Review on the structural analysis of fluoride-phosphate and fluoro-phosphate glasses

Fluoride phosphate/fluorophosphate (FP) glasses form an intriguing material's basis for diverse applications in optics, photonics and energy storage. The optimization of the materials' properties with regard to the application envisioned demands an understanding of the key concepts describing their local structures on different distance regimes. Because of the inevitable losses of fluoride during melting or annealing, the synthesis and elemental analytical characterization present important and critical challenges. In this contribution we summarize the current state of the literature regarding the structural characterization of these glasses. Foci are the detailed analysis of vibrational spectra with special emphasis on Raman scattering results and multinuclear solid-state NMR spectroscopy. While vibrational spectra give important information about the local environments (short range order) advanced multi-dimensional and multi-resonance methodology can extend the informational content of solid-state NMR into the sub-nanometer distance range, including second and third coordination spheres. Exemplary results will be presented for (a) aluminum fluoride based simple model systems, (b) fluoride phosphate glasses containing heavy transition metals or post-transition metal elements, and (c) rare-earth doped fluoride phosphate glasses for laser applications. The latter application field documents the utility of correlated spectroscopic studies using both methods in understanding the structural foundations of photophysical properties.