Anisotropic Thermal Expansion of an Adaptive Ni Metal-Organic Framework with 1D Helical Chains

Metal-organic frameworks (MOFs) are gradually becoming an ideal platform for investigating negative thermal expansion (NTE) owing to their designability and framework flexibility. In this work, we studied the thermal expansion (TE) phenomenon of the orthorhombic [Ni(L-cam)(4,4'-bpe)]_n (L-cam = L-camphoric acid and 4,4'-bpe = 4,4'-vinylenedipyridine) with 1D helical chains through thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), variable-temperature powder X-ray diffraction (VT-PXRD), and variable-temperature single-crystal X-ray diffraction (VT-SCXRD). The synergistic effect of the lateral thermal vibrations of bpe and the spring-like stretching thermal motion elucidated the mechanism of NTE along the c-axis. The positive thermal expansion (PTE) along the b-axis can be attributed to the spring-like stretching thermal motion of the helical chains. The a-axis was nearly unchanged, exhibiting zero thermal expansion (ZTE). The PTE happened in the unit cell volume due to the larger deformation along the b-axis compared to the c-axis. This study deepens our comprehension of the NTE of 3D MOFs and provides a point of view for the design and synthesis in the development and investigation of other NTE materials.