Slow relaxation and boson peak studies of glass-forming ionic liquids at low temperatures*

The relaxation processes and boson peak (BP) behavior of two ionic liquids were investigated by means of QENS and INS at different temperatures (180 K – 300 K) and wave vector transfers (0.3 A^-1 – 2.0 A^-1), aiming to investigate the spatially heterogeneously dynamic contribution to the glass and melting transitions, and their relationship with the BP. The samples, tributylmethylammonium bis(trifluoromethylsulfonyl)imide and trimethylbutylammonium bis(trifluoromethylsulfonyl) were studied under heating from the glass to the liquid state after an almost instantaneous cooling to 180 K.
We used the KWW stretched exponential to probe the relaxation time and the DHO function to analyze the BP and crystal vibrational modes. The BP intensity increases and the BP energy decreases by increasing the temperature. We obtained the relaxation times of the glassy, crystal, supercooled liquid and liquid states between 0.1 ns and 2 ns. The results were compared with previous structural analyses. Besides the diffusion process of the liquid state, our study correlates the relaxation processes with the spatial heterogeneity and provides direct evidence on the structural origins of the slow relaxations of ionic glass.