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, αa = −2.4(7) M K–1, αc = 11.3(3) M K–1, and αV = 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.