On the crystal structure of the vaterite polymorph of CaCO3: A calcium-43 solid-state NMR and computational assessment

The vaterite polymorph of CaCO3 has puzzled crystallographers for decades in part due to difficulties in obtaining single crystals. The multiple proposed structures for the vaterite polymorph of CaCO3 are assessed using a combined Ca-43 solid-state nuclear magnetic resonance (SSNMR) spectroscopic and computational approach. A combination of improved experimental and computational methods, along with a calibrated chemical shift scale and Ca-43 nuclear quadrupole moment, allow for improved insights relative to our earlier work (Bryce et al., J, Am. Chem. Soc. 2008, 130, 9282). Here, we synthesize a Ca-43 isotopically-enriched sample of vaterite and perform high-resolution quadrupolar SSNMR experiments including magic-angle spinning (MAS), double-rotation (DOR), and multiple-quantum (MQ) MAS experiments at magnetic field strengths of 9.4 and 21.1 T. We identify one crystallographically unique Ca2+ site in vaterite with a slight distribution in both chemical shifts and quadrupolar parameters. Both the experimental Ca-43 electric field gradient tensor and the isotropic chemical shift for vaterite are compared to those calculated with the gauge-including projector-augmented-wave (GIPAW) DFT method in an attempt to identify the model that best represents the crystal structure of vaterite. Simulations of Ca-43 DOR and MAS NMR spectra based on the NMR parameters computed for a total of 1 8 structural models for vaterite allow us to distinguish between these models. Among these 18, the P3(2)21 and C2 structures provide simulated spectra and diffractograms in best agreement with all experimental data, (C) 2014 Elsevier Inc. All rights reserved