Negative Thermal Expansion in the Noncarboxylate Based Metal–Organic Framework Cd(trz)Cl

Negative thermal expansion (NTE), as a counterintuitive phenomenon, has attracted considerable interest owing to its great scientific and technical value. Owing to the inherent structural flexibility, metal–organic frameworks (MOFs) have recently been recognized as an ideal platform for the study of NTE. However, the reported NTE MOFs are mainly restrained in the carboxylate based ones. Herein, we found that the noncarboxylate MOF Cd(trz)Cl (trz = 1,2,4-trizaole) can also exhibit intrinsic volumetric NTE, and a transition from NTE to PTE at high temperatures can be observed. The combined in situ synchrotron powder X-ray diffraction, Raman spectra measurements, and DFT calculations shed light on the fact that the NTE mainly originates from the synergistic effect of geometric flexibility and the enhanced out-of-plane rotation behavior of the trz ligand. This study not only demonstrates a new NTE material but also can help to extend the scope of NTE to the more diverse noncarboxylate based MOFs.