Molecular Packing-Dependent Thermal Expansion Behaviors in Metal Squarate Frameworks

Packing mode of the molecules in materials has a great influence on materials' physical and chemical properties. Herein, we investigated three metal squarate frameworks (Zn_sq_C, Cd_sq_C, and Cd_sq_R) with similar crystal structures. While owing to the difference in the packing mode of the squarate ligand, two kinds of structures can be achieved (cubic phase and trigonal phase). Synchrotron powder X-ray diffraction analyses show that the three compounds exhibit dramatically different thermal expansion behaviors, including positive thermal expansion (PTE), zero thermal expansion (ZTE), and negative thermal expansion (NTE). By directly structural comparison, it is found that, owing to the unique "eclipsed packing mode", the trigonal phase (Cd_sq_R) can undergo a "wine-rack" distortion upon heating, which makes a great contribution to the NTE behavior, while this "wine-rack" distortion is forbidden in the "staggered packing" cubic phase. Furthermore, single-crystal X-ray diffraction and synchrotron X-ray total scattering analyses shed light on that the difference in magnitude of rotation of the squarate ligand may be responsible for the different thermal expansion behaviors. The present study shows how the proper alignment of the building blocks can be used to tailor materials' thermal expansion behavior and can provide guidance for the rational design of novel NTE or ZTE materials.