Density of mechanisms within the flexibility window of zeolites

TL;DR

This paper models zeolite frameworks as mechanical trusses to analyze their flexibility, revealing that extensively flexible structures are most common at minimal density, which impacts their entropy and potential for synthesis.

Contribution

It introduces a mechanical truss model for zeolites and uncovers the density-dependent distribution of flexible mechanisms, highlighting their significance in zeolite stability and synthesis.

Findings

Flexible mechanisms peak at minimal density in zeolites.

25 out of 197 frameworks show extensive flexibility.

Flexibility influences zeolite synthesis and stability.

Abstract

By treating idealized zeolite frameworks as periodic mechanical trusses, we show that the number of flexible folding mechanisms in zeolite frameworks is strongly peaked at the minimum density end of their flexibility window. 25 of the 197 known zeolite frameworks exhibit an extensive flexibility, where the number of unique mechanisms increases linearly with the volume when long wavelength mechanisms are included. Extensively flexible frameworks therefore have a maximum in configurational entropy, as large crystals, at their lowest density. Most real zeolites do not exhibit extensive flexibility, suggesting that surface and edge mechanisms are important, likely during the nucleation and growth stage. The prevalence of flexibility in real zeolites suggests that, in addition to low framework energy, it is an important criterion when searching large databases of hypothetical zeolites for potentially useful realizable structures.