  The use of the osmotic thermodynamic model, combined with a series of methane
and carbon dioxide gas adsorption experiments at various temperatures, has
allowed shedding some new light on the fascinating phase behavior of flexible
MIL-53(Al) metal-organic frameworks. A generic temperature-loading phase
diagram has been derived; it is shown that the breathing effect in MIL-53 is a
very general phenomenon, which should be observed in a limited temperature
range regardless of the guest molecule. In addition, the previously proposed
stress model for the structural transitions of MIL-53 is shown to be
transferable from xenon to methane adsorption. The stress model also provides a
theoretical framework for understanding the existence of lp/np phase mixtures
at pressures close to the breathing transition pressure, without having to
invoke an inhomogeneous distribution of the adsorbate in the porous sample.
