M3(Au,Ge)19 and M(3.25)(Au,Ge)18 (M = Ca, Yb): distinctive phase separations driven by configurational disorder in cubic YCd6-type derivatives.

Exploratory syntheses in the M-Au-Ge (M = Ca, Yb) systems have led to the discovery of two cleanly separated non-stoichiometric phases M(3)Au(approximately 14.4)Ge(approximately 4.6) (I) and M(3.25)Au(approximately 12.7)Ge(approximately 5.3) (II). Single crystal X-ray studies reveal that both (space group Im3) feature body-centered-cubic packing of five-shell multiply endohedral clusters that resemble those in the parent YCd(6) (= Y(3)Cd(18)) and are akin to approximate phases in other quasicrystal systems. However, differences resulting from various disorders in these are distinctive. The innermost cluster in the M(3)Au(approximately 14.4)Ge(approximately 4.6) phase (I) remains a disordered tetrahedron, as in the YCd(6) parent. In contrast, its counterpart in the electron-richer M(3.25)Au(approximately 12.7)Ge(approximately 5.3) phase (II) is a "rattling" M atom. The structural differentiations between I and II exhibit strong correlations between lattice parameters, cluster sizes, particular site occupancies, and valence electron counts.