Polytypes of long-period stacking structures synchronized with chemical   order in a dilute Mg-Zn-Y alloy
  A series of structural polytypes formed in an Mg-1at.%Zn-2at.%Y alloy has been identified, which are reasonably viewed as long-period stacking derivatives of the hcp Mg structure with alternate AB stacking of the close-packed atomic layers. Atomic-resolution Z-contrast imaging clearly revealed that the structures are long-period chemical-ordered as well as stacking-ordered; unique chemical order along the stacking direction occurs as being synchronized with a local faulted stacking of AB'C'A, where B' and C' layers are commonly enriched by Zn/Y atoms. 
E. Abe, A. Ono, T. Itoi, M. Yamasaki, Y. Kawamura
cond-mat.mtrl-sci