Pressure-Induced Loss of Long-Range Structural Order in MFM300(Al): An X-ray Diffraction and Raman Spectroscopic Study

Abstract

The structural flexibility of MFM-300(Al), a non-amine metalorganic framework (MOF) with efficient greenhouse gas trapping properties, is investigated under external pressure by synchrotron powder X-ray diffraction and Raman spectroscopy. This experimental work sheds light on the mechanical stability of MFM-300 by carefully examining the onset of the breathing transition and amorphization under pressure. The results are compared with the reported ab initio and force-field calculations on MFM-300. The results suggest the evidence for two-phase transitions below 2 GPa. The first transition at similar to 0.6 GPa is deduced by the disappearance of several Raman modes along with the appearance of a new mode similar to 150 cm(-1), which is consistent with our X-ray diffraction data. The second phase transition at 1.5 GPa is confirmed by the discontinuities in the Raman mode progression with pressure corresponding to OH bending along the Al-O-Al plane and ring breathing modes. There are clear changes in the X-ray diffraction above similar to 1.5 GPa, confirming the second phase transition. These experimental observations are in good agreement with the reported force field calculations, which suggest a breathing-type transition in MFM-300 below 2 GPa from a large pore to a narrow pore structure through an intermediate phase near 1 GPa. Diffraction data at 9 GPa show the disappearance of almost all Braggs peaks, except a few, indicating the occurrence of amorphization in this pressure range. Our Raman data show that MFM-300(Al) undergoes amorphization around 11 GPa; however, upon decompression, it reverts to the ambient large-pore phase. This is unique in the family of wine-rack structured MOFs. This suggests that the framework flexibility is not a priori limitation for MOFs' diverse applications.