Plastic crystal behaviour in tetraethylammonium dicyanamide

The plastic crystal tetraethylammonium dicyanamide ([N_2,2,2,2][dca]) has been investigated with an emphasis on structure and dynamics in the plastic phase. It was found that almost all of the volume expansion occurs at the II → I transition, with no volume expansion at the melt transition (as normally observed for crystals). The conductivity of this material shows a rapid increase at temperatures below the II → I transition, reaching values ~ 10^{− 3} S/cm in Phase I, and 0.1 S/cm in the melt. The NMR measurements show that there is a sudden onset of rotational motions of the cations at the plastic transition; below this temperature the cations appear static. The rotational motion of the cation in Phase I has been discussed in terms of isotropic tumbling.<br />