Phase Stability in the Systems AeAl<sub>2</sub><sub>-</sub><i><sub>x</sub></i>Mg<i><sub>x</sub></i> (Ae = Ca, Sr, Ba): Electron Concentration and Size Controlled Variations on the Laves Phase Structural Theme
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The systems AeAl2-xMgx (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu2 → MgNi2 → MgZn2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl2-xMgx (0 < x ≤ 2) and BaAl2-xMgx (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu2 → MgNi2 → MgZn2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu2 structure at low x. For Ae = Ba a binary compound BaAl2 does not exist, but more Ba-rich Ba7Al13 forms. The reinvestigation of the crystal structure of Ba7Al13 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 Ba21Al40 (space group P31m, Z = 1, a = 10.568(1) Å, c = 17.205(6) Å). In Ba21Al40 a size match problem between Ba and Al present in Ba7Al13 is resolved. The structure of Ba7Al13 (Ba21Al40) can be considered as a Ba excess variant of the hexagonal MgNi2 Laves phase type structure. An incommensurately modulated variant of the MgNi2 structure is obtained for phases BaAl2-xMgx with x = 0.8−1. At even higher Mg concentrations a structural change to the proper MgZn2 type structure takes place.
