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 [<i>Cmc</i>2₁, <i>oS</i>60, <i>Z</i> = 4, <i>a</i> = 4.5030(7) Å, <i>b</i> = 20.085(3) Å, <i>c</i> = 16.207(3) Å, wR2 = 0.0451, 1470 <i>F</i>² values, 93 variables]. The La₄Mg₇Ge₆ phase represents a new structure type with a monoclinic unit cell [<i>C</i>2/<i>m</i>, <i>mS</i>34, <i>Z</i> = 2, <i>a</i> = 16.878(3) Å, <i>b</i> = 4.4702(9) Å, <i>c</i> = 12.660(3) Å, β = 122.25(3)°, wR2 = 0.0375, 1466 <i>F</i>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