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.

Rare-earth squarate frameworks with topology

As a typical planar 4-connected ligand that possesses D_4h symmetry, the squarate ligand is expected to construct some interesting topologies. Here, we report that the assembly of the squarate ligand with rare-earth ions can produce a series of (4, 8)-connected frameworks with the "smallest" scu type topology. Among these compounds, the Tb based analogue not only possesses a good proton conductivity, but also exhibits luminescence responses toward MnO₄^- and Cr₂O₇^2-, making it a candidate for multifunctional materials.

Two-Dimensional Negative Thermal Expansion in a Facile and Low-Cost Oxalate-Based Metal-Organic Framework

Two-dimensional negative thermal expansion (NTE) is achieved in a tetragonal oxalate-based metal-organic framework (MOF), CdZrSr(C₂O₄)₄, within a temperature range from 123 to 398 K [space group I4̅m2, α_a = -2.4(7) M K^-1, α_c = 11.3(3) M K^-1, and α_V = 6.4(1) M K^-1]. By combining variable-temperature X-ray diffraction, a high-resolution synchrotron X-ray pair distribution function, and thermogravimetry-differential scanning calorimetry, we shows that NTE within the ab plane derives from the oriented rotation of an oxalate ligand in zigzag chains (-CdO₈-ox-ZrO₈-ox-)_∞. That is simplified to the Zr atom rotating with an unchanged Zr···Cd distance as the radius, which also gives rise to the deformation of a hingelike connection along the c axis and results in its positive thermal expansion. By virtue of the facile and low-cost oxalate ligand, the present NTE MOF may show application prospects in the future.

Biaxial negative thermal expansion in Zn[N(CN)2]2

A 2D-layered network Zn[N(CN)2]2, is reported in which the transverse vibrations of C atoms and the rotation of ZnN4 tetrahedra dominate its biaxial NTE behavior.

Halogen regulation triggers NLO and dielectric dual switches in hybrid compounds with green fluorescence

An effective strategy of using halogens to modify organic–inorganic hybrid materials to obtain NLO switching characteristics, which is expected to be used for the directional adjustment of NLO switch activity.