Thermal and physical properties of an archetypal organic ionic plastic crystal electrolyte

The thermal and mechanical properties of the ionic plastic crystal N-methyl-N-propylpyrrolidinium hexafluorophosphate have been investigated and the effect of adding a miscible polymer on the mechanical properties is reported. The physical properties of the pure plastic crystal are discussed in detail and for the first time the change in volume with temperature for an organic ionic plastic crystal is reported. An increase in volume in conjunction with increased conductivity supports the hypothesis that ion conduction within the plastic crystal proceeds via defects. For phase I and melting, the magnitude of the volume increase does not appear to be in accord with the subtle change in conductivity. This is suggested to be due to the presence of layer defects, which allow for correlated ionic motion, which does not increase the conductivity. Addition of polymer to the plastic crystal significantly increases the mechanical strength, decreases the conductivity, but has little effect on the phase behaviour, further supporting the hypothesis of vacancy-mediated conduction.