Charge ordering of LuFe₂O₄ observed by an anomalous X-ray dispersion

We observed a superlattice reflection for triangular layered oxide LuFe₂O₄ by using anomalous X-ray dispersion with synchrotron radiation source. We found that superlattice spot (1/3, 1/3, 4.5) shows a large intensity variation upon a slight change of incident X-ray energy (wave length) from 7.114 keV (1.7437 Å) to 7.1448 keV (1.7353 Å) across the Fe absorption edge. This phenomenon is explained by superlattice structure of Fe²⁺ and Fe³⁺ on triangular lattice. The existence of the superlattice structure due to the charge ordering of iron ions on triangular lattice indicates that LuFe₂O₄ possesses local spontaneous polarization.