Crystal polymorphism of a room-temperature ionic liquid, 1,3-dimethylimidazolium hexafluorophosphate: Calorimetric and structural studies of two crystal phases having melting points of ∼50 K difference

► 1,3-Dimethylimidazolium hexafluorophosphate has two polymorphic crystals, α and β. ► The presence of crystal β has remained unknown for this sample. ► It lacks conformational flexibility in the associated ions. ► Among all ILs, it shows the largest difference between the melting points (50 K).