Abstract
In the ternary system Ba-Al-Ge new intermetallic compounds which are lying on or close to the binary section BaAl 2 -BaGe 2 were synthesized from the elements and characterized on the basis of X-ray single crystal data. The Al-content x in the compounds BaAl x Ge 2−x forming the AlB 2 structure type ranges from x = 1.4 [BaAl 1.4 Ge 0.6 space group P6/mmm, a = 443.5(1), c = 512.4(1) pm, Z = 1, R1 = 0.0222] to the stoichiometric ordered compound BaAlGe [space group P6̅m2, a = 434.9(1), c = 513.6 pm, Z = 1, R1 = 0.0252]. In the two new Ge-rich barium intermetallics Ba 5 Al 2 Ge 7 [space group C2/m, a = 859.8(4), b = 1031.5(4), c = 1847.8(6) pm, β = 103.23(3)°, Z = 4, R1 = 0.0553] and Ba 7 Al 4 Ge 9 [space group Fmm2, a = 1032.7(5), b = 2559(2), c = 862.1(4) pm, Z = 4, R1 = 0.1197] complex Al/Ge polyanions are present, which consist of (1) Al/Ge-ribbons of condensed planar sixmembered rings comparable to the anions in Ba 3 Al 2 Ge 2 , and (2) [Ge/Al] 5 clusters comparable to the anions in the tetrelides Ba 3 M 5 . The building units (1) and (2) are connected via Al-Ge bonds to form complex ribbons in the case of Ba 5 Al 2 Ge 7 and sheets in the case of Ba 7 Al 4 Ge 9 . The electron count in the two compounds supports an interpretation of the structures according to the Zintl concept and the Wades rules. The small formal electron excess, caused by the incomplete transfer of charge from Ba towards the Al/Ge polyanions, decreases with the Ge content of the compound.