Compositional Inhomogeneity and Tuneable Thermal Expansion in Mixed-Metal ZIF-8 Analogues
A F Sapnik, H S Geddes, E M Reynolds, H H-M Yeung and A L Goodwin
Chemical Communications (2018, in press)

We study the structural and thermomechanical effects
of cation substitution in the compositional family of
metal–organic frameworks Zn1–xCdx(mIm)2 (HmIm = 2-
methylimidazole). We find complete miscibility for all
compositions x, with evidence of inhomogeneous distributions of Cd and Zn that in turn affect framework aperture characteristics. Using variable-temperature X-ray powder diffraction measurements, we show that Cd substitution drives a threefold reduction in the magnitude of thermal expansion behaviour. We interpret this effect in terms of an increased density of negative thermal expansion modes in the more flexible Cd-rich frameworks.