Abstract
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.
Highlights
Nowadays, hybrid organic–inorganic materials play a major role in the development of advanced functional materials
Details of SCXRD, synchrotron X-ray powder diffraction (SXPD), magnetic and heat capacity measurements, Electrochemical impedance spectroscopy (EIS), neutron powder diffraction (NPD) measurements are thoroughly described in the Electronic supplementary information (ESI).†
The title compound transits to the liquid state at a considerably higher temperature (192 C; see DSC analysis in Fig. S7, ESI†) than other tetrachloridoferrate complexes with less symmetric imidazolium cations, since the melting point depends on the symmetry of the organic cation, among other factors
Summary
Tetrahaloferrate(III) complexes are attractive due to their magnetic and crystal behavior. Speci cally, imidazolium-based tetrahaloferrate(III) compounds present long-range magnetic order at low temperature which lead to a highly organized 3-D structure in their condensed phase. (coulombic), to speci c anisotropic forces (such as hydrogen bonding, halogen bonding, dipole–dipole, magnetic dipole and electron pair donor/acceptor interactions) Some of these interactions are strongly in uenced by the temperature, while others are not affected due to the anisotropy of both the unit cell and the H-bonding network. As a result, these materials exhibit unusual anisotropic thermal expansion processes, which can be related to the ability of the organic moiety to present translational and reorientational phenomena that govern properties such as magnetism and solid-state ionic conduction. We report the synthesis, crystal characterization, ionic conductivity and magnetic properties of a new tetrahaloferrate molten salt containing the 1,2,3-trimethylimidazolium cation and tetrachloridoferrate anion, (trimim)[FeCl4]. Details of SCXRD, SXPD, magnetic and heat capacity measurements, EIS, NPD measurements are thoroughly described in the ESI.†
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