Transparent fluorotellurite glasses were prepared by melt-quenching in the ternary system TeO2-Nb2O5-PbF2. The synthesis conditions were adjusted to minimize fluorine loss monitored as HF release. It was found that 10 mol% of Nb2O5 is the optimum content for PbF2 incorporation up to 35 mol% in the tellurite matrix without loss of glass forming ability. Such glass compositions exhibit a wide optical window from 380 nm to about 6 μm. Crystallization properties were carefully investigated by thermal analysis and compositions with higher PbF2 contents exhibit preferential precipitation of lead oxyfluoride Pb2OF2 at lower temperatures. The lead oxyfluoride crystallization mechanism is also governed by a volume nucleation, barely reported in tellurite glasses. Eu3+ doping of these glass compositions also promotes a more efficient nucleation step under suitable heat-treatments, resulting in transparent Eu3+-doped glass-ceramics whereas undoped glass-ceramics are translucent. Finally, Eu3+ spectroscopy pointed out a progressive, more symmetric surrounding around the rare earth ions with increasing PbF2 contents as well as higher quantum efficiencies. These new fluorotellurite glass compositions are promising as luminescent hosts working in the middle infrared.
Keywords: crystallization; europium; fluoride; glass; glass-ceramic; tellurite.