The relationship between orientational disorder and pressure: The case study of succinonitrile

Plastic phases can be considered as an intermediary state between the crystalline state and the liquid state, in which the molecules maintain their positions, but they exhibit orientational disorder. Whereas plastic phases have been studied as a function of temperature, the effect of pressure on these systems is largely unknown. The pressure temperature phase diagram of succinonitrile has been obtained by a topological approach and by direct measurement. Both approaches demonstrate that the temperature range of the plastic phase increases with pressure. This in first instance counterintuitive result can be explained by a faster increase of the diffusional energy barrier than the rotational barrier with pressure, or in other words, the liquid phase becomes even more unstable than the plastic phase with pressure. These conclusions have been confirmed by comparing the succinonitrile results with a large group of other compounds exhibiting plastic crystal behavior. (C) 2014 Elsevier B.V. All rights reserved