Plastic phase transitions in N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfony)imide

The salt N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR12TFSI) undergoes several solid-solid phase transitions before melting at 91 degrees C. At -120 degrees C, the crystal structure consists of ordered ions (phase IV). After the solid-solid phase transition at -85 degrees C, however, all of the PYR12+ and TFSI- ions become disordered, forming a plastic crystalline phase (phase III). We report here the PYR12TFSI 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.