Compositional Inhomogeneity and Tuneable Thermal Expansion in Mixed-Metal ZIF-8 Analogues

<div>We study the structural and thermomechanical effects</div><div>of cation substitution in the compositional family of</div><div>metal–organic frameworks Zn1􀀀xCdx(mIm)2 (HmIm = 2-</div><div>methylimidazole). We find complete miscibility for all</div><div>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.</div>