Structure and dynamics in an organic ionic plastic crystal, N-ethyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) amide, mixed with a sodium salt

We present for the first time, the solid state phase behaviour of the organic ionic plastic crystal (OIPC) N-methyl-N-ethyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide, [C(2)mpyr][NTf2], upon mixing with the sodium salt, Na[NTf2]. Whereas the behaviour of OIPCs mixed with lithium salts has been well established, the influence of adding a sodium salt has not previously been reported. The phase diagram is presented for Na[NTf2] compositions between 1 and 50 mol%, and shows a eutectic composition at 15 mol% with a eutectic temperature at 63 degrees C. In contrast to the lithium doping of the same OIPC, the conductivity does not increase significantly at, or below room temperature, however the material containing 40 mol% Na[NTf2] has an ionic conductivity of 10(-4) S cm(-1) in the solid state at 60 degrees C, which is more than 3 orders of magnitude higher than the pure OIPC. Synchrotron XRD, solid-state NMR and SEM all indicate the presence of two distinct phases across all of the compositions studied. One phase is identical to that of the pure [C(2)mpyr][NTf2] and the other phase is a mixed cation compound, quite distinct from the pure Na[NTf2] material. The higher eutectic temperature (63 degrees C) in the sodium based system compared to lithium (30 degrees C) leads to purely solid state conductivity over the entire composition range up to 63 degrees C.