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

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journal contribution
posted on 16.04.2019, 00:00 by Lena Marie Funke, Oliver Janka, Rainer Pöttgen, Leonid Glebov, Michael Ryan Hansen, Hellmut Eckert
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.