Sr3Al2Ge4, Ca10Al6Ge9 and Ca20Al6Ge13.: New aluminium germanides

In the ternary systems Ca-Al-Ge and Sr-Al-Ge three germanides with new structure types have been synthesized from stoichiometric ratios of the elements. Their crystal structures were determined using single crystal X-ray data. In the structure of Sr3Al2Ge4 (monoclinic, space group C2/m, a = 1267.6(4), b = 416.2(2), c = 887.4(3) pm, beta = 110.37(2)degrees, Z = 2, R1 = 0.0354) Al-Ge sheets with Al in tetrahedral (i.e. Al-) and Ge in threefold psi-tetrahedral (i.e. Ge-) coordination against Ge are present. Thus, the compound can be classified as an electron precise Zintl phase. This finding is verified by the result of a band structure calculation (within the FP-LAPW approach), that shows a distinct minimum of the total density of states at the Fermi level. The structure of Ca10Al6Ge9 (trigonal, space group R (3) over barm, a = 1398.45(14), c = 2107.4(3) pm, Z = 6, R1 = 0.0613) contains complicated sheets of trigonal planar building units [AlGe3] and [AlGe4] tetrahedra. The compound Ca-20[Al3Ge6](2)[Ge] (hexagonal, space group P6(3)/m, a = 1600.9(2), c = 458.48(7) pm, Z = 1, R1 = 0.0282) shows two planar trimers of [AlGe3] triangles of formula [Al3Ge6] besides isolated Ge atoms (i.e. Ge4-). The overall electron count of the latter compounds, that contain trigonal planar coordinated Al atoms and considerable multiple bond character of the Al-Ge bonds, shows a very small deviation from the Zintl concept, comparable to the one observed in other aluminium-germanides like SrAlGe.