Crystallographic Evidence of Size-Dependent Bond Flexibility in MOF Nanocrystals

Size-dependent electronic, magnetic, and optical behavior suggests that metal-organic frameworks (MOFs) 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 offers crystallographic insight into size-dependent bond flexibility. Rietveld refinement reveals size-dependent “colossal” positive thermal expansion upon downsizing the crystalline domains from 178 nm to 9 nm, with a six-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.