Transition from isotropic positive to negative thermal expansion by local Zr₆O₈ node distortion in MOF-801

The chemical designability and diversity of metal-organic frameworks (MOFs) endow them with plenty of anomalous properties, such as negative thermal expansion (NTE). Herein, we investigated the thermal expansion behaviors of the well-known MOF-801, which has been widely used in water adsorption and gas separation. The analyses of variable temperature powder X-ray diffraction and Rietveld refinements revealed a fascinating transition from positive thermal expansion to NTE. Further in situ Raman spectra and pair distribution function investigations shed light on the transition being attributed to the local Zr6O8 node distortion rather than a long-range phase transition. Our findings will enhance the comprehension of NTE and contribute to the effective utilization of MOF-801 over a broad temperature range.