## 3 Results and Discussion
### Luminescence Behavior of Fabricated Sol-Gel Materials  
Figure 3 shows the photoluminescence excitation spectra (PLE) of prepared xerogels and CaF\({}_{2}\):Eu\({}^{3+}\) glass-ceramics. The spectra were monitored at \(\lambda_{\text{em}}\) = 611 nm wavelength corresponding to the \({}^{5}\)D\(0\rightarrow{}^{7}\)F\({}_{2}\) optical transition of Eu\({}^{3+}\) ions. The recorded excitation lines were identified as the intra-configurational electronic transitions inside 4\({}^{6}\) manifold of Eu\({}^{3+}\) optically active ions. The bands were assigned to transitions from the \({}^{7}\)F\({}_{0}\) ground level to the excited states: \({}^{5}\)D\({}_{4}\) (363 nm), \({}^{5}\)G\({}_{J}\), \({}^{5}\)L\({}_{J}\) (372 nm-389 nm), \({}^{5}\)L\({}_{6}\) (395 nm-xerogel, 394 nm/398 nm-CaF\({}_{2}\):Eu\({}^{3+}\) GCs) and \({}^{5}\)D\({}_{2}\) (465 nm-xerogel, 464 nm-CaF\({}_{2}\):Eu\({}^{3+}\) GCs). In addition, the weak bands according to the \({}^{7}\)F\({}_{0}\rightarrow{}^{5}\)D\({}_{3}\) (416 nm) and \({}^{7}\)F\({}_{0}\rightarrow{}^{5}\)D\({}_{1}\) (525 nm) transitions were recorded. Since, the \({}^{7}\)F\({}_{1}\) level is thermally populated at room temperature, a satellite line corresponding to the \({}^{7}\)F\({}_{1}\rightarrow{}^{5}\)D\({}_{1}\) (535 nm) was also observed. It was observed that for xerogels the \({}^{7}\)F\({}_{0}\rightarrow{}^{5}\)L\({}_{6}\) excitation band has only one maximum, meanwhile for prepared CaF\({}_{2}\):Eu\({}^{3+}\) GC samples such a line is a double peak with two splitted components located at 394 nm and 398 nm. Since Eu\({}^{3+}\) ions could be distributed between silicate sol-gel host and CaF\({}_{2}\) nanocrystals in fabricated GCs, such splitting into two components could indicate that Eu\({}^{3+}\) ions are residing in two different frameworks, as was suggested by [PERSON] for sol-gel LaF\({}_{3}\):Eu\({}^{3+}\) GCs [31]. Thus, 394 nm component could be associated with their location within silicate network, meanwhile 398 nm component could be related with location of Eu\({}^{3+}\) in fluoride nanocrystals. Based on paper by [PERSON] et al. [32], the excitation spectra of pure CaF\({}_{2}\):Eu\({}^{3+}\) crystal phase revealed only one component of the \({}^{7}\)F\({}_{0}\rightarrow{}^{5}\)L\({}_{6}\) band. Therefore, it may confirm the hypothesis that visible splitting of indicated excitation band could be related with distribution of Eu\({}^{3+}\) ions between CaF\({}_{2}\) nanocrystals and silicate sol-gel host. Since the most intense excitation line corresponded to the \({}^{7}\)F\({}_{0}\rightarrow{}^{5}\)L\({}_{6}\) transition for fabricated sol-gel samples, the appropriate wavelengths were used to perform the emission measurements. The photoluminescence spectrum (PL) registered for precursor xerogels is shown in Figure 4. Generally, the recorded spectrum consisted from five emission bands assigned to the transitions from the \({}^{5}\)D\({}_{0}\) excited state into the \({}^{7}\)F\({}_{l}\) levels: \({}^{7}\)F\({}_{0}\) (578 nm), \({}^{7}\)F\({}_{1}\) (591 nm), \({}^{7}\)F\({}_{2}\) (616 nm), \({}^{7}\)F\({}_{3}\) (649 nm) as well as \({}^{7}\)F\({}_{4}\) (697 nm). Since the relative intensities of Eu\({}^{3+}\) emission bands were strictly dependent on symmetry in their local framework, the emission spectra gave valuable information about the nearest  
Figure 3: Photoluminescence excitation spectra (PLE) spectra registered for fabricated xerogels and CaF\({}_{2}\):Eu\({}^{3+}\) GCs. framework around them in host matrix [32]. It was observed that for silicate xerogels an emission band assigned to the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) electric-dipole transition was more intense compared to a line assigned to the \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) magnetic-dipole transition. The \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) transition was very sensitive to the symmetry in the local vicinity around Eu\({}^{3+}\) ions and it was hypersensitive in nature.