  The atomic structure of Guinier-Preston (G.P.) zone in an Mg-Zn-Gd alloy has
been determined by combining atomic resolution high angle annular dark field
(HAADF) scanning transmission electron microscopy (STEM) and first-principles
calculation. The G.P. Zone consists of three atomic layers paralleling to
(0001) plane of Mg matrix, where two outer layers enrich with Gd while the
middle layer enriches with Zn. Moreover, it is found that most of the
structures reported previously are unstable and should converge to the present
result after atomic relaxation, even though the atomic species and positions in
the middle layer vary in those models.
