<p>Pyrrolidinium
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 ionic liquids 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 (P<i><sub>1n</sub></i>TFSI, n=1,2,4). Brillouin spectral
features such as the number of acoustic modes, their shape, and linewidth
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 P<i><sub>11</sub></i>TFSI and P<i><sub>12</sub></i>TFSI are a consequence of a
high degree of disorder and plasticity present in the system. Anomalous
temperature-dependent behavior of linewidth and asymmetric (Fano) line shape of
acoustic modes observed in certain phases of P<i><sub>1n</sub></i>TFSI 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.</p>
Inversion of light-scattering measurements for particle size and optical constants: theoretical study
