Zigzag Charge Ordering in alpha'-NaV2O5
  23Na NMR spectrum measurements in alpha'-NaV2O5 with a single- crystalline sample are reported. In the charge-ordered phase, the number of inequivalent Na sites observed is more than that expected from the low-temperature structures of space group Fmm2 reported so far. This disagreement indicates that the real structure including both atomic displacement and charge disproportionation is of lower symmetry. It is suggested that zigzag ordering is the most probable. The temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions. It is thus concluded that the charge ordering transition is first-order. 
Tetsuo Ohama, Atsushi Goto, Tadashi Shimizu, Emi Ninomiya, Hiroshi   Sawa, Masahiko Isobe, Yutaka Ueda