Evidence of secondary relaxations in the dielectric spectra of ionic liquids

We investigated the dynamics of a series of room temperature ionic liquids based on the same 1-butyl-3-methyl imidazolium cation and different anions by means of broadband dielectric spectroscopy covering 15 decades in frequency (10^-6--10⁹0.3em0exHz), and in the temperature range from 4000.3em0exK down to 350.3em0exK. Ionic conductivity is observed above the glass transition temperature Tg, and below, two relaxation processes appear, with the same features as the secondary relaxations typically observed in molecular glasses. The activation energy of the secondary processes of these ionic systems, and their dependence on the anion, are different. The slower process shows the characteristics of an intrinsic Johari-Goldstein relaxation, in particular an activation energy E_β=24kBTg is found, as observed in molecular glasses.