Multiple Phase Transitions and Temperature-Dependent Ionic Conductivity of the Plastic Crystal Trioctylammonium Triflate Studied by Dielectric Spectroscopy and Calorimetry

Hydrophilic trioctylammonium triflate ([TOA][OTf]), a protic organic ionic plastic crystal (POIPC), is studied by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) in a temperature range from −100°C to 120 °C. Below the melting temperature,
${T}_{m}~\approx ~18 ^{\circ} {\mathrm {C}}$
, [TOA][OTf] shows five solid phases, separated by sharp thermal transitions as detected by DSC. BDS spectra are analyzed using various strategies for data analysis. Phase changes are discernible in temperature-dependent permittivity and dielectric loss data; however, the derivatives
${{\partial \epsilon ^{\prime }}}/ {{\partial }\text {ln}{\omega }}$
and
${{\partial }\text {ln}{\varepsilon ^{\prime }}}/ {{\partial {T}}}$
appear to be much more sensitive to confirm all calorimetric phase transitions consistently. Fitting of
${{\partial \epsilon ^{\prime }}} /{{\partial }\text {ln}{\omega }}$
spectra with symmetric Cole–Cole functions reveals two conductivity relaxation processes, the relaxation time of which (
${\tau }_{\text {fast}}$
and
${\tau }_{\mathrm {slow}}$
) is separated by about two decades over the whole temperature range. The dc conductivity (
${\sigma }_{\mathrm {dc}}$
) is computed from
${\varepsilon ^{\prime \prime }}$
spectra for the solid phases I–IV and in the melt. Within all solid phases,
${\sigma }_{\text {dc}}$
shows Arrhenius behavior with typically decreasing activation energy toward higher temperature except for phase III. In the melt,
${\sigma }_{\text {dc}}$
obeys Vogel–Fulcher–Tammann (VFT) behavior as expected favoring a “vehicle” mechanism for ionic charge transport. Surprisingly, the ratio
${\tau }_{\text {fast}}/ {\tau }_{\text {slow}}$
shows an exceptional sensitivity to phase changes, either by stepwise changes or changes in its temperature dependence. From the presence of two distinct conductivity relaxation modes, we conclude that the crossover from subdiffusive to diffusive charge transport proceeds via two characteristic length scales, where the longer one likely relates to apolar aggregates of nanophase-separated cation tails as proposed by various authors. Contact angle measurements support this hypothesis. Finally, we discuss the temperature dependence of
${\sigma \tau {T}}$
and
${\sigma \tau }$
in the framework of the Debye–Stokes–Einstein (DSE) and Barton–Nakajima–Namikawa (BNN) relation. Here,
${\sigma }_{\text {dc}}{\tau }_{\text {slow}}{T}$
exclusively “sees” the transition III
$\to $
II, which implies that this transition originates from a reorganization of the nanophase-separated apolar aggregates.