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

The frequency range below-100 cm-I 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 pres sure effects on the intermolecular dynamics of a typical ionic liquid, 1-buty1-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, [NTf2]. The glass transition temperature of [Pip(14)]1[NTf2] at atmospheric pressure is T-g = 198 K, and the pressure of glass tran sition at room temperature is Pg = 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 IoEs, IBP, and the BP frequency, coop, 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 omega(BP) obtained here for [PiP(14)] [[NTf2] are discussed in light of known results for other glass formers Published under license by AIP Publishing