  Mesocrystals are nanostructured materials consisting of individual
nanocrystals having a preferred crystallographic orientation. On mesoscopic
length scales, the properties of mesocrystals are strongly affected by
structural heterogeneity. Here, we report the detailed structural
characterization of a faceted mesocrystal grain self-assembled from 60 nm sized
gold nanocubes. Using coherent X-ray diffraction imaging, we determined the
structure of the mesocrystal with the resolution sufficient to resolve each
gold nanoparticle. The reconstructed electron density of the gold mesocrystal
reveals its intrinsic structural heterogeneity, including local deviations of
lattice parameters, and the presence of internal defects. The strain
distribution shows that the average superlattice obtained by angular X-ray
cross-correlation analysis and the real, multidomain structure of a mesocrystal
are very close to each other, with a deviation less than 10 percent. These
results will provide an important impact to understanding of the fundamental
principles of structuring and self-assembly including ensuing properties of
mesocrystals.
