Transparent Glasses and Glass-Ceramics in the Ternary System TeO₂-Nb₂O₅-PbF₂

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.