Reentrant Negative Linear Compressibility in MIL-53(Al) over an Ultrawide Pressure Range

Anomalous compression behaviors such as negative linear compressibility (NLC) and negative area compressibility (NAC) are emerging functionalities essential for utilities under extreme conditions. Metal–organic frameworks (MOFs) present a large family of NLC/NAC materials under the well-recognized “wine-rack” mechanism. Nevertheless, the working pressure of MOFs is limited to typically no more than 3 GPa. Herein, we report the observation of a rare reentrant NLC behavior of MIL-53(Al) over an ultrawide pressure region up to 41.6 GPa. MIL-53(Al) with the classic wine-rack topology first exhibits an NLC behavior along the c-axis in the pressure region of 0–2.7 GPa, followed by a normal compression process between 2.7 and 12.9 GPa. After that, it is surprising to find that the reentrant NLC property remains at a record-high pressure (>40 GPa) among the previously reported MOFs. The mechanisms of the distinct compression behaviors of MIL-53(Al) in the three stages are studied by combining the X-ray diffraction analyses, Raman spectra, and DFT calculation. Particularly, the photoluminescence of Cr3+ is used as a unique probe for the subtle changes of the local coordination environments of Al3+ under compression. Both the absorption spectra and photoluminescence of Cr3+-doped MIL-53(Al) not only verify the three-stage compression processes but also reveal a hidden structural/electrical phase transition around 22.0 GPa. These findings provide an in-depth understanding of the structure–property relationship of abnormal compression behavior at the local-structure level.