Temperature evolution of (quinuclidinium)[FeCl4]: a plastic/polar magnetic hybrid compound with a giant dielectric constant

Abstract

The temperature evolution of the physical properties of the (quinuclidinium)[FeCl₄] compound shows an unprecedented series of phase, structural and magnetic transitions. Above 390 K, a plastic phase (also termed as “rotator phases”) is observed (phase I). Between 390 K and 280 K, an intermediate phase solved in a non-centrosymmetric space group has been characterized. Crystallographic studies show that the compound crystallizes in the Pbc2₁ polar space group (phase II), and therefore the phase transition between I and II involves a paraelectric-to-polar phase transition. This result is confirmed by complex dielectric permittivity. Moreover, the real part of the dielectric permittivity shows a giant increase above 390 K, reaching a maximum value of 10⁵ that is notably larger than any other hybrid compound previously reported. Below 280 K, a third structural phase transition is observed, which involves the doubling of the unit cell and a change of symmetry due to the blocking of counter-ions, phase III crystallizing in the Pbca centrosymmetric space group. Below 3.5 K, long-range magnetic order is detected. Neutron diffraction below this order temperature suggests breaking of symmetry and the magnetic structure in phase IV was solved in the P2₁′2₁′2₁ Shubnikov space group, giving rise to a ferrimagnetic structure that allows a net ferromagnetic signal along the c-axis.