Phase Diagram of ZIF-4 Computed via Well-tempered Metadynamics

TL;DR

This study uses well-tempered metadynamics simulations to map the phase diagram of ZIF-4, revealing the mechanisms of phase transitions and the topology of the phase diagram in relevant temperature and pressure ranges.

Contribution

It provides the first computational exploration of ZIF-4's phase diagram, elucidating transition pathways and the role of collective degrees of freedom in phase changes.

Findings

Porous ZIF-4 transforms into ZIF-4-cp via pore closure.

ZIF-4-cp transitions to ZIF-4-cp-II at higher pressures.

Simulations reveal the phase diagram topology and transition mechanisms.

Abstract

Well-tempered metadynamics simulations are employed to explore the phase diagram of ZIF-4, a porous crystalline metal-organic framework of industrial relevance. Despite the vast amount of experimental efforts, the phase diagram that includes ZIF-4 and its related polymorphs has not yet been fully determined. For example, the crystalline phase called ZIF-4-cp is not experimentally observed when high pressure ramps are applied. Our simulations shed light into the phase diagram topology and allow us to further look into the collective degrees of freedom that drive the phase transitions in the T=150-450 K and P=0-200 MPa region. The porous ZIF-4 phase transforms into ZIF-4-cp through pore closure, while the latter has a phase transition at higher pressures regimes to ZIF-4-cp-II, a transformation which involves subtle changes in swing dihedral angles.