Low-frequency Raman spectra of a glass-forming ionic liquid at low temperature and high pressure

The frequency range below ∼100 cm^-1 of the Raman spectrum of a glass-forming liquid exhibits two features that characterize the short-time (THz) dynamics: the quasi-elastic scattering (QES) tail and the boson peak (BP). In this work, we follow temperature and pressure effects on the intermolecular dynamics of a typical ionic liquid, 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, [Pip₁₄][[NTf₂]. The glass transition temperature of [Pip₁₄][[NTf₂] at atmospheric pressure is T_g = 198 K, and the pressure of glass transition at room temperature is P_g = 1.1 GPa. Raman spectra obtained while cooling the liquid or heating the glass exhibit hysteresis in QES and BP intensities, I_QES and I_BP. The dependence of I_QES, I_BP, and the BP frequency, ω_BP, with pressure up to the glass transition is steeper than the temperature dependence due to the stronger pressure effect on density within the GPa range. The temperature and pressure behaviors of the parameters I_QES, I_BP, and ω_BP obtained here for [Pip₁₄][[NTf₂] are discussed in light of known results for other glass-formers.