Superconducting boron allotrope featuring pentagonal bipyramid at ambient pressure

Elemental boron has evoked substantial interest owing to its chemical complexity in nature. It can form multicenter bonds due to its electron deficiency, which induces the formation of various stable and metastable allotropes. The search for allotropes is attractive for finding functional materials with fascinating properties. Using first-principles calculations with evolutionary structure search, we have explored boron-rich K-B binary compounds under pressure. A series of dynamically stable structures (Pmm2 KB5, Pmma KB7, Immm KB9, and Pmmm KB10) containing boron framework with open channels are predicted, which can possibly be synthesized under high pressure and high temperature conditions. After the removal of K atoms, we obtain four novel boron allotropes, o-B-14, o-B-15, o-B-36, and o-B-10, which exhibit dynamical, thermal, and mechanical stability at ambient pressure. Among them, o-B-14 contains an unusual B-7 pentagonal bipyramid and appears in a bonding combination of seven-center-two-electron (7c-2e) B-B pi bonds, which is the first time to be identified in three-dimensional boron allotropes. Interestingly, our calculation reveals that o-B-14 can act as a superconductor with a T-c value of 29.1 K under ambient conditions.

Automated summary

Elemental boron has attracted considerable interest due to its electron deficiency, which allows for the formation of multicenter bonds and various stable and metastable allotropes. Through first-principles calculations and structure search, boron-rich K-B binary compounds have been investigated, leading to the prediction of dynamically stable structures containing boron framework with open channels. Furthermore, four novel boron allotropes were discovered to possess dynamical, thermal, and mechanical stability at ambient pressure, with o-B-14 exhibiting an unusual bonding combination and potential superconductivity.