## 3 Results and Discussion
### Structural and Thermal Characterization of Sol-Gel Materials
The CaF\({}_{2}\) crystal phase was obtained during thermal decomposition of Ca(CF\({}_{3}\text{COO)}_{2}\) as follows:  
\[\text{Ca(CF}_{3}\text{COO)}_{2}\rightarrow\text{CaF}_{2}+\text{CF}_{3}\text{ CFO}+\text{CO}_{2}+\text{CO}\]  
The decomposition of Ca(CF\({}_{3}\text{COO)}_{2}\) was observed in DSC curve as an exothermic peak with maximum at \(\sim\)318 \({}^{\circ}\)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 \({}^{\circ}\)C was chosen to perform the controlled ceramization process of precursor xerogels in order to precipitate the CaF\({}_{2}\) 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 \(\sim\)1664 cm\({}^{-1}\) was also detected and it was associated to residual Si-OH groups. For heat-treated samples, the shoulders located at \(\sim\)1193 cm\({}^{-1}\) and \(\sim\)1134 cm\({}^{-1}\) were related to Si-O-Si siloxane bridges and SiO\({}_{4}\) tetrahedrons in Q\({}^{4}\) units, respectively. Otherwise, the vibration modes originated from SiO\({}_{4}\) tetrahedrons inside Q\({}^{3}\) (1051 cm\({}^{-1}\)) and Q\({}^{2}\) units (951 cm\({}^{-1}\)) were also identified. Indeed, it growth in density from 1.936 g/cm\({}^{3}\) before controlled ceramization up to 2.201 g/cm\({}^{3}\) 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 \({}^{\circ}\)C per 10 h. The XRD patterns are in good accordance with the diffraction lines of regular CaF\({}_{2}\) 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\({}^{\circ}\), 46.9\({}^{\circ}\), 55.6\({}^{\circ}\), 68.6\({}^{\circ}\) and 75.8\({}^{\circ}\) were identified as (111), (200), (311), (400) and (311) reflexes of CaF\({}_{2}\) phase, respectively. Observed broadening of diffraction lines clearly indicates that CaF\({}_{2}\) phase crystallized in nanometric range and the average crystals size was estimated using the Scherrer formula:  
\[\text{D}\ =\ \frac{\text{K}\lambda}{\beta\cos\theta} \tag{1}\]  
in which D is related to the crystal size, K is a constant value (for our calculations it was taken K = 1), \(\lambda\) is the X-ray wavelength, \(\beta\) is a half width of analyzed diffraction peak and \(\theta\) is the diffraction angle. The mean value of CaF\({}_{2}\) nanocrystals was equaled to 11.7 nm \(\pm\)1.0 nm. Due to similar ionic radii of Ca\({}^{2+}\) (1.00 A) [24] and Eu\({}^{3+}\) (1.07 A) [25], Ca\({}^{2+}\) cations in CaF\({}_{2}\) crystal lattice could be effectively substituted by trivalent Eu\({}^{3+}\) ions and therefore, a very slight shift of diffraction lines was identified. Figure 2b presents TEM image of fabricated glass-ceramics. The size of CaF\({}_{2}\) nanocrystals was consistent with average crystal size estimated from Scherrer equation. The CaF\({}_{2}\) nanocrystals with comparable size (about 10 nm) were obtained by [PERSON] et al.