Electronic ferroelectricity from charge ordering in RFe2O4

We report our recent discovery of novel ferroelectricity arising from the polar ordering of Fe³⁺ and Fe²⁺ in a mixed valence triangular lattice oxide LuFe₂O₄, where the electric polarization is not a result of 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 great potential for the future application of ferroelectrics.