The Bearings from Rare-Earth (RE = La, Lu, Sc, Y) Cations on the Oxygen Environments in Aluminosilicate Glasses: A Study by Solid-State 17O NMR, Molecular Dynamics Simulations, and DFT Calculations

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• 作者：Aleksander Jaworski ; Baltzar Stevensson ; Mattias Edén
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：June 23, 2016
• 年：2016
• 卷：120
• 期：24
• 页码：13181-13198
• 全文大小：846K
• 年卷期：0
• ISSN：1932-7455

文摘

Aluminosilicate (AS) glasses incorporating trivalent cations of rare-earth (RE) elements exhibit a significant structural disorder and manifest building blocks incommensurate with conventional glass structure models. We present a comprehensive experimental and computational study of the O speciations in RE₂O₃–Al₂O₃–SiO₂ glasses with RE = {La³⁺, Y³⁺, Lu³⁺, Sc³⁺}, where the cations are ordered according to increasing field-strength. The coexisting ¹⁷O^[p]–Si_p–mAl_m moieties were quantified by magic-angle-spinning (MAS) ¹⁷O nuclear magnetic resonance (NMR) experiments and atomistic molecular dynamics (MD) simulations. Experimental ¹⁷O quadrupolar products (C_Qη) and isotropic chemical shifts (δ_iso) agreed well with predictions from density functional theory with the projector augmented wave (PAW) and gauge including PAW approaches, respectively. We highlight an observed strong influence of both {δ_iso, C_Qη} NMR parameters on the average number of ¹⁷O^[p]–RE³⁺ contacts (q̅) and establish simple correlations between q̅ and each of δ_iso and C_Qη that encompass all ¹⁷O^[p]–Si_p–mAl_m moieties with 1 ≤ p ≤ 3. The quadrupolar product of each O^[p]–Si_p–mAl_m motif depends linearly on the fractional ionicity of the bonds to the ¹⁷O site, which is readily calculated from the parameter set {m, p, q̅}, with q̅ extracted from the MD-generated glass models. We rationalize and discuss the stability of each O^[p]–Si_p–mAl_m moiety using bond valence sums evaluated on the MD-derived RE AS glass models: all comprise non-negligible populations of “unconventional” O species, such as “free” O^2– ions (O^[0] coordinations), and “oxygen triclusters” (O^[3]–SiAl₂ and O^[3]–Al₃). The triclusters preferentially connect high-coordination Al^[5]/Al^[6] species via edge-sharing, where the participation in corner or edge shared polyhedra is reflected in the {δ_iso, C_Qη} ¹⁷O NMR parameters.