Ferroelectricity and a 1/3-Magnetization Plateau in the Polar Magnet LiFeV₂O₇ with Fe³⁺ Ions in a Twisted Diamond Chain

We investigated the thermodynamic properties and spin exchanges of the polar magnet LiFeV₂O₇ through experiments and density functional theory calculations. LiFeV₂O₇ crystallizes in a polar monoclinic Cc structure, where Fe³⁺ (S = 5/2) ions form diamond chains along the [101] direction, each surrounded by four neighboring chains. The compound undergoes long-range antiferromagnetic ordering at T_N ≈ 5 K, which decreases under an external magnetic field. Below T_N, it exhibits multiferroic behavior, and a 1/3-magnetization plateau above a critical field of 4.3 T. Neutron diffraction measurement at 1.5 K reveals a collinear magnetic structure with k = (0, 1, 0) with unequal spin moments on monomer and dimer sites of the diamond chains (3.41(1) vs 1.56(1) μ_B). These unique magnetic properties stem from strongly antiferromagnetic dimer spin exchange and weakly antiferromagnetic dimer-monomer spin exchanges. This results in a small energy gap between the magnetic ground and excited states below T_N and induces thermal spin fluctuations at the monomer and dimer sites reducing their moments. The further reduction of the moment at the dimer sites is ascribed to quantum spin fluctuations of the dimer sites.