Crystal Polymorphism of 1-Butyl-3-methylimidazolium Hexafluorophosphate: Phase Diagram, Structure, and Dynamics

The ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), is one of the most representative ILs. Despite its relatively simple ion structure and popularity, [C4mim]PF6 shows complex and confusing thermal phase behaviours, which stem from crystal polymorphism associated with cation conformational change and large thermal hysteresis. To the best of our knowledge, [C4mim]PF6 is the most investigated IL in terms of phase diagram, whereas full understanding has not yet been achieved due to its complexity. Here we review the current status of understanding of the phase diagram and structure/dynamics of each crystalline phase. Presently, depending on temperature and pressure, five structurally different polymorphic crystals have been reported as α, β, γ, δ, and δ’ in addition to some unspecified phases implied by calorimetric studies. Particularly for the α, β and γ phases, the structure and dynamics are well investigated by Raman, NMR, and X-ray scattering techniques.