Tailoring Optical Properties of Rare-Earth Doped Oxyfluoride Glass-Ceramics by Tuning Crystal Structure

It is known that oxyfluoride glasses can be used as the host for fluoride crystals that feature promising optical properties. This work demonstrates that the optical properties of the Er³⁺-Yb³⁺ co-doped oxyfluoride precursor glass-ceramic (PGC) can be tailored by changing crystal structure through chemical variation (LaF₃/BaF₂ = 0, 1/3; YF₃/LaF₃ ratios = 0, 2/7, 4/7, 6/7, 1) and heat treatment. The strong correlations between the crystallization behavior, microstructure, and optical properties are revealed. It is found that the precipitated crystals such as BaF₂, Ba₂LaF₇, Ba_2-xLa_1-xY_xF_7-2x (0 < x <1), and BaYF₅ are uniformly distributed in the glass matrices of PGC. Upon heat-treatment, the crystallinity of PGC increases, whereas the crystal type remains unchanged. Both the up-conversion luminescence and optical transmittance of the samples strongly depend on the substitution of one type of fluoride for another. Moreover, by taking advantage of the effect of crystal structure on the cross-relaxation propensity of Er³⁺ ions, it is feasible to tune the up-conversion luminescence color from green to light yellow. It is unveiled that the enhanced cross-relaxation arises from lattice shrinkage. This work provides guidance for optimizing the optical properties of rare-earth-doped oxyfluoride glass ceramics.