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 Tc=35 K. Consequently, the charge ordering pattern is the zigzag-type ladders 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+-delta_c/2} is observed below T_C, showing continuous variation of delta_c as a function of temperature. 
H. Nakao (1), K. Ohwada (1), N. Takesue (1), Y. Fujii (1), M. Isobe   (2), Y. Ueda (2), M. v. Zimmermann (3), J. P. Hill (3)...