We
demonstrate anion (chloride and fluoride ions) transfer in an
organic ionic plastic crystal (OIPC), diethyldimethylammonium d-camphorsulfonate, and the role of defects in ion conduction.
The phase transitions, crystalline structures, dynamics, and ion transfer
mechanisms of the pure material and the doped mixtures were investigated
using a combination of differential scanning calorimetry, X-ray diffraction,
solid-state nuclear magnetic resonance spectroscopy, and electrochemical
impedance spectroscopy. The doped mixtures show minor modifications
in thermal behaviors and solid phase structures to the host material.
The ion mobility of the pure material in the plastic phase was assigned
mainly to cations. The fluoride salt-doped mixture has drastically
enhanced conductivity at all tested temperatures, and the chloride
salt-doped mixture displays a strong temperature-dependent behavior.
Ionic conductivity measurements suggest transfer mechanisms through
crystalline phases. The pure material and doped mixtures exhibit similar
defect volumes and concentrations, and both factors are phase-dependent,
as determined using positron annihilation lifetime spectroscopy. The
conductivity displays dependence on not only the defect volume but
also the target ion volume. The relationships between the defect volume
and the conductivity qualitatively follow the Cohen–Turnbull
free volume model, while critical volumes were very large. For the
first time in OIPCs, the size of the target ions was found to significantly
influence ionic conductivity.