Phase diagram of ZIF-4 from computer simulations

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, even though it is known to be stable under temperature and pressure conditions within the studied range. Our simulations shed light on 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 in higher pressure regimes to ZIF-4-cp-II, a transformation which involves subtle changes in the orientation of four member rings with respect to unit cell vectors.