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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.

Accurate electron density mapping is quite a common practice for crystals cooled at low temperature and accurately measured. This is not true for species under external perturbation, due to complicated experimental conditions. Studying molecular crystals in excited states is a challenge, Coppens(2009), and a purely experimental electron density mapping is not possible at present. So far, the same limitation affected molecular crystals at high pressure, with only few attempts to use theoretical multipoles to fit experimental data, Fabbiani (2011). Here we report on the first unconstrained multipolar model, refined for syn-l,6;8,13 biscarbonyl[14]annulene (BCA) at P=7.7 GPa. BCA was the subject of a low temperature data collection by Destro (1995). The molecule (close to C2v symmetry) has a fair aromaticity, but it progressively localizes double and single bonds as a function of pressure. At 7.7 GPa the geometrical distortion is quite evident and mirrored by the electron density. The experiment, carried out at Diamond Light Source, was possible combining: a) high energy (40 Kev) to overcome the resolution problems caused by diamond anvil cells and reduce absorption and extinction; b) microfocused beam (30 micron) to minimize spurious X-ray diffusion; c) two crystals in the DAC, to increase data coverage; d) sufficient pressure to quench atomic motion. The final agreement is obviously worse than what typically obtained at ambient pressure. However, the model is satisfactory because: a) the deformation density is sensible and in agreement with the calculated one; b) the distribution of residuals is normal and no significant error is evident. The study proves that aromatic molecules are more reactive when squeezed, in keeping with the recent theoretical study by Hoffmann et al. on benzene. The Figure shows the static deformation density of BCA in 3D, obtained from a multipolar model refined against the experimental structure factors.