Abstract
We present an investigation of the organic ionic plastic crystal choline triflate, both in its pure state and in mixtures with triflic acid. High ionic conductivity behavior was observed in these materials in the solid state with Arrhenius behavior evident in all cases suggesting a single, thermally activated conduction mechanism of ionic transport. The activation energies (E-a) in phases II and I were very low (similar to 16-17 kJ mol(-1)), consistent with plasticlike mechanical properties throughout the wide range of temperatures (0-130 degrees C) investigated here. Positron annihilation lifetime spectroscopy (PALS) indicated that the thermally generated defects play a significant role in the high ionic conductivities in these phases. High-resolution powder XRD studies showed a structural transformation from monoclinic to cubic at the II -> I phase transformation in all samples. The acid-containing samples exhibited an additional phase (phase III) and displayed structural transformations from orthorhombic to monoclinic to cubic with increasing temperature in the phase III -> II -> I transition. This system exhibited unusual behavior, with the addition of acid leading to apparently less mobility of the matrix ions (as seen by NMR spectroscopy and the decrease in low-temperature conductivity) despite a higher proton reduction activity than previously reported. The combination of high-resolution XRD, PALS, and solid-state NMR techniques at varying temperature suggests a thermally activated, defect-assisted conductivity mechanism of ionic transport in these materials.