Organic
ionic plastic crystals (OIPCs) are a ductile soft material
where the composing ions are in isotropic free rotation, while their
positions are aligned in order. The rotational motion in its plastic
phase promotes ion conduction by decreasing the activation energy.
Here, we report novel OIPCs comprised of tetracyanoborate ([TCB]−) and various organic cations. In particular, the OIPC
composed of [TCB]− and spiro-(1,1′)-bipyrrolidinium
([spiropyr]+) cations can transform into its plastic phase
at ultralow temperature (Tp = −55 °C) while maintaining a high melting point
(Tm = 242 °C). Replacement
of the cation with either tetraalkylammonium or phosphonium and comparing
their phase behavior, the high Tm was attributed to the relatively small interionic distance
between [spiropyr]+ and [TCB]−. At the
same time, the low Tp was realized by the restricted vibrational mode of the spirostructure,
allowing the initiation of isotropic rotational motion with less thermal
energy input.