Short- and Medium-Range Order in Photothermal Refractive Glass Revealed by Solid-State NMR Techniques

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive Na₂O–ZnO–Al₂O₃–SiO₂ glass, containing NaF and KBr additives, along with cerium, silver, tin, and antimony oxide dopants. After heating above 500 °C, UV-exposed regions of this glass produce permanent refractive index changes, resulting from precipitation of NaF nanocrystals. Short- and medium-range order of this glass system is studied via multinuclear single- and double-resonance solid-state nuclear magnetic resonance (NMR) spectroscopy in regular PTR glass and in model glasses with simplified compositions. The results, when combined with data from energy-dispersive X-ray spectroscopy, indicate that the NaF component modifies the standard aluminosilicate framework, producing small amounts of F-bonded five- and six-coordinated aluminum species. The fluoride speciation is obtained from ¹⁹F magic-angle spinning NMR spectra, supported by ¹⁹F­{²⁷Al} and ¹⁹F­{²³Na} dipolar recoupling experiments. The majority of fluoride within the PTR glass is found within Na-dominated local environments, which also interact strongly with the aluminum. ²³Na­{¹⁹F} rotational echo double resonance reveals that about 1/3 of the Na⁺ ions have fluoride ions in their first coordination spheres.