New Ternary Germanides La4Mg5Ge6 and La4Mg7Ge6: Crystal Structure and Chemical Bonding

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 [Cmc21, oS60, Z = 4, a = 4.5030(7) Å, b = 20.085(3) Å, c = 16.207(3) Å, wR2 = 0.0451, 1470 F2 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) Å, b = 4.4702(9) Å, c = 12.660(3) Å, β = 122.25(3)°, wR2 = 0.0375, 1466 F2 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.