Crystallographic Evidence of Size-Dependent Bond Flexibility in Metal–Organic Framework Nanocrystals

Size-dependent electronic, magnetic,
and optical behavior suggests
that metal–organic frameworks become softer materials as their
particle sizes decrease, but direct evidence is lacking. Here, we
report variable-temperature powder X-ray diffraction data of Fe(1,2,3-triazolate)2 particles that offer crystallographic insight into size-dependent
bond flexibility. Rietveld refinement reveals size-dependent positive
thermal expansion upon downsizing the crystalline domains from 178
to 9 nm, with a 6-fold increase from 16 MK–1 to
96 MK–1. This behavior occurs in tandem with size-dependent
elongation of metal–ligand bonds and increasing thermal displacement
parameters, consistent with pronounced metal-linker bond lability.
We propose that these effects, as well as size-dependent annealing
of crystallite sizes, originate from the high charge density and surface
stress of smaller particles. Taken together, these results provide
structural evidence that size reduction serves as a synthetic route
to controlling the dynamic response of materials to external stimuli.