Sr14[Al4]2[Ge]3: Eine Zintl-Phase mit isolierten [Ge]4–- und [Al4]8–-Anionen / Sr14[Al4]2[Ge]3: A Zintl Phase with Isolated [Ge]4− and [Al4]8− Anions

Abstract

The new ternary intermetallic compound Sr14[Al4]2[Ge]3 was synthesized from stoichiometric ratios of the elements. The crystal structure (trigonal, space group R3̅, a = 1196.58(2), c = 4010.33(7) pm, Z = 6, R1 = 0.0574) was determined using single crystal X-ray data. The structure contains two crystallographically independent tetrahedral [Al4] anions with Al-Al distances in the range from 269.7 to 273.6 pm. Taking into account the Zintl concept and the isosteric analogy to white phosphorus, their formal charge is−8. Both of these tetrahedra are surrounded by 16 Sr cations. The three isolated Ge4− anions per formula unit (isosteric to the noble gases) are coordinated by nine Sr cations. According to the ionic description Sr14[Al4]2[Ge]3̅↦14Sr2+ +2[Al4]8− +3[Ge]4− the title compound is an electron-precise Zintl phase. This interpretation is supported by the results of a FP-LAPW band structure calculation, which show a distinct minimum of the total density of states at the Fermi level. Attempts to synthesize the analogous compounds in the systems Sr-Ga-Ge and Ca-Ga-Ge resulted in the formation of new members of the Ca11Ga7 structure type family. In the case of Ca-Al-Ge only the stable binary border compounds Ca2Ge and CaAl2 were formed in respective experiments.