Transparent oxyfluoride glass ceramics co-doped with Er3+ and Yb3+ – 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 Tg. 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.