Intrinsic flexibility of porous materials: theory, modelling and the flexibility window of the EMT zeolite framework

Framework materials have structures containing strongly bonded polyhedral
groups of atoms connected through their vertices. Typically the energy cost for
variations of the inter-polyhedral geometry is much less than the cost of
distortions of the polyhedra themselves – as in the case of silicates, where the
geometry of the SiO4 tetrahedral group is much more strongly constrained than
the Si—O—Si bridging angle. As a result, framework materials frequently
display intrinsic flexibility, and their dynamic and static properties are strongly
influenced by low-energy collective motions of the polyhedra. Insight into these
motions can be obtained in reciprocal space through the ‘rigid unit mode’
(RUM) model, and in real-space through template-based geometric simulations.
We briefly review the framework flexibility phenomena in energy-relevant
materials, including ionic conductors, perovskites and zeolites. In particular we
examine the ‘flexibility window’ phenomenon in zeolites and present novel
results on the flexibility window of the EMT framework, which shed light on the
role of structure-directing agents. Our key finding is that the crown ether,
despite its steric bulk, does not limit the geometric flexibility of the framework.