Article pubs.acs.org/JPCC
Scandium and Yttrium Environments in Aluminosilicate Glasses Unveiled by 45Sc/89Y NMR Spectroscopy and DFT Calculations: What Structural Factors Dictate the Chemical Shifts? Aleksander Jaworski,† Thibault Charpentier,‡ Baltzar Stevensson,† and Mattias Edén*,† †
Physical Chemistry Division, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden ‡ NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette cedex, France S Supporting Information *
ABSTRACT: 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 45 Sc, 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. intermixing7,17−19 (i.e., nonadherence to Loewenstein Al[4]− O−Al[4] avoidance23), and exotic structural motifs such as Al− NBO bonding and free O2− ions over large compositional regions.7,18,20,21 As for many physical properties of RE AS glasses, the extent of these “structural anomalies” tend to increase with the RE3+ CFS, notably so the elevated Al average coordination numbers. Indeed, the structural strengthening from the network cross-linking of the higher-coordination AlO5/AlO6 moieties was recently attributed to account for the well-known enhanced Vickers hardness of RE−Al−Si−O glasses for growing RE3+ CFS.7,9 The present report concerns the 89Y and 45Sc environments and their NMR parameters in Y2O3−Al2O3−SiO2 and Sc2O3− Al2O3−SiO2 glasses. Experimental characterizations of the short-range structural organization of RE AS glasses have mainly been addressed by magic-angle spinning (MAS) 29 Si/27Al NMR spectroscopy,6−8,10−17,19,24,25 and to a lesser extent by 17O NMR.14,16,20,21,25 However, reports on the direct
1. INTRODUCTION Ternary RE2O3−Al2O3−SiO2 glasses (where RE is a rare-earth element) combine the favorable properties of being chemically inert while manifesting remarkably high glass transition temperatures, low thermal expansion coefficients, and high microhardness values,1−9 all of which stem from their unusual structural organization over both short (
