Transparent oxyfluoride glass ceramics co-doped with Er and Yb - Crystallization and upconversion spectroscopy

Transparent glass ceramics in the system SiO₂-B₂O₃-PbO-CdO-PbF₂-CdF₂-YbF₃-ErF₃ showing infrared to visible anti-Stokes (upconversion) luminescence are studied in the present work. The glass compositions have been optimized in order to reduce the melting temperature and, hence, to obtain laboratory scale samples with good optical quality. Erbium-doped nanoscale Pb₄Yb₃F₁₇ crystals are precipitated in the precursor glasses during annealing at temperatures 30-40 K above T_g. A kinetically self-constrained growth explains the nano sizes of the crystals. Both the Stokes and anti-Stokes luminescence spectra of glasses could be explained with clustering of the Yb³⁺ and Er³⁺ ions in fluorine-rich regions. At the annealing temperature these regions act as nucleation precursors. The crystal growth further enhances the local concentration of these ions. Consequently, a series of energy transfer and energy cross relaxation processes occurs between adjacent rare earth ions leading to the observed luminescence spectra of the glass ceramics studied.