Yet Another Case of Lithium Metal Atoms and Germanium Atoms Sharing Chemistry in the Solid State: Synthesis and Structural Characterization of Ba₂LiGe₃

Abstract

Several Ba–Li–Ge ternary phases are known and structurally characterized, including the title compound Ba2LiGe3. Its structure is reported to contain [Ge6]10– anions that exhibit delocalized bonding with a Hückel‐like aromatic character. The Ge atoms are in the same plane with the Li atoms, and if both types of atoms are considered as covalently bonded, [LiGe3]4– honeycomb‐like layers will result. The latter are separated by slabs of Ba2+ cations. However, based on the systematic work detailed herein, it is necessary to re‐evaluate the phase as Ba2Li1–xGe3+x (x < 0.05). Although small, the homogeneity range is clearly demonstrated in the gradual change of the unit cell for four independent samples. Subsequent characterization by single‐crystal X‐ray diffraction methods shows that the Ba2Li1–xGe3+x structure, responds to the varied number of valence electrons and the changes are most pronounced for the refined lengths of the Li–Ge and Ge–Ge bonds. Indirectly, the changes in the Ge–Li/Ge distances within layers affect the stacking too, and these changes can be correlated to the variation of the c‐cell parameter. Chemical bonding analysis based on TB‐LMTO‐ASA level calculations affirms the notion for covalent character of the Ge–Ge bonds; the Ba–Ge and Li–Ge interactions also show some degree of covalency.

Applicability of the Zintl Concept to Understanding the Crystal Chemistry of Lithium-Rich Germanides and Stannides

“…The case of Li 5– x Ge 2 is clearly in between the “end” members. Another example of intermediate bond order and intricate contribution of Li to π-bonding concerning Ge–Ge bonds is the structure of Ba 2 LiGe 3 , or rather Ba 2 Li 1– x Ge 3+ x , where the bond order is fractional, with the hexagonal [Ge 6 ] 10– units showing characteristics of Hückel-aromaticity . Examples of intermediate bond order in stannides also exist, with Ca 9– x Li 2 Sn 10 being just a more recent one …”

“…Another example of intermediate bond order and intricate contribution of Li to π-bonding concerning Ge−Ge bonds is the structure of Ba 2 LiGe 3 , or rather Ba 2 Li 1−x Ge 3+x , where the bond order is fractional, with the hexagonal [Ge 6 ] 10− units showing characteristics of Huckel-aromaticity. 50 Examples of intermediate bond order in stannides also exist, with Ca 9−x Li 2 Sn 10 being just a more recent one. 70 One can surmise that in all structures, π*-contributions to the Ge−Ge bond are expected to increase with increasing the Li content.…”

“…48 The existence of Ge defects is another possibility; the hypothesis that the homogeneity range can be due to Li/Ge disorder is also a viable alternative, brought to light by the demonstration that lithium atoms can share position with germanium atoms in some structures, such as Ba 8 Li x Ge 46−x (x < 5.33), 49 and Ba 2 Li 1−x Ge 3+x (x < 0.1). 50 To distinguish the eventual scenarios Li 5−x Ge 2+x vs Li 5−x Ge 2 , we decided to reinvestigate the isotypic Li 5 Sn 2 phase as well, employing the same techniques, single-crystal X-ray diffraction for multiple crystals, supported by the Rietveld refinement of synchrotron X-ray powder diffraction and neutron powder diffraction data. The results from the structural work are discussed in the context of the Zintl concept, 51,52 which also proves helpful in rationalizing the structures.…”

“…These results indicate that Li 5 Ge 2 , most likely, is not a line compound, a case that could be analogous to Li 5 Si 2 , which von Schnering et al showed to host Li defects . The existence of Ge defects is another possibility; the hypothesis that the homogeneity range can be due to Li/Ge disorder is also a viable alternative, brought to light by the demonstration that lithium atoms can share position with germanium atoms in some structures, such as Ba 8 Li x Ge 46– x ( x < 5.33), and Ba 2 Li 1– x Ge 3+ x ( x < 0.1) …”

Results from Exploratory Work in Li-Rich Regions of the AE-Li-Ge Systems (AE = Ca, Sr, Ba)

“…In contrast, the shortest Li-Ge distance (2.72 Å) is close to the second shortest Li-Ge distance (2.80 Å). Both Li-Ge distances are longer than those in many ternary compounds: BaLiGe 2 (2.70 Å) [41], Ba 2 LiGe 3 (2.58 Å) [42,43], RE 3 Li 4 Ge 4 (2.62-2.69 Å) [44] and RE 2 Li 2 Ge 3 (2.56-2.62) Å) [45] but close to the values reported for some ternary compounds such as LiZnGe (2.78 Å) [46]. Such longer Li-Ge distances suggest enhanced metallic characters compared with the reported alkaline-earth metal lithium germanides and rare-earth metal lithium germanides.…”