title: Homologous critical behavior in the molecular frameworks Zn(CN)2 and Cd(imidazolate)2.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, United Kingdom.
authors.forename: Ines E
authors.initials: IE
authors.lastname: Collings
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Cairns
authors.forename: Amber L
authors.initials: AL
authors.lastname: Thompson
authors.forename: Julia E
authors.initials: JE
authors.lastname: Parker
authors.forename: Chiu C
authors.initials: CC
authors.lastname: Tang
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Tucker
authors.forename: Jadna
authors.initials: J
authors.lastname: Catafesta
authors.forename: Claire
authors.initials: C
authors.lastname: Levelut
authors.forename: Julien
authors.initials: J
authors.lastname: Haines
authors.forename: Vladimir
authors.initials: V
authors.lastname: Dmitriev
authors.forename: Philip
authors.initials: P
authors.lastname: Pattison
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Goodwin
mesh_terms: D002104:Cadmium / Q000737:chemistry*; D003486:Cyanides / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry*; D033362:Powder Diffraction; D014961:X-Ray Diffraction; D015032:Zinc / Q000737:chemistry*
chemical_list: D003486:Cyanides; D007093:Imidazoles; D009942:Organometallic Compounds; D002104:Cadmium; D015032:Zinc