Magnetic properties of a non-centrosymmetric polymorph of FeCl ₃

The growth of single crystals of FeCl₃, through sublimation and from the melt, is presented alongside a thorough investigation of their magnetostructural properties through a combination of DC magnetization and AC magnetic susceptibility measurements, single crystal X-ray diffraction (SCXRD), neutron powder diffraction (NPD) and small-angle neutron scattering (SANS). A new chiral polymorph of FeCl₃ is identified, crystallizing in the non-centrosymmetric space group <i>P</i>3₁. NPD and SANS reveal that a weakly first-order magnetic phase transition occurs from a paramagnetic phase with significant short-range correlations to an antiferromagnetic phase at <i>T</i> _N = 8.6 K, best described by the magnetic propagation vector <b><i>k</i></b> = (1/2, 0, 1/3) which differs from the previously reported magnetic structure of the well-known centrosymmetric polymorph (space group <i>R</i>3̄). We show that disordered crystallographic models including a large number of stacking faults are required to accurately reproduce the scattering observed in NPD patterns, preventing full determination of the magnetic structure. The magnetic field and temperature-dependent behavior of the intensities of the <b><i>k</i></b> = (1/2, 0, 2/3) and (1/2, 0, 5/3) magnetic Bragg peaks measured by SANS suggest that a field-induced spin reorientation occurs at <i>H</i> = 40 kOe when <b><i>H</i></b> ‖<i>c</i>-axis and at a significantly lower field of <i>H</i> ≈ 25 kOe when <b><i>H</i></b> ⊥<i>c</i>-axis. Above these magnetic fields in both cases the spins lie predominantly in the basal plane. The long-range magnetic ordering and the field-induced transitions observed in the neutron scattering experiments coincide with anomalies observed in the magnetisation <i>versus</i> both temperature and applied field along the principal crystal directions.