Structural modulations in multiferroic tetragonal tungsten bronze K_xMn_xFe_1+xF₃

Multiferroic materials showing coupling of the different order parameters (ferroelectric, ferromagnetic, ferroelastic) are interesting not only from a fundamental perspective, but also from a technological point of view, e.g. for to the development of new storage technologies. However, the coexistence of (ferro)magnetism and ferroelectricity is considered a rare phenomenon. Whilst this may be true for perovskite oxides, where empty d-shells favor the off-centering of ions but counteract magnetism, this intrinsic limitation can be avoided by moving to different structure types, and/or away from oxides. An example of non-perovskite, non-oxide multiferroic systems are the tetragonal tungsten bronze (TTB) fluorides K_xM²⁺_xM³⁺_1+xF₃ (x = 0.4 – 0.6), which show coexistence of electric and magnetic ordering ¹. Here we present a detailed structural study on a series of TTB fluorides, K_xMn_xFe_1+xF₃ (x = 0.4 – 0.55). KMnFeF₆ has been previously described as tetragonal P4₂bc and orders ferrimagnetically below T = 148 K ². Additional satellite reflections were found in transmission electron microscopy experiments and attributed to ferroelastic domains arising from tilting of MF₆ octahedra, but the reported bulk powder XRD measurements indicated only tetragonal symmetry ³. We used high-resolution powder diffraction techniques to reinvestigate the crystal structure as a function of temperature in comparison with DSC data. Our results reveal a structural distortion to orthorhombic symmetry (Ccc2) at room temperature, which diminished when moving to the end members of the series (x → 0.4 and x → 0.6). Although structurally subtle, this distortion may indicate a ferroelectric state, similar to K_xFeF₃, where ferroelectricity is observed only in the orthorhombic phase. On heating, an anomaly in the c-axis lattice parameter accompanies a phase transition to centrosymmetric P4₂/mbc around 320 – 350 K, marking the transition from ferroelectric – paraelectric state.

Investigation of the Orbital and magnetic orderings in VF3

We report on a magnetic and structural investigation of layered antiferromagnetic system vanadium (III) fluoride. VF3 crystallizes in a distorted ReO3 structure (R-3c) with rotated undistorted VF6 octahedra. The V3+ cations are arranged in a triangular lattice with the possibility of exhibiting magnetic frustration. Polycrystalline samples of VF3 were investigated using heat capacity, dielectric, magnetic susceptibility, synchrotron and neutron powder diffraction methods. Combining our results, we report the first evidence for a first order phase transition resulting from the ordering of the t2g orbitals below 105-110 K. This transition reduces the symmetry to C2/c. We further confirm that VF3 undergoes a long-range antiferromagnetic order at ∼19 K in accordance with literature [1]. The antiferromagnetic order results in a magnetic structure with the magnetic moments alternating between a parallel and b parallel alignments in the ab plane.