Structural and optical properties of rare earth doped low melting point phosphate glasses

A Low Melting Point Phospate Glass (LMMPG) has been doped with five rare earth (RE) cations, specifically Nd, Sm, Eu, Tb and Er, as well as double doping with Eu-Tb, which encompass most of the 4f period. They were prepared by a melt-quenching method, using a phosphate glass with chemical composition (mol.%) 45 P 2 O 5 , 10 CaO, and 45 Na 2 O, where 1 mol.% of calcium oxide was substituted with the corresponding rare earth oxide. Whereas melting point of glasses is approximately 550°C, doped LMPPG beads were obtained from melt at 900°C, in order to reduce the viscosity which is a key factor to remove bubbles. However, such a low temperature prevented us to employ RE oxides, as refractory RE oxides, as they may be not completely solved, introducing optical defects in the solid bodies. Instead, RE cations were supplied by means of chlorides, which decompose at temperatures very close to the melting point of the LMPPG, preventing the formation of RE oxides. As a result, beads exhibit a high degree of transparency which has a beneficial effect in the luminescent properties. Quantitative structural characterization by 31 P NMR, FTIR, and ATD determines that RE cations strongly bond to the phosphate chain edges. This interaction enhances the glass stability as well as allowing a perfect dispersion of these cations inside the matrix, minimizing quenching and self-absorption processes. Additionally, it has been showed, that multiple doping is also accepted by the amorphous matrix, in such manner that different color hues can be obtained. This versatile and high quality doped glasses could be employed as visible and infrared light sources, anti-counterfeit materials, and other photonic applications which requires efficient emitters based on RE.