## 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 Eu\({}^{3+}\)-doped xerogels with nominal composition (in molar ratio): TEOS:C\({}_{2}\)H\({}_{5}\)OH:H\({}_{2}\)O: CH\({}_{3}\)COOH = 1:4:10:0.5 (90 wt.%) CF\({}_{3}\)COOH:Ca(CH\({}_{3}\)COO)\({}_{2}\):Eu(CH\({}_{3}\)COO)\({}_{3}\) = 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(CH\({}_{3}\)COO)\({}_{2}\) and Eu(CH\({}_{3}\)COO)\({}_{3}\) 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 \({}^{\circ}\)C for seven weeks to form transparent and colorless xerogels. To fabricate the glass-ceramics containing CaF\({}_{2}\) nanocrystals, the xerogels were heat-treated in a muffle furnace FCF 5 5 SHP (Czylok, Jastrzebie-Zdroj, Poland) at 350 \({}^{\circ}\)C. The temperature was raised by 10 \({}^{\circ}\)C/min until the direct temperature (350 \({}^{\circ}\)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 \({}^{\circ}\)C/min and the curves were registered within temperature range from 40 \({}^{\circ}\)C to 500 \({}^{\circ}\)C. To verify the crystallization of the CaF\({}_{2}\) phase, the X-ray diffraction (XRD) analysis was carried out using an X'Pert Pro diffractometer supplied by PANalytical (Almelo, The Netherlands) with Cu K\(\alpha\) radiation. The TEM microscopy was also used and the CaF\({}_{2}\) 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 CaF\({}_{2}\):Eu\({}^{3+}\) 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 \(\pm\)0.1 nm. Decay curves were detected with the accuracy of \(\pm\)2 \(\upmu\)s. All structural and photoluminescence measurements were performed at room temperature.