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

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.