Homologous Critical Behavior in the Molecular Frameworks Zn(CN)2 and Cd(imidazolate)2

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc­(II) cyanide (Zn­(CN)2) and cadmium­(II) imidazolate (Cd­(im)2), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn­(CN)2 undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn­(CN)2. We find that the fundamental mechanical response exhibited by Zn­(CN)2 is mirrored in the temperature-dependent behavior of Cd­(im)2. Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology.