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 273K≤T≤318K the dc conductivity is characterized by an activation energy of Ea1=0.50eV and the activation energy decreases to Ea2=0.17eV at temperatures 318K≤T≤358K. 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 280K as well as at 340K. 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.