SiO 2 -LiGdF 4 [64,65] glass ceramics and the key role played by the nature of the RE-dopant ions and Li co-dopants ions in the stabilization of orthorhombic GdF 3 phase: the nanocrystals size increase from 9 nm to 25 nm is accompanied by strong lattice distortion evidenced by XRD peaks shift towards higher angles [65] (Figure 15). Under 980 nm IR light pumping, we observed up-conversion luminescence signal assigned to the Er 3+ ions ( Figure 6), with more than one order of magnitude higher in Li co-doped glass ceramic. Photoluminescence spectra recorded in Eu 3+ doped SiO2-GdF3 glass ceramic (left) and Tb 3+ doped SiO2-GdF3 glass ceramic excited at Gd 3+ or Eu 3+ /Tb 3+ excitation wavelength peaks (reproduced from reference [63]).
Recently we used Fujuhara's approach [5] for the synthesis of Eu-doped and Yb/Er co-doped SiO2-GdF3 and SiO2-LiGdF4 [64,65] glass ceramics and the key role played by the nature of the RE-dopant ions and Li co-dopants ions in the stabilization of orthorhombic GdF3 phase: the nanocrystals size increase from 9 nm to 25 nm is accompanied by strong lattice distortion evidenced by XRD peaks shift towards higher angles [65] (Figure 15). Under 980 nm IR light pumping, we observed up-conversion luminescence signal assigned to the Er 3+ ions ( Figure 6), with more than one order of magnitude higher in Li codoped glass ceramic. Figure 15. X-ray diffraction patterns of the Yb/Er co-doped SiO2-GdF3 xerogel (curve a) and glass ceramics undoped (curve b) and Li (1%) co-doped (c) [65]; the PDF file of orthorhombic GdF3 is shown for comparison.
For RE 3+ -doped SiO2-CeF3 sol-gel glass-ceramics with RE = Eu, Sm, Yb/Er, structural and morphologic characterization has showed precipitation of the CeF3 nanocrystals [66]. The analysis of the intense red-orange emissions due to Eu 3+ and Sm 3+ transitions and the excitation spectra confirmed the partition of a large fraction of these ions into the precipitated CeF3 nanocrystals. A cross-relaxation process occurs between Eu 3+ and Ce 3+ ions and 5 5 Figure 15. X-ray diffraction patterns of the Yb/Er co-doped SiO 2 -GdF 3 xerogel (curve a) and glass ceramics undoped (curve b) and Li (1%) co-doped (c) [65]; the PDF file of orthorhombic GdF 3 is shown for comparison.
For RE 3+ -doped SiO 2 -CeF 3 sol-gel glass-ceramics with RE = Eu, Sm, Yb/Er, structural and morphologic characterization has showed precipitation of the CeF 3 nanocrystals [66]. The analysis of the intense red-orange emissions due to Eu 3+ and Sm 3+ transitions and the excitation spectra confirmed the partition of a large fraction of these ions into the precipitated CeF 3 nanocrystals. A cross-relaxation process occurs between Eu 3+ and Ce 3+ ions and therefore the emissions from higher 5 D 1 and 5 D 0 energy levels are inhibited by the phonon-assisted energy transfer between the ions. In addition, in the Yb/Er codoped samples, the presence of Ce 3+ ions as a phonon-assisted cross-relaxation channel is responsible for a strong emission at 1.5 µm, which also leads to a drastic inhibition of the up-conversion emission.

Thernary and More Complex Oxyfluoride Glass Ceramic
The sol-gel route has been used to obtain other transparent glass ceramic compositions comprising thernary fluoride nanocrystalline Me1Me2F4 phases such as: SiO 2 -NaYF 4 [67], SiO 2 -NaLaF 4 [68], SiO 2 -NaGdF 4 [69], SiO 2 -KLaF 4 [70], SiO 2 -KYF 4 [71], SiO 2 -LiYF 4 [30,72,73] and their properties have been investigated. In these cases, the crystallization mechanism seems to be more complicated and the decomposition of metal trifluoroacetates is likely to be accompanied by some chemical reaction between metal and fluorine partners, followed by nanocrystalline phase precipitation within the glassy matrix. It was observed that the nature of the final precipitate crystalline phase is strongly dependent on the molar ratio between trivalent ion (Y, Gd, La) and alkali metals ions (Li, Na or K), and in general mixtures of fluorides were obtained (for LaF-NaLaF 4 [68] and YF 3 -LiYF 4 [30,72,73]) or different phase mixtures of the same compound (cubic and/or hexagonal NaGdF 4 or KLaF 4 nanocrystals [69,70]). Hence, an optimization of the initial composition and processing parameters (time and temperature) was required to obtain precipitation of the desired crystalline phase.
Optical properties of the Eu 3+ ions were used to investigate local structure and xerogel to glass ceramic transformations in SiO 2 -NaYF 4 [67] and SiO 2 -LiYF 4 [30] glass ceramics. For Eu 3+ -doped SiO 2 -NaYF 4 glass ceramics, cubic-NaYF 4 nanocrystals of about 4-10 nm size precipitated during the thermal processing [67]. Site selective spectroscopy measurements allowed discerning ions in the amorphous silica glassy phase from those preferentially partitioned into a like-crystalline environment. Hence, a noticeable enhancement of the 613 nm luminescence from 3.1 to 5.7 ms by changing from amorphous surroundings (exciting at 464 nm) to those ions partitioned into nanocrystals (exciting at 392 nm) was observed. Moreover, the red to green emissions ratio ( 5 D 0 -7 F 2 / 5 D 0 -7 F 1 ) diminishes with the heat treatment indicating the partition of Eu 3+ ions in the nanocrystals. In the SiO 2 -LiYF 4 [72,73] glass ceramic, the precipitation of the LiYF 4 nanocrystals was observed only for high Li excess (up to four molar Li/Y ratio). For a lower molar ratio, a glass-ceramic containing a mixture of YF 3 and LiYF 4 crystals or only YF 3 phase (i.e., for stoichiometric ratio) was obtained [73]. The enhancement of Eu 3+ -lifetime values from 0.22 ms in the xerogel to 8.68 ms and the diminishing of the red to green emissions ratio were associated to the xerogel to glass ceramic transformation and Eu 3+ ions partition in the nanocrystals [30]. Moreover, the group-theoretical analysis of the photoluminescence spectra has indicated that the Eu 3+ ions incorporation occurs dominantly inside the LiYF 4 nanocrystals with lower symmetry (C 2v ) sites than in polycrystalline pellet (D 2d ).
The investigations of the Nd 3+ -doped SiO 2 -NaLaF 4 glass ceramic showed both LaF 3 /NaLaF 4 crystalline phases for different initial compositions and annealing temperatures; the crystallization of NaLaF 4 was only promoted for Na deficiency in the precursors and higher temperatures calcination, above 650 • C [68]. The incorporation of Nd 3+ ion into NaLaF 4 and LaF 3 nanocrystals was confirmed by site-selective emission and excitation spectra ( Figure 16). cies for the red and green UC luminescences are much lower in the glass and η = 3.5%) compared to the pellet (η = 46% and η = 21%) [73], but sim co-doped glass ceramic containing CaF2 nanocrystals [40]. Moreover, th red and green up-conversion emission bands can be varied as a functio temperature and pump power resulting in color tunable up-conversion p The investigations of the Nd 3+ -doped SiO2-NaLaF4 glass ceram LaF3/NaLaF4 crystalline phases for different initial compositions and ann tures; the crystallization of NaLaF4 was only promoted for Na deficiency and higher temperatures calcination, above 650 °C [68]. The incorporation NaLaF4 and LaF3 nanocrystals was confirmed by site-selective emissio spectra ( Figure 16). For the SiO 2 -NaGdF 4 glass ceramic, the precipitation of cubic and/or hexagonal NaGdF 4 nanocrystals with a size ranging between 4 and 24 nm was observed, depending on the Na:Gd ratio and processing conditions (temperature and time) [69]. The Na:Gd ratio was optimized to obtain the crystallization of β-NaGdF 4 phase, more adequate for luminescent applications and for a molar ratio 0.95:1, the precipitation of β-NaGdF 4 (JCPDS 027-0699) phase was observed after treatment at 550 • C. Luminescence results showed Eu 3+ ions' incorporation mainly in NaGdF 4 NCs, and an efficient energy transfer Gd 3+ to Eu 3+ was observed. Electron microscopy investigations of SiO 2 -KLaF 4 glass ceramics confirmed the coexistence of cubic (α-phase) and hexagonal (β-phase) KLaF 4 phases, the last one being favored for high temperature calcination [70]. The spectral features of the Nd 3+ ions dopants confirmed the incorporation of Nd 3+ ions in both crystalline phases, with emission of Nd 3+ predominantly in the β-KLaF 4 hexagonal phase.
The UV light excitation of Ce 3+ /Tb 3+ /Eu 3+ triply-doped SiO 2 -KYF 4 glass ceramics is accompanied by an efficient ET processes between Ce 3+ and Tb 3+ /Eu 3+ ions, followed by their characteristic green and reddish-orange emission, respectively [71]. The emitted color can be tuned by varying the content of Eu 3+ ions and/or the excitation wavelength, and white light generation was reached ( Figure 17). In SiO 2 -KYF 4 transparent glassceramics comprising Yb 3+ , Er 3+ , Tm 3+ co-doped KYF 4 nanocrystals, white light generation was obtained by simultaneous red, green, and blue additive up-conversion emissions of the RE 3+ -ions dopants [75]. companied by an efficient ET processes between Ce 3+ and Tb 3+ /Eu 3+ ions, followed by their characteristic green and reddish-orange emission, respectively [71]. The emitted color can be tuned by varying the content of Eu 3+ ions and/or the excitation wavelength, and white light generation was reached ( Figure 17). In SiO2-KYF4 transparent glass-ceramics comprising Yb 3+ , Er 3+ , Tm 3+ co-doped KYF4 nanocrystals, white light generation was obtained by simultaneous red, green, and blue additive up-conversion emissions of the RE 3+ -ions dopants [75]. Figure 17. Emission spectra of 0.3Ce 3+ /0.3Tb 3+ /0.6Eu 3+ (mol %) triply-doped SiO2-KYF4 glass ceramics under UV excitation of Ce 3+ by comparison to the direct excitation of Eu 3+ and Tb 3+ ions, showing the Eu 3+ and Tb 3+ luminescence peaks (reproduced from reference [71]).
More complex sol-gel glass ceramics, containing RE 3+ -doped BaYF 5 and BaGdF 5 nanocrystals embedded in silica matrix, have been produced by appropriate annealing of the corresponding precursor xerogel [76,77]. Optical measurements confirmed the distribution of a significant fraction of RE 3+ ions into the precipitated BaYF 5 fluoride nanocrystals environment [76]. For RE 3+ -doped (Eu 3+ , Sm 3+ , Dy 3+ or Tb 3+ ) glass ceramics containing BaGdF 5 nanocrystals, intense visible emissions through efficient energy transfer from Gd 3+ to RE 3+ ions were observed, which leads to consider these materials as potential emitting phosphors for color converted UV LED [77]. Sol-gel glass ceramics comprising Tm 3+ co-doped SiO 2 -BaY 0.78−x Gd x Yb 0.2 Tm 0.02 F 5 (x = 0-0.78) nanocrystals showed UC luminescence in the NIR, visible, and UV range assigned to the Tm 3+ -dopant ions distributed into nanocrystalline environments [78]. The substitution of Y 3+ by Gd 3+ leads to a continuous decrease of the UV emission intensities due to an effective energy transfer between Tm 3+ and Gd 3+ ions.
As RE 3+ -doped Lu-based fluorides are investigated for their applications as scintillators due to high absorption cross-section for any kind of radiation, the sol-gel route has been used to prepare RE-doped SiO 2 -Sr 2 LuF 7 glass ceramics and their structural and optical properties investigated [79]. The luminescent features of Eu 3+ ions, used as structural probes, revealed the distribution of RE 3+ ions in both glass matrix and fluoride nanocrystals. Under 980 nm laser excitation of Yb 3+ -Tm 3+ , Yb 3+ -Er 3+ and Yb 3+ -Ho 3+ co-doped glass ceramics, intense UV, vis and NIR up-conversion emissions were observed.

Oxychloride Glass Ceramic
Sol-gel glass ceramics containing Er 3+ -doped BaCl 2 nanocrystals of about tens of nm size embedded in a silica matrix were prepared but by replacing TFA with trichloroacetic acid (CCl 3 COOH) [80]. The crystallization process relies on thermal decomposition of Ba-trichloroacetate at about 300 • C followed by subsequent growth into BaCl 2 -nanocrystals at 745 • C; Er 3+ -ions are incorporated during the nanocrystals' growth. Under 810 nm laser light pumping, it shows green (( 2 H 11/2 , 4 S 3/2 ) → 4 I 15/2 ) and red ( 4 F 9/2 → 4 I 15/2 ) Er 3+ upconversion luminescences, much weaker (only about 10%) compared to NaYF 4 :Er 3+ (18%). Glassy nanocomposites containing Eu 3+ -doped LaOCl nanocrystals of about tens of nm size embedded in a silica matrix were prepared with the sol-gel route using trichloroacetic acid (CCl 3 COOH) as a chlorination agent [81]. It was shown that the LaOCl nanocrystalline phase precipitation is the result of the lanthanum chloride hydrolytic and oxidative reactions. As the annealing temperature increases, nanocrystals grow up to tens of nm size and Eu 3+ -ions are gradually incorporated inside the LaOCl nanocrystals, with a (C 4v ) local coordination symmetry.

Perspectives and Applications
In summary, the synthesis and investigations of sol-gel derived oxyfluoride glass ceramics are based and limited to the synthesis initially developed by Fujihara et al. [5][6][7], using TMOS and/or TEOS as the SiO 2 precursor and TFA as the fluorine source and active fluoride crystalline phase fraction of about 5%, much lower than in melt-quenched glass ceramics. Most of the research efforts were dedicated to oxyfluoride glass ceramics, being focused primarily on optical (luminescence) properties of the RE-doped nanocrystals. Therefore, future research efforts are expected to be dedicated to the discovery of new glass ceramic materials with multifunctional properties (optical, electric, and magnetic) for new and improved applications.
Transparent glass ceramic waveguides offer specific characteristics of capital importance in photonics and the sol-gel approach has proved to be a very convenient and flexible way to deposit glass ceramic thin films on a variety of substrates for different applications, such as planar waveguides or integrated optics. New glass ceramic thin films comprising RE-doped oxide semiconducting nanocrystals such as SnO 2 , ZrO 2 , CeO 2 , HfO 2 have been already obtained and optical properties studied [82][83][84][85][86]. The investigations of both optical and electric properties would be interesting for both basic and applied research.
New developments of the glass ceramic films can include patterned glass ceramic thin films that can be produced by using laser-induced crystallization [87] allowing the development of active integrated optical circuits. Moreover, a higher crystallized fraction of the nominal active fluoride crystalline phase is expected to improve the optical properties. Recent studies have shown that crack-free SiO 2 -LaF 3 glass ceramic films with a crystalline LaF 3 crystalline fraction of 18 wt% can be obtained [18].
The investigations of glass ceramics with Gd 3+ -based fluorides nanocrystals embedded in the glass matrix were limited to the analysis of luminescence properties of the co-dopant RE-ions [61][62][63]. The magnetic properties related to Gd 3+ ions and the influence of the magnetic field on optical properties, in particular on the energy transfer processes, have been overlooked/missed. Such novel, multifunctional magneto-optical materials allow the intertwining between the magnetism and photonics and might offer new opportunities for magneto-optical devices.
Sol-gel synthesis has been used to obtain not only thin films but also monolithic silica and glass ceramics [88,89] opening the opportunity to produce monolithic scintillating glass ceramics based on BaF 2 , CaF 2 , CeF 3 and BaCl 2 . Proper co-doping with broad blue absorbing sensitizer ions (Bi 3+ , Sn 3+ , etc.) capable to transfer their excitation energy to neighboring activator ions might develop potential spectral down-conversion applications.
The optical properties investigations of glass ceramics were focused only on trivalent ions related ones and divalent Eu 2+ and Sm 2+ ions were omitted. Previous investigations [90,91] have shown the incorporation of the reduced Eu 2+ and Sm 2+ ions in sol-gel glasses (not ceramic ones) under moderate conditions of temperature and atmosphere in two steps, glass-formation, and their reduction to the bivalent state by calcination in reducing atmosphere. Eu 2+ and Sm 2+ ions have attracted significant attention because they have a great potential for various photonics-related applications or radiation detection. The Sm 2+ ion shows persistent spectral hole burning (PSHB) and potential application for high density optical memories. On the other hand, Eu 2+ doping is crucial for scintillators detectors or X-ray storage phosphor for digital imaging applications. However, the incorporation of such bivalent ions into the precipitated nanocrystals from the glass ceramics still remain an open problem.
A new approach for sol-gel glass ceramics was recently proposed by Cruz M.E. et al. (2020) [92], where dispersed nanocrystals were incorporated in the silica glass matrix. This approach allows the incorporation in the silica glass matrix of a much broader range of nanocrystalline phosphors such as Eu 2+ -doped persistent phosphors [93] or Sm 3+ -doped BaFCl for multilevel optical data storage applications [94].
As the optical properties are strongly influenced by the hydroxyl ions (present even after high temperatures calcination), new non-aqueous sol-gel approaches are highly desirable [95,96]. A first step was made by using hydrofluoric acid catalyzed sol-gel process [95] and the non-aqueous fluorolytic sol-gel synthesis of metal nano-fluorides [96].

Conclusions
In conclusion, sol-gel derived glass ceramic materials based on stabilized rare-earth doped nanoparticles embedded in a glass matrix were demonstrated as novel, attractive materials for photonics applications. However, the advantages of sol-gel chemistry and thin films' deposition ability have not been fully exploited for advances in both basic and applied research. Therefore, the development of new glass ceramic materials with multifunctional properties for new and improved applications is highly desirable.
Author Contributions: The manuscript was written by M.S. in collaboration with C.S. and C.B. All the authors contributed to discussions and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Romanian Ministry of Research and Innovation (MCI) through the PN19-03 Core Program of NIMP (2020).

Conflicts of Interest:
The authors declare no conflict of interest.