## 1 Introduction  
Transparent glass-ceramics (GCs) containing fluoride nanocrystals fabricated using high-temperature melt-quenching or low-temperature sol-gel method are suitable modern materials for lasers, optical waveguides, solid-state lighting and numerous photonic applications [1; 2; 3; 4; 5]. Depending on chemical composition and technological conditions, nano- or micro-crystals are usually well-formed during temperature-controlled crystallization of precursor glasses or xerogels. The heat-treatment process often introduces transformation from amorphous systems to transparent oxyfluoride glass-ceramic materials and rare earths play the role as optically active ions. The coordination sphere around rare earth ions changes drastically during this structural transformation, giving important contribution to the luminescence characteristics.  
Among binary and ternary rare-earth fluoride nanophosphors [6], calcium fluoride CaF\({}_{2}\) belongs to the most important and perspective nanoparticles, which could be successfully formed during the heat treatment process. CaF\({}_{2}\) fluoride nanocrystals were well formed from Na\({}_{2}\)O/K\({}_{2}\)O/CaO/CaF\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) glass [7] and their mean crystallite sizes were in the range from 8 to 10.4 nm and did not get larger with time or increasing the annealing temperature. However, synthesis, structure and properties of transparent glass-ceramics containing CaF\({}_{2}\) nanocrystals depended critically on treating temperature and glass composition, where SiO\({}_{2}\) was substituted by CaO/CaF\({}_{2}\)[8]. A special attention has been paid to crystallization processes and spectroscopic properties of erbium-doped transparent glass ceramics containing CaF\({}_{2}\) nanocrystals [9; 10; 11]. In this CaF\({}_{2}\):Er\({}^{3+}\) glass-ceramic system, near-IR luminescence at 1.53 \(\upmu\)m [12; 13] and up-conversion processes [14; 15] of Er\({}^{3+}\) ions have been examined in detail. The results for the crystallization processes and fluorescence properties of Nd\({}^{3+}\)-doped glass-ceramics with CaF\({}_{2}\) nanocrystals have been also presented and discussed [16; 17]. Recently, several research groups have been focused on glass-ceramic materials containing CaF\({}_{2}\) nanocrystals for numerous photonic applications. [PERSON] et al. [18] suggest that CaF\({}_{2}\):Dy\({}^{3+}\) nano-glass-ceramics are promising for white light generation. Moreover, the oxyfluoride glass-ceramics containing CaF\({}_{2}\):Sm\({}^{3+}\) crystals have become a new fast erasable dosimetric detector material for micro-beam radiation cancer therapy applications at the Canadian Synchrotron [19].  
In this work, oxyfluoride silicate xerogels containing europium ions were heat-treated at 350 \({}^{\circ}\)C to obtain transparent glass-ceramics containing crystalline fluoride nano-phase CaF\({}_{2}\). Luminescence spectra of Eu\({}^{3+}\) ions and their decays in transparent glass-ceramic samples have been studied and compared to precursor silicate xerogels. Our previously published work indicates that sol-gel glass-ceramics containing SrF\({}_{2}\):Eu\({}^{3+}\) fluoride nanocrystals are attractive materials for reddish-orange luminescence applications [20].