Crystal and magnetic structure of the (trimim)[FeBr4] molten salt: A temperature dependence study

Abstract

A new imidazolium-based halometallate complex with formula (trimim)[FeBr4] (trimim: 1,2,3-trimethylimidazolium) has been synthesized and characterized as a function of the temperature. The structural phase transitions, magnetic properties and magnetic structure of this compound have been thoroughly studied. The crystal structure evolution from 1.8 to 400 K has been investigated by a combination of synchrotron X-ray and neutron powder diffraction, as well as single crystal X-ray and neutron diffraction. Four different phases were found: phase I, from 400 to 350 K, with the orthorhombic Cmc21 space group; phase II, from 350 to 160 K, which was modelled with the monoclinic P21/m space group; and phase III, stable down to 13 K and showing the monoclinic Cm space group. The transition from phase III to II involves the appearance of disorder in the organic cations. Magnetometry measurements indicate a long-range magnetic order below 13 K, which was confirmed by single crystal and powder neutron X-ray diffraction analyses. The occurrence of new reflections that were not possible to index with the phase III unit cell suggests a symmetry breaking, giving rise to the phase IV. A joint refinement (single-crystal and powder data) revealed that the latter phase exhibits the magnetic Shubnikov Ccc space group. This model consists of a strictly antiferromagnetic structure with ferromagnetic interactions within the unit cell, which is compatible with the magnetometry results.