Crystal structure, magneto-structural correlation, thermal and electrical studies of an imidazolium halometallate molten salt: (trimim)[FeCl4]

A novel imidazolium halometallate molten salt with formula (trimim)[FeCl4] (trimim: 1,2,3-trimethylimidazolium) was synthetized and studied with structural and physico-chemical characterization. Variable-temperature synchrotron X-ray powder diffraction (SXPD) from 100 to 400 K revealed two structural transitions at 200 and 300 K. Three different crystal structures were determined combining single crystal X-ray diffraction (SCXD), neutron powder diffraction (NPD), and SXPD. From 100 to 200 K, the compound exhibits a monoclinic crystal structure with space group P21/c. At 200 K, the former crystal system and space group are retained, but a disorder in the organic cations is introduced. Above 300 K, the structure transits to the orthorhombic space group Pbcn, retaining the crystallinity up to 400 K. The study of the thermal expansion process in this temperature range showed anisotropically evolving cell parameters with an axial negative thermal expansion. Such an induction occurs immediately after the crystal phase transition due to the translational and reorientational dynamic displacements of the imidazolium cation within the crystal building. Electrochemical impedance spectroscopy (EIS) demonstrated that this motion implies a high and stable solid-state ionic conduction (range from 4 × 10−6 S cm−1 at room temperature to 5.5 × 10−5 S cm−1 at 400 K). In addition, magnetization and heat capacity measurements proved the presence of a three-dimensional antiferromagnetic ordering below 3 K. The magnetic structure, determined by neutron powder diffraction, corresponds to ferromagnetic chains along the a-axis, which are antiferromagnetically coupled to the nearest neighboring chains through an intricate network of superexchange pathways, in agreement with the magnetometry measurements.


Experimental procedures
Nuclear Magnetic Resonance (NMR) 1 H and 13 C NMR spectra were recorded on a Bruker DPX400 400 MHz nuclear magnetic resonance spectrometer.

Electrospray Ionization Mass spectrometry (ESI-MS)
ESI-MS analyses were carried out on a Bruker ESI-TOF MicroTOF II.

Fourier Transform Infrared Spectroscopy (ATR FT-IR)
ATR FT-IR measurements were performed on a Bruker Alpha Series FT-IR spectrometer equipped with an attenuated total reflectance (ATR) module. The ATR FT-IR spectra were recorded by collecting 24 scans of a sample in the ATR module.

Thermal analysis
A Setaram calorimeter (DSC131) was used for the thermal analyses from 20 to 250 ºC under nitrogen atmosphere at a heating rate of 10 K/min. For this experiment, ca. 5 mg of sample were used, and blank runs were performed.

Single-crystal X-ray diffraction and structure determination:
The crystal structure of (trimim) [FeCl 4 ] was determined by single crystal X-ray diffraction at 150 K. Data were collected using Mo-K radiation (0.71073 Å) in an Agilent Technologies Supernova diffractometer. A single crystal of the compound with approximate dimensions 0.11 mm  0.19 mm  0.23 mm was employed. The data reduction was performed with the CrysAlis PRO program. 1 Data were corrected for Lorentz and polarization effects. All the structures were solved by direct methods using the SIR92 program 2 and refined by full matrix least-squares on F 2 including all reflections (SHELXL97). 3 All non-hydrogen atoms were refined anisotropically. H atoms were included at calculated positions and treated as riding atoms with isotropic thermal motion related to that of its parent atom. All the calculations for these structures were performed using the WINGX crystallographic software package. 4 Table S1 gathers the crystal data and structure refinement parameters. It deserves to note that PLATON 5 Addsym routine detects a possible pseudo-translation (c/2) at 0 -0.004 1/2, but the nonfits (ca. 8%) include C5 atom of the imidazolic ring, which implies to rule out the proposition. In fact, its consideration leads to persistently high R1 values (12-13%) and the resulting structure require modelling a disorder of imidazolium cation in two positions. Furthermore, the thermal parameters of the disordered atoms are not clearly defined and require hard restrains to make the refinement stable. All in all, proposed pseudo-translation has been disregarded, in such a way that the herein reported structure is featured by lacking of any disordered entity and by exhbiting reasonable R-factors (R1 = 5.3%) and anisotropic thermal displacement parameters.
Variable temperature synchrotron X-Ray powder data collection: Synchrotron X-ray powder diffraction (SR-XRPD) measurements were performed at the high resolution station of the MSPD beamline (BL04) at ALBA synchrotron. 6 The sample was introduced into a 0.7 mm glass capillary and measured in transmission at an energy of 20 keV (0.61872 Å wavelength determined from a Si NIST-640d reference) using the microstrip Mythen-II detector (six modules, 1280 channels/module, 50 µm/channel, sample-to-detector distance 550 mm). Temperature was controlled using an Oxford Cryosystems Series 700 Cryostream. Data from 2 to 45º (2θ) were collected during a 5 K/min ramp from 400 to 100 K every 30 s. The crystal structures were solved by the direct-space methodology implemented in TALP 6 software using intensities extracted with DAJUST 7 program. The final restrained Rietveld refinement was performed with the computer RIBOLS18 8 program.

Electrochemical Impedance Spectroscopy (EIS):
In order to study the ionic conductivity of the material, a 0.53 x 0.27 cm pellet was prepared. This pellet was not sintered since the compound decomposes at high temperature (ca. 130°C). The electrical properties were determined for the plane-parallel sample, performing ac complex impedance measurements with a Solartron 1260 Impedance Analyzer. The measured frequency range was 10 -2 -10 6 Hz, with 10 mV signal amplitude. The behavior of the material was studied between room temperature (R.T.) and 400 K (above this temperature, the material becomes corrosive and damages the current collectors). The impedance diagrams were analyzed and fitted by the software Zview.

Magnetization measurements
DC magnetic susceptibility measurements were performed using a Quantum Design PPMS magnetometer whilst heating from 2 to 300 K under an applied magnetic field of strength at 1 and 10 kOe. Magnetization as a function of field (H) was measured using the same magnetometer in the -50 ≤ H/kOe ≤ 50 at 2 K after cooling the sample in zero field.

Neutron diffraction experiments:
Neutron powder diffraction measurements were performed on D2B and D1B powder diffractometers at the Institute Laue-Langevin (ILL, Grenoble, France). Ca. 2 g of (trimim)[FeCl 4 ] were used for the experiments, which were placed in a cylindrical vanadium container and held in a liquid helium cryostat. The highresolution powder diffractometer D2B (λ=1.5942 Å) was used to obtain extensive and accurate structural data at 250 K over a large angular angle 5  2 160º. High flux and medium resolution diffractometer D1B, operated at λ = 2.525 Å, was used to study the evolution of the sample in the temperature range 10-250 K. Besides, neutron diffraction patterns were measured at 250, 10 and 1.5 K in the angular range 5  2 128º. The collected data allowed us to solve and refine the magnetic structure.           The R Bragg after last refinement cycle was of 6.85%. S10 Fig. S12. Ionic conductivity as a function of temperature obtained from the fit to the impedance spectra. Fig. S13. Fit to the Curie-Weiss law (solid red line) to the magnetic susceptibility data at 10 kOe. S11