X-ray anomalous scattering investigations on the charge order in $alpha^prime$-NaV$_2$O$_5$

Anomalous x-ray diffraction studies show that the charge ordering in $alpha^prime$-NaV$_2$O$_5$ is of zig-zag type in all vanadium ladders. We have found that there are two models of the stacking of layers along emph{c-}direction, each of them consisting of 2 degenerated patterns, and that the experimental data is well reproduced if the 2 patterns appears simultaneously. We believe that the low temperature structure contains stacking faults separating regions corresponding to the four possible patterns.