Crystallite shape and packing of paramagnetic microcrystalline powders from the measurement and calculation of bulk magnetic susceptibility shifts in solid-state NMR

The size and shape of crystallites strongly influence the properties of functional materials yet remain challenging to access experimentally. In paramagnetic solids, the bulk magnetic susceptibility (BMS) contributes a part to the measured nuclear magnetic resonance (NMR) shift tensor, which depends strongly on particle shape and packing. Here, we use a Fourier-space approach to quantify BMS shifts in LiFePO4 and use the 7Li magic-angle spinning NMR spectra to directly probe the crystallite shape and size. By disentangling internal and external BMS shift contributions from the crystallite of interest and its neighbors, we show that particle shape anisotropy leaves a clear signature in the NMR line shape. Combining BMS modeling with quantum-chemical local shift parameters yields excellent agreement with experimental spectra and enables the extraction of new morphological insights, highlighting solid-state NMR as a quantitative probe of crystallite morphology in paramagnetic materials.