Intrinsic volumetric negative thermal expansion in the "rigid" calcium squarate

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.