## Fluoride loss in fluoride-phosphate glasses: dependence of composition and glass structure on preparation conditions

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Abstract
11-021 Doris Möncke Möncke, D.(National Hellenic Research Foundation); Ehrt, D.(Friedrich-Schiller-Universität Jena); Fluoride phosphate (FP) glasses are atypical glass forming systems which have been developed and studied in Jena systematically since the 1980s [Ehrt, Phys.Chem.Glasses B56 (2015) 217]. Strong ionic bonds result in very low melt viscosities, often at low temperatures. The high tendency toward devitrification of pure fluoroaluminates glasses can be overcome by even small additions of phosphates. Characteristic for FP glasses are a wide transmission range, low OH content, high positive anomalous partial dispersion, low (non)linear refractive indices, athermal behavior ($\Delta$n/$\mathrm{\Delta }$T), and a high solubility for rare earth ions. These properties, and in doped glasses broad absorption and emission bands or fluorescence lifetimes, can be tailored in a wide range by variation of the ${\mathrm{AlF}}_{3}:{\mathrm{P}}_{2}{\mathrm{O}}_{5}$ ratio. FP glasses containing Nd, Er or Yb and have been tested successfully as amplifier or laser glasses. The structure of FP glasses consists of chains of $\mathrm{Al}\left(\mathrm{F},\mathrm{O}{\right)}_{6}$ octahedra bonded via oxygen to phosphate groups. The occurrence of P-F bonds depends on the melting temperature and the composition. F-loss during melting can be substantial and will modify the glass composition, structure and properties. Parameters influencing F-loss include melting parameters and raw materials used. Different reactions occur when melting under air or when melting under reducing conditions in carbon crucibles under argon. Through quantitative analysis together with spectroscopic methods (NMR, Raman, IR, etc.) we gained a better understanding of the glass structure and regarding compositional and structural variations with melting temperature, the crucible material, melting atmosphere or the raw materials employed. As shown by Ehrt et al, it is possible to prepare laser glasses with a refractive index homogeneity in the 6th decimal, using a closed Pt crucible with temperature controlled down pipe, for successfully use in the petawatt laser (POLARIS).
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