The new ternary germanides La4Mg5Ge6 and La4Mg7Ge6: crystal structure and chemical bonding

Description

The synthesis, structural characterization, and chemical-bonding peculiarities of the two new polar lanthanum magnesium germanides La4Mg5Ge6 and La4Mg7Ge6 are reported. The crystal structures of these intermetallics were determined by single-crystal X-ray diffraction analysis. The La4Mg5Ge6 phase crystallizes in the orthorhombic Gd4Zn5Ge6 structure type [Cmc2(1), oS60, Z = 4, a = 4.5030(7) angstrom, b = 20.085(3) angstrom, c = 16.207(3) angstrom, wR2 = 0.0451, 1470 F-2 values, 93 variables]. The La4Mg7Ge6 phase represents a new structure type with a monoclinic unit cell [C2/m, mS34, Z = 2, a = 16.878(3) angstrom, b = 4.4702(9) angstrom, c = 12.660(3) angstrom, beta = 122.25(3)degrees, wR2 = 0.0375, 1466 F-2 values, 54 variables]. Crystallographic analysis together with linear muffin-tin orbital band structure calculations reveals the presence of strongly bonded 3D polyanionic [Mg-Ge] networks balanced by positively charged La atoms in both stoichiometric compounds. The La4Mg5Ge6 compound is related to Zintl phases, showing a prominent density of states pseudogap at the Fermi level. A distinctive feature of the La4Mg5Ge6 structure is the presence of Ge-Ge covalent dumbbells; in La4Mg7Ge6, the higher Mg content generates a polyanionic network consisting exclusively of Mg-Ge heterocontacts. Nevertheless, the frameworks of the two phases are structurally similar, as is highlighted in this work.

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