  Under the application of electrical currents, metal nanocrystals inside
carbon nanotubes can be bodily transported. We examine experimentally and
theoretically how an iron nanocrystal can pass through a constriction in the
carbon nanotube with a smaller cross-sectional area than the nanocrystal
itself. Remarkably, through in situ transmission electron imaging and
diffraction, we find that, while passing through a constriction, the
nanocrystal remains largely solid and crystalline and the carbon nanotube is
unaffected. We account for this behavior by a pattern of iron atom motion and
rearrangement on the surface of the nanocrystal. The nanocrystal motion can be
described with a model whose parameters are nearly independent of the
nanocrystal length, area, temperature, and electromigration force magnitude. We
predict that metal nanocrystals can move through complex geometries and
constrictions, with implications for both nanomechanics and tunable synthesis
of metal nanoparticles.
