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

摘要

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 LiB(15) to Li_5B, over a pressure range from 1 atm to 300 GPa. At P = 1 atm, stability is found for the experimentally known LiB_(0.8-1.0') LiB_(3,) and Li_3B_(14) phases. As the pressure rises, the latter two structures are no longer even metastable, while the LiB_(18-1.0) 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: LiB_(4,) Li_3B_v, Li_2B, and Li_5B. At the boron-rich end, one obtains structures expectedly containing polyhedral motifs, and geometries are governed by Wade-Mingos electron counts; LiB_4 has a BaAl_4 structure. In the center and on the lithium-rich side of the phase diagram, Zintl-phase considerations, i.e., bonding between B~(n-) entities, give us insight into the structures-tetrahedral B~ networks in LiB; B pairs to isolated bonds in Li_5B.