## 3 Results and Discussion  
### 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 CaF\({}_{2}\):Eu\({}^{3+}\) 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\({}^{-1}\) (Q\({}^{n}\) units in SiO\({}_{4}\) tetrahedrons: Q\({}^{4}\), Q\({}^{3}\) and Q\({}^{2}\), respectively) indicates that polycondensation reaction occurred. In addition, we identified the presence of vicinal or geminal Si-OH groups (\(\sim\)3664 cm\({}^{-1}\)), hydrogen-bonded Si-OH moieties (\(\sim\)3398 cm\({}^{-1}\)) and hydrogen-bonded OH groups (\(\sim\)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 \(\sim\)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 \({}^{\circ}\)C up to 500 \({}^{\circ}\)C (inset of Figure 1). The thermal degradation profile with two distinguishable stages was recorded in temperature intervals: 35 \({}^{\circ}\)C-151 \({}^{\circ}\)C (1) and 151 \({}^{\circ}\)C-348 \({}^{\circ}\)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%. Figure 1: Infrared spectroscopy (IR-ATR) spectra of sol-gel materials before and after controlled heat-treatment. Inset shows the thermogravimetric analysis (TG)/differential scanning calorimetry (DSC) curves recorded for prepared xerogels. Beyond the residual acetic acid (used as a catalyst), such acid was also produced during the chemical reaction between TFA and acetate salts:  
\[\text{Ca(CH}_{3}\text{COO)}_{2}+2\text{CF}_{3}\text{COOH}\rightarrow\text{Ca( CF}_{3}\text{COO)}_{2}+2\text{CH}_{3}\text{COOH}\]  
\[\text{Eu(CH}_{3}\text{COO)}_{3}+3\text{CF}_{3}\text{COOH}\rightarrow\text{Eu( CF}_{3}\text{COO)}_{3}+3\text{CH}_{3}\text{COOH}\]  
As was shown, \(\text{CF}_{3}\text{COO}^{-}\) anions could effectively coordinate RE\({}^{3+}\) cations, therefore, the trifluoroacetate ions were chemically bonded. In fact, TFA was introduced as fluorination agent, which allowed for successful precipitation of CaF\({}_{2}\) 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(CF\({}_{3}\text{COO)}_{2}\) and the formation of CaF\({}_{2}\) nanocrystals. The formation of the CaF\({}_{2}\) 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].