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

Abstract

The synthesis, structural characterization, and chemical-bonding peculiarities of the two new polar lanthanum-magnesium germanides La(4)Mg(5)Ge(6) and La(4)Mg(7)Ge(6) are reported. The crystal structures of these intermetallics were determined by single-crystal X-ray diffraction analysis. The La(4)Mg(5)Ge(6) phase crystallizes in the orthorhombic Gd(4)Zn(5)Ge(6) structure type [Cmc2(1), oS60, Z = 4, a = 4.5030(7) Å, b = 20.085(3) Å, c = 16.207(3) Å, wR2 = 0.0451, 1470 F(2) values, 93 variables]. The La(4)Mg(7)Ge(6) 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 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 La(4)Mg(5)Ge(6) compound is related to Zintl phases, showing a prominent density of states pseudogap at the Fermi level. A distinctive feature of the La(4)Mg(5)Ge(6) structure is the presence of Ge-Ge covalent dumbbells; in La(4)Mg(7)Ge(6), 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.

Synthesis and crystal chemistry of new ternary pnictides containing lithium—adding structural complexity one step at a time

“…A very close structural analog is the RE 4 Mg 7 Ge 6 family (La 4 Mg 7 Ge 6 structure type), 50,51 which also crystallizes with the same space group C2/m, but is not isotypic. A very close structural analog is the RE 4 Mg 7 Ge 6 family (La 4 Mg 7 Ge 6 structure type), 50,51 which also crystallizes with the same space group C2/m, but is not isotypic.…”

Polar‐Covalent Bonding Beyond the Zintl Picture in Intermetallic Rare‐Earth Germanides

“…In previous studies R ‐ M ‐Ge ( R =rare earth metal; M =other metal) systems were characterized from the point of view of phase equilibria and intermediate ternary compound formation . Among these, the R 2 M Ge 6 constitute a continuously growing family, which has been studied with respect both to crystal structure peculiarities, and physical properties .…”

Phase equilibria in the La–Mg–Ge system at 500°C and crystal structure of the new ternary compounds La11Mg2Ge7 and LaMg3−xGe2

“…Among these, the most probable parent types were CaIn 2 (P6 3 /mmc-fb) and LaLi 3 Sb 2 (P-3m1-d 2 ba). Based on our knowledge and previous investigations on R-Mg-Ge compounds [1][2][3][4] it is known that neither Mg/Ge nor R/Ge show a tendency to give statistical mixtures. That is why the LaLi 3 Sb 2 -model containing 4 Wyckoff sites was chosen as possible structural model.…”

“…Crystal and electronic structure properties of the ternary compounds τ 3 -La 4 Mg 5 Ge 6 and τ 4 -La 4 Mg 7 Ge 6 were discussed in [4]. These phases are involved in two three-phase fields (τ 2 + τ 3 + τ 4 and τ 3 + τ 4 + τ 5 )…”

“…These compounds provide a valuable dataset to study the interplay between composition, crystal structure and chemical bonding peculiarities of intermetallics. In this framework, we focused on the La-Mg-Ge system, with the aim to enrich the largely unexplored family of Mg-containing germanides, and thus the crystal structure and chemical bonding of La 4 Mg 5 Ge 6 and La 4 Mg 7 Ge 6 were elucidated [4]. The whole composition range of the chosen system was then searched; in this work results on phase equilibria and several new ternary La-Mg-Ge compounds are discussed.…”