Physics Behind the Guest-Assisted Structural Transitions of a Porous Metal-Organic Framework Material

MIL53(Cr) is one of the metal-organic framework (MOF)-type materials which shows the most spectacular breathing behavior upon adsoption of various types of fluids. The previously reported thermodynamic models did not allow a subtle analysis of the factors and machanisms that govern the structural transition of the framework in play. Here, we demonstrate that probing the interplay between the host and the guest molecules is crucial to capturing the physics behind such a transition, which has never been addressed so far. It is thus emphasized that the host/guest interactions preliminarily induce a soft mode in the host framework, which is a prerequirement for initiating the structural transition of the MIL-53(Cr) solid. It follows a displacive type of mechanism, which involves the occurrence of a new metastable phase that has been neither experimentally evidenced nor predicted. Such an observation questions the bistability of the MIL-53(Cr) solid upon adsorption reported so far in the literature.