Ba5Al2Ge7 und Ba7Al4Ge9: Zwei neue intermetallische Phasen mit ungewöhnlichen Al-Ge-Anionen / Ba5Al2Ge7 and Ba7Al4Ge9: Two New Intermetallic Phases with Unusual Al-Ge Anions

Abstract

In the ternary system Ba-Al-Ge new intermetallic compounds which are lying on or close to the binary section BaAl2 -BaGe2 were synthesized from the elements and characterized on the basis of X-ray single crystal data. The Al-content x in the compounds BaAlxGe2−x forming the AlB2 structure type ranges from x = 1.4 [BaAl1.4Ge0.6 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 Ba5Al2Ge7 [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 Ba7Al4Ge9 [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 Ba3Al2Ge2, and (2) [Ge/Al]5 clusters comparable to the anions in the tetrelides Ba3M5. The building units (1) and (2) are connected via Al-Ge bonds to form complex ribbons in the case of Ba5Al2Ge7 and sheets in the case of Ba7Al4Ge9. 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.