Impedance spectroscopy studies of proton conductivity in imidazolium malonate

Imidazolium malonate njstands out from other proton conducting compounds of nitrogen containing heterocyclic bases with dicarboxylic acids due to a presence of both ordered and disordered imidazolium in the crystal lattice. Our impedance spectroscopy studies of electric conductivity revealed that in the range 273 K <= T <= 318 K the dc conductivity is characterized by an activation energy of E-a1 = 0.50 eV and the activation energy decreases to E-a2 = 0.17 eV at temperatures 318 K <= T <= 358 K. Single crystal X-ray diffraction studies showed that imidazolium malonate has the same triclinic structure with P-1 space group and rather high thermal displacements factors at 280 K as well as at 340 K. The results suggest that the low-temperature activation energy can be ascribed to phonon assisted 180 flipping of ordered imidazolium cations, observed earlier in NMR experiments. The 'fast-like diffusion' of protons in the high temperature range we consider to be due to the presence of disordered imidazolium cations enhanced by high thermal displacements. (C) 2017 Elsevier B.V. All rights reserved.