Molecular solids at extreme pressure

The term ‘Extreme Pressure’ could mean a number of different things depending on your field of study. What is extreme for biologists interested in life in deep sea trenches, barely registers on the scale used by many planetary scientists. The last 300 years (or so) of chemistry has largely been dominated by the use of temperature to make or modify materials, and pressure beyond a few tens of atmospheres is encountered rarely, if at all. Nevertheless, pressure is a very powerful thermodynamic variable which is being applied increasingly to molecular systems.

For molecular materials we can usually vary temperature only within a few hundred degrees, whereas the difference between atmospheric pressure and 1 GPa represents a factor of 10⁴. 1 GPa (= 9869 atm = 10 kbar) is a rather modest figure by the standards of modern high-pressure science, and is far from being esoteric or involving undue personal risk: in fact, guacamole is processed at 0.8 GPa. In the papers published in this special issue of CrystEngComm the term ‘high pressure’ refers to the region between about 0.1 and 20 GPa. Contributions by pressure × volume terms to free energy in this range can be equivalent to covalent bond energies, and substantial structural changes in molecular solids can be induced by these pressures.

High-pressure science is a mature area, having been used for decades by mineralogists and physicists. Over the last ten years, high-pressure research has branched from these origins into a number of different fields, facilitated by advances in experimental techniques. In particular, the study of organic and metal–organic systems is an area of increasing interest and study by the crystal engineering community.

Scientists interested in molecular solids aim to understand structure–property relationships. Changes in magnetism, colour, metal coordination, pore content and transitions to other polymorphs can all be induced by pressure and the source of the change directly correlated with changes in structure. Gaining a predictive understanding of the molecular solid state is one of the big questions still to be solved in chemistry, and high pressure is one of the most powerful tools at our disposal. From the computational viewpoint, for example, since intermolecular potentials and other computational protocols used for modelling of the solid state are usually optimised with reference to ambient-pressure structural data, so their application to high-pressure problems is a very demanding test of their validity.

The majority of papers in this issue deal with work in the organic solid state covering pressure-induced phase transitions, polymorphism and control of hydrogen bonding. A number also address the field of inorganic and metal–organic chemistry with a focus on tuning properties using pressure. Working in the field of high-pressure structural chemistry is rather like having the first Bunsen burner and having to decide what to heat first: this themed issue offers an overview of current work in this emerging area of crystal engineering by leading researchers in the field.

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