Phase stability in the systems AeAl2-xMgx (Ae = Ca, Sr, Ba): Electron concentration and size controlled variations on the Laves phase structural theme

The systems AeAl 2 - x Mg x (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu 2 → MgNi 2 → MgZn 2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl 2 - x Mg x (0 < x ≤ 2) and BaAl 2 - x Mg x (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu 2 → MgNi 2 → MgZn 2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu 2 structure at low x. For Ae = Ba a binary compound BaAl 2 does not exist, but more Ba-rich Ba 7 Al 1 3 forms. The reinvestigation of the crystal structure of Ba 7 Al 1 3 by selected area and convergent beam electron diffraction in a transmission electron microscope revealed a superstructure, which subsequently could be refined from single X-ray diffraction data. The formula unit of the superstructure is Ba 2 1 Al 4 0 (space group P31m, Z= 1, a = 10.568(1) A, c = 17.205(6) A). In Ba 2 1 Al 4 0 a size match problem between Ba and Al present in Ba 7 Al 1 3 is resolved. The structure of Ba 7 Al 1 3 (Ba 2 1 Al 4 0 ) can be considered as a Ba excess variant of the hexagonal MgNi 2 Laves phase type structure. An incommensurately modulated variant of the MgNi 2 structure is obtained for phases BaAl 2 - x Mg x with x = 0.8-1. At even higher Mg concentrations a structural change to the proper MgZn 2 type structure takes place.