X-Ray Anomalous Scattering Study of a Charge-Ordered State in NaV2O5

Charge ordering of V⁴⁺ and V⁵⁺ in NaV₂O₅ has been studied by an x-ray diffraction technique using anomalous scattering near a vanadium K-absorption edge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below T_C = 35 K. The charge ordering pattern is the fully charged zigzag-type ladder with the unit cell 2a×2b×4c, but not the chain-type originally proposed for the spin-Peierls state. Charge disproportionation suggested in our model as the average valence V^4.5+/-δ_c/2 is observed below T_C, showing continuous variation of δ_c as a function of temperature.