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 diffr action technique using anomalous scattering near a vanadium K-absorption ed ge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below T-C = 35 K. T he charge ordering pattern is the fully charged zigzag-type ladder with the unit cell 2a X 2b X 4c, but not the chain-type originally proposed for the spin-Peierls state. Charge disproportionation suggested in our model as th e average valence V4.5+/-deltac/2 is observed below T-C, showing continuous variation of delta (c) as a function of temperature.