Impedance spectroscopy studies of proton conductivity in imidazolium malonate

Imidazolium malonate ɱstands 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 Ea1 = 0.50 eV and the activation energy decreases to Ea2 = 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.