Inelastic neutron scattering study on boson peaks of imidazolium-based ionic liquids

Abstract Low energy excitations of 1-alkyl-3-methylimidazolium ionic liquids (ILs) have been investigated by means of neutron spectroscopy. In the spectra of inelastic scattering, a broad excitation peak referred to as a “boson peak” appeared at 1–3 meV in all of the ILs measured. The intensity of the boson peak was enhanced at the Q positions corresponding to the diffraction peaks, reflecting the in-phase vibrational nature of the boson peak. Furthermore the boson peak energy ( E BP ) was insensitive to the length of the alkyl-chain but changed depending on the radius of the anion. From the correlation among E BP , the anion radius, and the glass transition temperature T g , we conclude that both E BP and T g in ILs are predominantly governed by the inter-ionic Coulomb interaction which is less influenced by the alkyl-chain length. We also found that the E BP is proportional to the inverse square root of the molecular weight as observed in molecular glasses.