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Issue 13, 2007
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First-principles calculations of solid-state 17O and 29Si NMRspectra of Mg2SiO4 polymorphs

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Abstract

The nuclear magnetic resonance (NMR) shielding and electric field gradient (EFG) tensors of three polymorphs of Mg2SiO4, forsterite (α-Mg2SiO4), wadsleyite (β-Mg2SiO4) and ringwoodite (γ-Mg2SiO4), have been calculated using a density functional theory (DFT) approach with a planewave basis set and pseudopotential approximation. These Mg2SiO4 polymorphs are the principal components of the Earth down to depths of 660 km and have been proposed as the hosts of water in the Earth’s upper mantle and transition zone. A comparison of our calculations with single-crystal spectroscopic data in the literature for the α-polymorph, forsterite, shows that both the magnitude and orientation of the shielding and EFG tensors for O and Si can be obtained with sufficient accuracy to distinguish subtle differences in atomic positions between published structures. We compare calculated 17O MAS NMR quadrupolar powder lineshapes directly with experimental lineshapes and show that we are able to reproduce them within the precision with which the NMR parameters may be determined from multi-parameter fitting. The relatively small amounts of sample available for the β- and γ-polymorphs, arising from the high pressures required for synthesis, has hindered the extraction of NMR parameters in previous work. The application of DFT calculations to these high-pressure polymorphs confirms previous spectral assignments, and provides deeper insight into the empirical correlations and observations reported in the literature. These first-principles methods are highly promising for the determination of local bonding in more complex materials, such as the hydrated forms of Mg2SiO4, by aiding analysis of their multinuclear NMR spectra.

Graphical abstract: First-principles calculations of solid-state 17O and 29Si NMR spectra of Mg2SiO4 polymorphs

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Submitted
13 Dec 2006
Accepted
12 Jan 2007
First published
12 Feb 2007

Phys. Chem. Chem. Phys., 2007,9, 1587-1598
Article type
Paper

First-principles calculations of solid-state 17O and 29Si NMR spectra of Mg2SiO4 polymorphs

S. E. Ashbrook, L. Le Pollès, C. J. Pickard, A. J. Berry, S. Wimperis and I. Farnan, Phys. Chem. Chem. Phys., 2007, 9, 1587
DOI: 10.1039/B618211A

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