¹⁷O and²⁹Si NMR Parameters of MgSiO₃Phases from High-Resolution Solid-State NMR Spectroscopy and First-Principles Calculations

Abstract

The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the 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. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (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 the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment.

Effect of pressure on structure of oxide glasses at high pressure: Insights from solid-state NMR of quadrupolar nuclides

“…11 B and 23 Na NMR studies also provided additional details of pressure induced coordination transformation in framework boron and non-framework sodium in the diverse oxide glasses [63,84,85,87,91]. We also note that detailed 17 O NMR studies of crystalline silicates synthesized at high pressure have been reported, providing new structural details and NMR parameters for key silicate mineral phases stable in the Earth's upper mantle [97,98]. Recent detailed 27 Al MAS NMR studies of crystalline MgSiO 3 perovskite and a high pressure silica polymorph at high magnetic field revealed the Al substitution mechanism in the perovskite and stishovite [99].…”

“…For instance, it has been shown that the calculated 17 O d iso for [n] (Si,Al)-O- [4] Si mainly increases with an increases in the coordination number (n) [93]. Note that the first principle calculations with the plain wave basis sets have also been recently utilized to yield NMR parameters for high pressure minerals including Mg-silicate polymorphs [97,100]. Diverse potentially useful structural information of clusters in oxide glasses at high pressure will be obtained from future quantum chemical calculations.…”

Solid-state NMR calculations for metal oxides and gallates: Shielding and quadrupolar parameters for perovskites and related phases

“…The r ref -values, gradients (m) and root mean square errors obtained from the fits are also shown, along with the standard errors in the former two quantities. (expt) indicates that the fixed experimental structure was used in the calculation [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] permitted by symmetry, and is qualitatively reproduced by the calculations: the value obtained from the fixed experimental structure being in marginally better agreement with experiment. As expected, the g Q -values at this site are zero, with the major axis of the EFG tensor oriented along the c-axis of the hexagonal cell, and the two minor axes confined to the ab-plane.…”

First-principles calculations of NMR parameters for phosphate materials

“…DFT–PAW/GIPAW) allows a better modelling of the NMR parameters measured experimentally. In the past 5 years, this theoretical approach, using NMR first‐principles calculation, has been generalised for the direct assignment of NMR parameters for different organic10–14, inorganic15–25 or amorphous26, 27 solid materials.…”