Na12Ge17: a compound with the zintl anions [Ge-4](4-) and [Ge-9](4-) - synthesis, crystal structure, and raman spectrum

Abstract

Na12Ge17 is prepared from the elements at 1025 K in sealed niobium ampoules. The crystal structure reinvestigation reveals a doubling of the unit cell (space group:P21/c; a = 22.117(3)Å, b = 12.803(3)Å, c = 41.557(6)Å, β = 91.31(2)°, Z = 16; Pearson code: mP464), furthermore, weak superstructure reflections indicate an even larger C-centred monoclinic cell. The characteristic structural units are the isolated cluster anions [Ge9]4— and [Ge4]4— in ratio 1:2, respectively. The crystal structure represents a hierarchical cluster replacement structure of the hexagonal Laves phase MgZn2 in which the Mg and Zn atoms are replaced by the Ge9 and Ge4 units, respectively. The Raman spectrum of Na12Ge17 exhibits the characteristic breathing modes of the constituent cluster anions at ν = 274 cm—1 ([Ge9]4—) and ν = 222 cm—1 ([Ge4]4—) which may be used for identification of these clusters in solid phases and in solutions. Raman spectra further prove that Na12Ge17 is partial soluble both in ethylenediamine and liquid ammonia. The solution and the solid extract contain solely [Ge9]4—. The remaining insoluble residue is Na4Ge4. By heating the solvate Na4Ge9(NH3)n releases NH3 and decomposes irreversibly at 742 K, yielding Na12Ge17 and Ge.