An EPR Study of Phase Transitions in Paramagnetic Ionic Liquids With [FeCl4] Anions

The PILs investigated, 1-butyl-3-methylimidazolium tetrachloroferrate ([C$_{4}$mim][FeCl$_{4}$] or [Bmim][FeCl$_{4}$]) and 1-ethyl-3-methylimidazolium tetrachloroferrate ([C$_{2}$mim][FeCl$_{4}$] or [Emim][FeCl$_{4}$]), are pure samples and, thus, exhibit novel thermomorphic behavior. Up to now, the paramagnetic behavior of such salts has been studied by observing small deviations from the Curie law at low temperatures. However, intuition indicates that electron paramagnetic resonance (EPR) spectroscopy is the right choice for studying PILs, although the method is rarely applied in the field of ionic liquids. The EPR group at CSUN, for years now, has been studying Heisenberg spin exchange in systems with a low concentration of magnetic species. This work attempts to apply their analysis to study spin exchange in high-concentration magnetic species. A Bruker EMX plus continuous-wave spectrometer was used to obtain the spectra for these salts in the temperature range 150 - 330 K. The EPR parameters were extracted from the spectra by using a least-squares fitting method. We found that the spectra of these pure PIL samples are dominated by the Heisenberg spin exchange (HSE) interaction, even at low temperatures, where dipole-dipole (DD) interactions are thought to prevail and influence the observed lineshapes mostly. For [Bmim][FeCl$_{4}$], repeated heating cycles reveal a notable phase transition at $T =$ 183 K corresponding to its vitrification, or glass transition. The EPR spectrum of this PIL shows one Lorentzian line throughout the entire observed temperature range, regardless of thermal treatment. This observed linewidth was used to obtain the linewidth due to HSE through the Anderson-Weiss theory. This result was then used to estimate the exchange integral of [Bmim][FeCl$_{4}$] in its liquid phase using the Curie-Weiss law resulting in a value of 0.03 K. Conversely, upon cooling, [Emim][FeCl$_{4}$] first crystallizes into modification \textbf{2} at $T =$ 270 K and then experiences a solid-solid transition into modification \textbf{1} at $T =$ 230 K. The EPR spectrum of this PIL shows one exchange-narrowed Lorentzian line in the liquid phase (330 K $ T $ 270 K) and in the solid modification \textbf{2} phase (270 K $ T $ 230 K). Furthermore, two exchange-narrowed EPR Lorentzian lines were observed below the solid-solid transition (150 K $ T $ 230 K). The two EPR signals likely originate from the two crystal structure modifications of [Emim][FeCl$_{4}$], which were studied by B\"{a}cker \textit{et al.}$^{49}$ previously. In the current study, the second EPR line appearing below $T =$ 230 K indicates that a large-fraction of the sample transforms into a new crystal structure (modification \textbf{1}) below the solid-solid transition during cooling. Simultaneously, the remaining part of the sample (modification \textbf{2}) produces its own EPR signal, which exists throughout the entirety of the solid-phase region, thus perturbing the observed EPR lineshape in this temperature range. Upon subsequent heating of [Emim][FeCl$_{4}$], its crystal modification \textbf{1} transitions into modification \textbf{2} at $T =$ 260 K before the sample finally melts at $T =$ 292 K. Using the observed EPR linewidth and the distance between nearest-neighbor Fe$^{3+}$ ions, the EPR linewidth due to HSE was calculated following the Anderson-Weiss theory. The HSE linewidth of each [Emim][FeCl$_{4}$] crystal modification was then used to estimate the exchange integral for each of these polymorphs using the Curie-Weiss law. The computed values of the exchange integral for the different phases of [Emim][FeCl$_{4}$] are 0.06 K for modification \textbf{1}, 0.24 K for modification \textbf{2}, and 0.04 K for the liquid phase. These results indicate that crystal modification \textbf{2} has a stronger superexchange coupling than that of modification \textbf{1}. This fact is also confirmed by looking at the crystallographic arrangement of [FeCl$_{4}$]$^{-}$ anions in both polymorphs, and analyzing their superexchange parameters. Lastly, the solid-solid transition and melting/crystallization transition in [Emim][FeCl$_{4}$] display a hysteretic behavior.