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Scandium and Yttrium Environments in Aluminosilicate Glasses Unveiled by 45Sc/89Y NMR Spectroscopy and DFT Calculations: What Structural Factors Dictate the Chemical Shifts?

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journal contribution
posted on 24.07.2017 by Aleksander Jaworski, Thibault Charpentier, Baltzar Stevensson, Mattias Edén
Aluminosilicate (AS) glasses incorporating rare-earth (RE) elements exhibit favorable mechanical and (magneto)­optical properties that reflect their unusual structural organization. Yet, experimental reports on the local RE3+ environments in AS glasses are very sparse. We examine the Y3+ and Sc3+ cations in Y2O3–Al2O3–SiO2 and Sc2O3–Al2O3–SiO2 glasses of variable RE/Al/Si contents by utilizing magic-angle spinning (MAS) 89Y and 45Sc nuclear magnetic resonance (NMR) experiments coupled with density functional theory (DFT) calculations of 89Y/45Sc NMR chemical shifts. The DFT models reveal {Y[p]} and {Sc[p]} coordination numbers (p) spanning 5 ⩽ p ⩽ 8 and 4 ⩽ p ⩽ 7, respectively, with {Y[6], Y[7]} and {Sc[5], Sc[6]} species dominating. Wide isotropic chemical shift ranges of 35–354 ppm (89Y) and 48–208 ppm (45Sc) are observed, as well as sizable shift anisotropies up to ≈370 ppm and ≈250 ppm for 89Y and 45Sc, respectively. Both the isotropic and anisotropic chemical shifts grow when the coordination number p is decreased for 89Y[p] as well as 45Sc[p]. Second to the coordination number, we demonstrate that the 89Y/45Sc isotropic chemical shifts are mainly influenced by the RE/Al/Si constellation in the second coordination sphere of Y and Sc, where the shift tends to increase for emphasized contacts with neighboring RE and Al species at the expense of Si. These DFT-derived trends are corroborated by a progressive 89Y deshielding observed in MAS 89Y NMR spectra for increasing Y and/or Al content of the glass. We also introduce heteronuclear MAS NMR experimentation involving the pairs of 89Y–27Al and 45Sc–29Si nuclides, utilized for probing the contacts between the Y3+/Sc3+ cations and the AS glass-network forming groups.