## 3 Results and Discussion
### Absorbance as a Function of Pressure  
Figure 3 displays the evolution of the infrared absorption between 600 and 1500 cm\({}^{-1}\) as a function of pressure in the range 0.1-10 GPa. Note that a background of interference fringes, due to multiple reflections between the two parallel diamond surfaces, is present over the whole spectral and pressure range. At low pressures, one can clearly observe the intense absorption  
Figure 2: Absorbance of 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI–TFO) in the mid infrared range, measured on cooling (**left panel**) and subsequent heating (**right panel**). lines of the anion centered around 639, 1031, 1163, 1225 and 1263 cm\({}^{-1}\), similar to the same bands measured at 330 K in the temperature-dependent experiments, which are reported for comparison in the lower part of Figure 3. Concerning the cation vibrations, one can observe the line around 758 cm\({}^{-1}\) and the CH bending bands above 1300 cm\({}^{-1}\). With the increasing of the applied pressure, \(p\), all bands shift towards higher frequencies, as expected, except for the absorption band centered around 639 cm\({}^{-1}\), that corresponds to the SO bond vibration [35]. This fact can be ascribed to a high strength of the SO double bond; such resilience to frequency shift of the SO vibration around 630 cm\({}^{-1}\) was observed also in \(N\)-trimethyl-\(N\)-propylammoniumbis(trifluoromenthanesulfonyl)imide and \(N\)-trimethyl-\(N\)-hexylammoniumbis(trifluoromenthanesulfonyl)imide, whose TFSI anion has two SO\({}_{2}\) groups with the same type of vibrations around 620 cm\({}^{-1}\)[23]. With increasing p, no splitting nor any abrupt disappearance of absorption lines is visible, suggesting that the sample does not undergo a crystallization process. In the literature, the occurrence of phase transitions as a function of pressure has been revealed in some cases by a change of the slope of the position of absorption peaks as a function of applied pressure. Most of the intense absorptions reported in Figure 3 do not appear as single bands, as they are composed by multiple contributions. Only the absorption line centered around 1031 cm\({}^{-1}\) for \(p\) = 0.1 GPa can be assimilated to a single band. A fit with a single Lorentzian curve for this contribution has been performed for all the measured pressure spectra and Figure 4 reports the frequency of such a vibration band, \(\
u_{\text{b}}\), as a function of p. It is well evident that the slope remains constant on the whole pressure range here investigated. Therefore, it seems unlikely that the sample undergoes any crystallization as a function of pressure. This behavior is quite different from the temperature dependence, which, on the contrary, indicates the transformation towards a crystalline state at low temperatures. By a fit of the data reported in Figure 4, one obtains a variation of the vibration frequency versus applied pressure of \(\partial\
u_{\text{b}}/\partial p=3.04\pm 0.02\) cm\({}^{-1}\)GP\({}^{-1}\). This value is close to those reported for 1-decyl-3-methylimidazolium tetrafluoroborate, 1-octyl-3-methylimidazoliumtetrafluoroborate, 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, 1-butyl-1-methylpyrrolidinium, tetrafluoroborateand 1-hexyl-3-methylimidazolium tetrafluoroborate [15; 53]. Figure 3: Absorbance of EMI–TfO in the mid infrared range measured at room temperature as a function of pressure. For comparison, two spectra recorded as a function of the temperature on heating in the same frequency range are reported in the lower part of the figure. To the best of our knowledge, only a couple of reports about the pressure phase diagram of TiO-based ILs are available [52; 54; 55].