An unexpected high-pressure stability domain for a lower density polymorph of benzophenone

For over a century, it was thought that the crystalline polymorph II of benzophenone does not possess a stable domain in the pressure–temperature phase diagram. With a combination of new experimental results and literature data, this case of crystalline dimorphism has finally been solved and it is shown that form II possesses a stable domain at high pressure and high temperature, even though its density is lower than that of form I, the stable form under ordinary pressure and temperature conditions. The phase diagram of benzophenone is a clear demonstration of the fact that to understand the phase behaviour of a chemical substance both the exchange of heat (due to the change in intermolecular interactions) and work (due to the change of volume at a given pressure) need to be taken into account.