From Wade–Mingos to Zintl–Klemm at 100 GPa: Binary Compounds of Boron and Lithium

Structural diversity and a variety of bonding schemes emerge as characteristics of the Li–B phase diagram in this ground-state theoretical investigation. We studied stoichiometries ranging from LiB15 to Li5B, over a pressure range from 1 atm to 300 GPa. At P = 1 atm, stability is found for the experimentally known LiB0.8–1.0, LiB3, and Li3B14 phases. As the pressure rises, the latter two structures are no longer even metastable, while the LiB0.8–1.0 structures change in geometry and narrow their range of off-stoichiometry, eventually coming at high pressure to a diamondoid NaTl-type LiB. This phase then dominates the convex hull of stability. Other phases emerge as stable points at some pressure: LiB4, Li3B2, Li2B, and Li5B. At the boron-rich end, one obtains structures expectedly containing polyhedral motifs, and geometries are governed by Wade–Mingos electron counts; LiB4 has a BaAl4 structure. In the center and on the lithium-rich side of the phase diagram, Zintl-phase considerations, i.e., bonding between Bn– entities, give us insight into the structures—tetrahedral B– networks in LiB; B pairs to isolated bonds in Li5B.