  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.
