## 3 Results and Discussion
### Luminescence Behavior of Fabricated Sol-Gel Materials
As was presented in Figure 5, the emission spectrum of CaF\({}_{2}\):Eu\({}^{3+}\) GCs also consisted of the characteristic bands of Eu\({}^{3+}\) ions corresponding to the intra-configurational \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{\rm J}\) transitions within the 4\({}^{6}\) 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 \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) 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 Eu\({}^{3+}\) 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 CaF\({}_{2}\) fluoride phase. The \({}^{7}\)F\({}_{\rm J}\) energy levels of Eu\({}^{3+}\) 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 [PERSON] et al. [39], Eu\({}^{3+}\) ions are located in C\({}_{4b}\) symmetry sites in the CaF\({}_{2}\) crystal lattice, despite the fact that Ca\({}^{2+}\) cations are located in O\({}_{h}\) symmetry sites. This effect could be explained by charge compensation, when divalent Ca\({}^{2+}\) cations in CaF\({}_{2}\) crystal lattice are substituted by trivalent Eu\({}^{3+}\) 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 Eu\({}^{3+}\) ion is located in the C\({}_{4b}\) site in the CaF\({}_{2}\) crystal  
Figure 5: PL spectra recorded for fabricated CaF\({}_{2}\):Eu\({}^{3+}\) GCs. The inset presents the luminescence decay curve of the \({}^{5}\)D\({}_{0}\) state of Eu\({}^{3+}\) (\(\lambda_{\rm exc}\) = 394 nm, \(\lambda_{\rm em}\) = 589 nm). lattice, the individual \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{7}\) (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 Eu\({}^{3+}\) ions are distributed both in amorphous silicate sol-gel network as well as CaF\({}_{2}\) nanocrystals. Hence, we assumed that the influence of the crystal field is largely masked. The photoluminescence decay curve of the \({}^{5}\)D\({}_{0}\) state of Eu\({}^{3+}\) 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 (\(\tau_{1}\) = 1.63 ms) and slow (\(\tau_{2}\) = 12.84 ms). The double-exponential decay clearly indicates that two different decay channels are involved in the total decay process from the \({}^{5}\)D\({}_{0}\) excited state of Eu\({}^{3+}\). The contribution of fast and slow components could be determined using fitting constants in the following equation:  
\[\tau_{\rm n},\%\ =\ \frac{\rm A_{n}}{\rm A_{n}+A_{m}}\cdot 100\% \tag{2}\]  
Since the A\({}_{1}\) and A\({}_{2}\) fitting constants were close to 1.9408\(\times\) 10\({}^{6}\) and 2.7539\(\times\) 10\({}^{6}\), respectively, a percentage contribution of \(\tau_{1}\) was equal to 41.34% and the contribution of \(\tau_{2}\) was 58.66%. The first of the luminescence lifetime, \(\tau_{1}\), was associated with Eu\({}^{3+}\) ions located in the silicate sol-gel host consisting of Q\({}^{3}\) units of SiO\({}_{4}\) tetrahedrons (\(\sim\)1051 cm\({}^{-1}\)) and residual Si-OH groups (3390 cm\({}^{-1}\), 3656 cm\({}^{-1}\)).