Plastic Phase Transitions in N-Ethyl-N-methylpyrrolidinium Bis(trifluoromethanesulfonyl)imide

The salt N -ethyl- N -methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 12 TFSI) undergoes several solid−solid phase transitions before melting at 91 °C. At −120 °C, the crystal structure consists of ordered ions (phase IV). After the solid−solid phase transition at −85 °C, however, all of the PYR 12 + and TFSI - ions become disordered, forming a plastic crystalline phase (phase III). We report here the PYR 12 TFSI phase III and IV structures determined from single-crystal diffraction data using the same crystal. The two structures are compared to elucidate the mechanism of the phase transition and gain a greater understanding of disordering modes of ions in the solid state.