Electronic Ferroelectricity from Charge Ordering in RFe2O4

We report a novel ferroelectricity arise from the polar ordering of Fe³⁺ and Fe²⁺ in a mixed valence triangular lattice oxide LuFe₂O₄, where the electric polarization do not arise from the ionic displacement. The polar ordering of Fe³⁺ and Fe²⁺ was confirmed with a resonant X-ray scattering study in SPring-8. The origin of such ordering is the competitive interaction between Fe³⁺ and Fe²⁺ in the triangular lattice, i.e. the charge frustration. The polar superlattice of Fe³⁺ and Fe²⁺ develops below 350 K where the electric polarization appears. The ferroelectricity arising from the polar charge ordering or the polar electron distribution may have a great potential for the future application of ferroelectrics.