Crystal structures of highly-ordered long-period stacking-ordered phases with 18R, 14H and 10H-type stacking sequences in the Mg–Zn–Y system

Abstract The crystal structures of highly-ordered Mg–Zn–Y long-period stacking-ordered (LPSO) phases with the 18 R -, 14 H - and 10 H -type stacking sequences, which are constructed by stacking structural blocks composed of six, seven and five close-packed atomic layers, respectively, have been investigated by atomic-resolution scanning transmission electron microscopy (STEM) and transmission electron microscopy. The in-plane order characterized by the periodic arrangement of Zn 6 Y 8 atomic clusters on the lattice points of a 2 3 a Mg × 2 3 a Mg two-dimensional primitive hexagonal lattice ( a Mg : the lattice constant of Mg) is developed in the central four consecutive atomic layers in each of the structural blocks for the 18 R -, 14 H - and 10 H -type Mg–Zn–Y LPSO phases. The random occupation of the central site of each Zn 6 Y 8 atomic cluster with Mg, Zn, Y or vacancy is suggested through the analysis of the ultra-high resolution annular-bright field STEM images. The space groups of the most stable polytypes of the 18 R -, 14 H - and 10 H -type Mg–Zn–Y LPSO phases at 500 °C are determined to be C 2/ c , P 6 3 22 and Cmce , respectively.