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 (2 KB, KB, KB, and KB) 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, -B, -B, -B, and -B, which exhibit dynamical, thermal, and mechanical stability at ambient pressure. Among them, -B contains an unusual B pentagonal bipyramid and appears in a bonding combination of seven-center-two-electron (7c-2e) B-B π bonds, which is the first time to be identified in three-dimensional boron allotropes. Interestingly, our calculation reveals that -B can act as a superconductor with a value of 29.1 K under ambient conditions.