Rich Polymorphism of a Metal–Organic Framework in Pressure–Temperature Space

We present an in situ powder X-ray diffraction
study on the phase stability and polymorphism of the metal–organic
framework ZIF-4, Zn­(imidazolate)2, at simultaneous high
pressure and high temperature, up to 8 GPa and 600 °C. The resulting
pressure–temperature phase diagram reveals four, previously
unknown, high-pressure–high-temperature ZIF phases. The crystal
structures of two new phasesZIF-4-cp-II and ZIF-hPT-IIwere
solved by powder diffraction methods. The total energy of ZIF-4-cp-II
was evaluated using density functional theory calculations and was
found to lie in between that of ZIF-4 and the most thermodynamically
stable polymorph, ZIF-zni. ZIF-hPT-II was found to
possess a doubly interpenetrated diamondoid topology and is isostructural
with previously reported Cd­(Imidazolate)2 and Hg­(Imidazolate)2 phases. This phase exhibited extreme resistance to both temperature
and pressure. The other two new phases could be assigned with a unit
cell and space group, although their structures remain unknown. The
pressure–temperature phase diagram of ZIF-4 is strikingly complicated
when compared with that of the previously investigated, closely related
ZIF-62 and demonstrates the ability to traverse complex energy landscapes
of metal–organic systems using the combined application of
pressure and temperature.