Intense red photoluminescence from Mn2+-doped (Na+; Zn2+) sulfophosphate glasses and glass ceramics as LED converters

We report on intense red fluorescence from Mn2+-doped sulfophosphate glasses and glass ceramics of the type ZnO-Na2O-SO3-P2O5. As a hypothesis, controlled internal crystallization of as-melted glasses is achieved on the basis of thermally-induced bimodal separation of an SO3-rich phase. Crystal formation is then confined to the relict structure of phase separation. The whole synthesis procedure is performed in air at <= 800 degrees C. Electron spin resonance and Raman spectroscopy indicate that Mn2+ species are incorporated on Zn2+ sites with increasingly ionic character for increasing concentration. Correspondingly, in the glasses, increasing MnO content results in decreasing network polymerization. Stable glasses and continuously increasing emission intensity are observed for relatively high dopant concentration of up to 3 mol.%. Recrystallization of the glass results in strongly increasing emission intensity. Dynamic emission spectroscopy reveals only on type of emission centers in the glassy material, whereas three different centers are observed in the glass ceramic. These are attributed to octahedrally coordinated Mn2+ in the residual glass phase and in crystalline phosphate and sulfate lattices, respectively. Relatively low crystal field strength results in almost ideal red emission, peaking around 625 nm. Excitation bands lie in the blue-to-green spectral range and exhibit strong overlap. The optimum excitation range matches the emission properties of GaN- and InGaN-based light emitting devices. (C) 2010 Optical Society of America