17O and 29Si NMR Parameters of MgSiO3 Phases from High-Resolution Solid-State NMR Spectroscopy and First-Principles Calculations

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• 作者：Sharon E. Ashbrook ; Andrew J. Berry ; Daniel J. Frost ; Alan Gregorovic ; Chris J. Pickard ; Jennifer E. Readman ; Stephen Wimperis
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：October 31, 2007
• 年：2007
• 卷：129
• 期：43
• 页码：13213 - 13224
• 全文大小：342K
• 年卷期：v.129,no.43(October 31, 2007)
• ISSN：1520-5126

文摘

The ²⁹Si and ¹⁷O NMR parameters of six polymorphs of MgSiO₃ were determined through acombination of high-resolution solid-state NMR and first-principles gauge including projector augmentedwave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO₃ is an important componentof the Earth's mantle that undergoes structural changes as a function of pressure and temperature. Forthe lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the ¹⁷O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. Theseassignments differ from those tentatively suggested in previous work on the basis of empirical experimentalcorrelations. The higher pressure polymorphs of MgSiO₃ (majorite, akimotoite, and perovskite) are stabilizedat pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the majorconstituent at depths >660 km. We present the first ¹⁷O NMR data for these materials and confirm previous²⁹Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transitionMAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested.The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider thevariation of NMR parameters (both shielding and quadrupolar) with local structure, including changes incoordination number, local geometry (bond distances and angles), and bonding. For example, we findthat, although there is a general correlation of increasing ¹⁷O chemical shift with increasing Si-O bondlength, the shift observed also depends upon the exact coordination environment.