Charge-Order Pattern of the Low-Temperature Phase of NaV2O5 Uniquely   Determined by Resonant X-Ray Scattering from Monoclinic Single Domain
  The present resonant x-ray scattering from each of monoclinically-split single domains of NaV2O5 has critically enhanced contrast between V4+ and V5+ ions strong enough to lead to unambiguous conclusion of the charge-order pattern of its low-temperature phase below Tc = 35 K. The zig-zag type charge-order patterns in the $ab$-plane previously confirmed have four kinds of configurations (A, A', B and B') and the stacking sequence along the c-axis is determined as the AAA'A' type by comparison with model calculations. By assigning the A and A' configurations to Ising spins, one can reasonably understand the previously discovered "devil's staircase"-type behavior with respect to the modulation of the layer-stacking sequences at high pressures and low temperatures, which very well resembles the global phase diagram theoretically predicted by the ANNNI model. 
Kenji Ohwada, Yasuhiko Fujii, Yuya Katsuki, Jiro Muraoka, Hironori   Nakao, Youichi Murakami, Hiroshi Sawa, Emi Ninomiya, Ma...