Structural transitions in MIL-53 (Cr): view from outside and inside.

We present a unified thermodynamic description of the breathing transitions between large pore (lp) and narrow pore (np) phases of MIL-53 (Cr) observed during the adsorption of guest molecules and the mechanical compression in the process of mercury porosimetry. By revisiting recent experimental data on mercury intrusion and in situ XRD during CO(2) adsorption, we demonstrate that the magnitude of the adsorption stress exerted inside the pores by guest molecules, which is required for inducing the breathing transition, corresponds to the magnitude of the external pressure applied from the outside that causes the respective transformation between lp and np phases. We show that, when a stimulus is applied to breathing MOFs of MIL-53 type, these materials exhibit small reversible elastic deformations of lp and np phases of the order of 2-4%, while the breathing transition is associated with irreversible plastic deformation that leads to up to ∼40% change of the sample volume and a pronounced hysteresis. These results shed light on the specifics of the structural transformations in MIL-53 (Cr) and other soft porous crystals (SPC).