The structure of long period stacking/order Mg–Zn–RE phases with extended non-stoichiometry ranges

We propose structural models of the unique long period stacking/order (LPSO) phases formed in Mg–Zn–RE alloys, based on Z-contrast scanning transmission electron microscopy observations and first principles calculations. The LPSO structures are long period stacking derivatives of the hcp Mg structure, and the Zn/RE distributions are restricted at the four close-packed atomic layers forming local fcc stacking (i.e. a local ABCA stacking). Chemical order is well developed for the LPSO phases formed in Mg97Zn1Er2 (14H type) and Mg85Zn6Y9 (18R type) alloys with pronounced superlattice reflections, and the relevant Zn/RE distributions clearly emerge in the Z-contrast atomic images. Initial ternary ordered models were constructed by placing all the atoms at the ideal honeycomb sites, leading to plausible space groups of P63/mcm for the 14H type and C2/m, P3112 or P3212 for the 18R type. The characteristic ordered features are well represented by local Zn6RE8 clusters, which are embedded in the fcc stacking layers in accordance with the L12 type short-range order. Energy favored structural relaxations of the initial model cause significant displacement of the Zn/RE positions, implying that strong Zn–RE interactions may play a critical role in phase stability. The LPSO phases seem to tolerate a considerable degree of disorder at the Zn and RE sites with statistical co-occupations by Mg, extending the non-stoichiometric phase region bounded along the Zn/RE equiatomic line from ∼Mg94.0Zn2.0Y4.0 to ∼Mg83.3Zn8.3Y8.3.