Zigzag Charge Ordering in α′-NaV 2O 5

23 Na NMR spectrum measurements in α ′ -NaV 2 O 5 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 F m m 2 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. It is also found that the temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions.