Brillouin light scattering study of microscopic structure and dynamics in pyrrolidinium salt based ionic liquids

Pyrrolidinium salts based ionic liquids are known to be good ionic conductors even in solid-state around room temperature, which is attributed to the highly disordered plastic crystalline phase. Moreover, these salts are characterized by multiple phase transitions which include plastic, structural glass, and glassy crystal phases with varying levels of molecular disorder. Temperature-dependent Brillouin light scattering is used to investigate the phase transitions in a series of alkylmethylpyrrolidinium Bis(trifluoromethanesulfonyl) imides (P1nTFSI, n = 1,2,4). Brillouin spectral features such as the number of acoustic modes, their shape, and linewidths provide the picture of different disordered phases resultant of dynamics at the microscopic scale. The longitudinal and transverse acoustic velocities in different phases are determined from the corresponding acoustic mode frequencies (Brillouin shift). Extremely low acoustic velocities in the solid phase of P11TFSI and P12TFSI are a consequence of a high degree of disorder and plasticity present in these systems. Anomalous temperature-dependent behavior of linewidths and asymmetric (Fano) line shape of acoustic modes observed in certain phases of P1nTFSI could be due to the strong coupling between the Brillouin central peak and the acoustic phonons. The present results establish that the Brillouin light scattering technique can be efficiently used to understand the complex phase behavior, microscopic structure, and dynamics of ionic liquids. •Structural dynamics at microscopic level in ionic liquids are probed using a non-contact technique of Brillouin light scattering (BLS).•BLS illustrates the high plasticity and ionic conductivity in the solid phase of ionic liquids.•The complex phase behavior of pyrrolidinium salts based systems is efficiently interpreted by BLS technique.•Anomalous phase behavior observed only in the plastic crystal phase as a result of strong coupling between the Brillouin central peak and the acoustic phonons.